DISEASES OF

and Related Plants

[jm^

Agriculture Handbook No. 144

U. S. DEPARTMENT OF AGRICULTURE

^

v.:t'V
CONTENTS

Page

Cabbage and other crucifers 1

Parasitic and nonparasitic diseases 1

Parasitic diseases and their control --- 2

General control practices 2

Clubroot 3

Diseases caused by nematodes- 7

Yellows 9

Black rot 17

Blackleg 19

Rhizoctonia disease 21

Bacterial soft rot 24

Watery soft rot and white

blight 25

Black leaf spot 26

Ring spot, or black blight 27

Powdery mildew 29

Downy mildew 29

White rust 31

Bacterial leaf spot of cauliflower 31

Black root of radish 32

Page

Parasitic diseases and their control—Continued

Mosaic 32

Damping-off 33

Leaf spots 34

Nonparasitic maladies 34

Low-potash injury 34

Magnesium-deficiency chlorosis 35

Boron-deñciency injury 35

Whiptail of cauliflower 35

Low-temperature injuries. _ 36

Tipburn 36

Flooding injury 36

Lightning injury 36

Intumescences 37

Transit and storage diseases 38

Freezing injury 38

Bacterial soft rot 40

Pythium head rot 40

Rhizopus soft rot 40

Gray mold rot 41

Black leaf speck 41

Walker, John Charles, 1893-

Diseases of cabbage and related plants [by J. C. Walker,

E. H. Larson, and A. L. Taylor. Washington, U. S. Govt.

Print. Off., 19583

41 p. nius. 24 cm. (U. S. Dept. of Agriculture. Agriculture

handbook no. 144)

Contains information formerly presented in Farmers' bulletin 1439

with the same title.

1. Cabbage—Diseases (and pestS) 2. Cruciferae—Diseases (and

pestS) ( Series)

SB608.C14W22 1958 635.34 Agr 58-342

U. S. Dept. of Agr. Libr. lAg84Ah no. 144

for Library of Congress t

This publication contains information formerly presented in Farmers' Bulletin

1439 of the same title.

Washington, D. C. Issued November 1958

For sale by the Superintendent of Documents, U. S. Government Printing Office

Washington 25, D. C. - Price 25 cents
Diseases of

CABBAGE

and Related Plants

By J. 0. WALKER, collaborator, Crops Research Division, Agricultural

Research Service, and professor of plant pathology, University of Wisconsin, R. H. LARSON, pathologist, Agricultural Research Service, and associate professor of plant pathology, University of Wisconsin, and A. L.

TAYLOR, nematologist. Crops Research Division, Agricultural Research

Service

CABBAGE AND OTHER CRUCIFERS

From the original wild stocks of

the cabbage group have come cultivated cabbage, cauliflower, broccoli,

brussels sprouts, kohlrabi, collards,

and kale. Other cultivated plants

closely related to these are turnip,

radish, rape, rutabaga, and horseradish. Among the related wild

plants, shepherds-purse, peppergrass, and mustard are of most frequent occurrence. Mustard is sometimes cultivated, but some species

grow so profusely under all conditions that they are more commonly

classed as weeds. The term "crucifers" as used in this handbook refers

collectively to all the vegetables and

weeds mentioned in this paragraph,

all of which belong to the botanical

family Cruciferae, so called from

the form of the four-petaled flower.

Many of the crucifers are subject

to the same diseases, so that the

methods of control of the diseases

of cabbage and its close relatives

can often be applied to other species

as well.

PARASITIC AND NONPARASITIC DISEASES

A large number of the diseases

discussed in this publication are

brought about by the attack of the

plant by nematodes and microscopic

organisms—^bacteria and fungi—or

ultramicroscopic agencies known as

viruses. This class of diseases collectively is referred to here as parasitic diseases. Their development is

influenced greatly by the climate

and by the condition of the soil, as

these environmental factors affect

not only the aggressiveness of the

parasite and progress of the disease

but also the susceptibility of the

plant it attacks.

There are many disease conditions in which no parasite is involved. They result from the detrimental influence of certain climatic

and soil conditions upon the crop.

This group is referred to as nonparasitic diseases. Outstanding

examples of nonparasitic diseases

of cabbage are soil-mineral-deficiency injuries (for example, potash starvation), flooding injury,

freezing injury, and tipburn.
2 AGR. HANDBOOK 14 4, U. S. DEPT. OF AGRICULTURE

PARASITIC DISEASES AND THEIR CONTROL

General Control Practices

Use of Disease-Free Seed

Blackleg, a fungus disease, and

black rot, a bacterial disease, are

more or less prevalent in all the

cabbage sections in the Central,

Eastern, and Southern States. The

germs of both maladies readily

overwinter on even mildly infected

seed heads, and they then attack

the seed plants the following season. From a diseased field it is

almost impossible to select seed

heads that are positively free from

these diseases. The most effective

means of ridding seed stock of

them, therefore, is to start with

clean or thoroughly disinfected

seed. By planting clean seed in a

clean seedbed and in a clean field,

these troublesome diseases may be

avoided and clean seed produced.

American cabbage growers, as a

rule, prefer to buy seed rather than

to grow their own. In general, this

custom is based on sound economic

principles and is likely to continue.

Seed growing is an industry m itself and requires specialized cultural methods and certain favorable

climatic conditions. Because of

these facts, most of the American

supply of cabbage seed is grown in

Pacific coast sections, which usually

appear to be practically free from

blackleg and black rot. A study of

climatic conditions as affecting

these diseases leads to the belief

that the dry season in midsummer

serves to discourage their development in those sections. Americangrown cauliñow^er seed is produced

chiefly in the coastal valleys of

California, where such seed crops

have so far been found to be free

from blackleg and black rot.

If the grower is certain his cabbage seed has been grown in Pacific

coast sections, it has been considered reasonably safe to plant it

without treatment; but recently the

blackleg organism has been found

on some lots of seed from those

western sections. It is safest, therefore, to use the hot-water seed

treatment for all members of the

cabbage group, turnip, rutabaga,

and rape. Semesan or thiram may

be applied as a dust following hotwater treatment, but the fungicides are not a substitute for the

hot-water treatment, since Semesan

or thiram serves only to protect

seed and young seedlings from

damping-off fungi in the soil.

Dusts containing copper are injurious to crucifer seed.

See warning about the handling

of mercury oompounds {p. 23),

Hot-Water Seed Treatment

Hot-water seed treatment must

be used with great care, since it

is likely to reduce germination

somewhat. Old seed is more likely

to suffer in this respect, and the

length of treatment must be slightly

reduced because of this fact. It is

important, therefore, to run a preliminary test on a few hundred

seeds, followed by a germination

test, before treating an entire lot.

The seed must be placed loosely in

a cheesecloth sack. It is then immersed in water held at 122° F.

for 25 to 30 minutes, depending

upon how long the particular lot

will stand being immersed without

the rate of germination being

greatly reduced. It is essential to

keep the water continuously agitated and to maintain the temperature by the frequent addition of hot

water without applying it directly

to the seed. At the end of the treatment, immerse the seed in cold

water, drain, and spread in a thin

layer to dry.
DISEASES OF CABBAGE

Plantbed Sanitation

Cabbage, cauliflower, and certain

other cruciferous crops are generally started in a plantbed. The

causal organisms of some of the

most destructive diseases may be

transferred to noninfested fields by

plants from the plantbed. The

mistake is often made of placing

the bed on an old cabbage field on

which diseases have been present.

Often manure for the plantbed is

taken from the heap on which diseased plants have been thrown to

compost or from animals that have

fed on diseased root crops. In

either case there is great danger of

having an infested plantbed, and

the transfer of diseased plants to

the field naturally means the transfer of the disease germs affecting

them.

Kale, rape, and coUards are commonly affected with black rot,

especially in the South; soil on

which they are growing or on which

they have grown during the previous 3-year period should be

avoided for the plantbed. When

it is necessary to use soil that may

contain the germs that cause diseases of crucifers, this soil should be

disinfected with live steam.

Care in Purchase of Plants

Extreme caution should be taken

in the purchase of plants. It is not

always possible to detect diseases at

the time of transplanting, even

though they are present in incipient

form. The only safe procedure,

therefore, is to make sure that disease-free seed was used and that

strict plantbed sanitation was practiced in growing the plants to be

purchased.

Crop Rotation

Repeated cropping of the soil

with the same kind of plant or

related ones favors the multiplication of disease organisms, as some

germs can overwinter in soil and

refuse. The lack of data as to just

how long is needed to starve out a

given organism, as well as regional

variations in climate and soil, makes

definite recommendation as to

length of rotation difficult. Blackleg and black rot organisms overwinter in the soil in the North to

some extent, but a 2- to 3-year rotation of the transplant field seems

to control these organisms satisfactorily. When the causal organism of yellows is once established, however, it is so persistent

that its reduction by means of rotation is out of the question. The

clubroot organism is also very persistent in the soil. Even long

rotations and complete eradication

of related wild plants that may harbor the organism are not sure means

of ridding the soil of the parasite.

Clubroot

A serious disease in Europe for

several centuries, clubroot is now

worldwide in its occurrence. It

probably causes the greatest losses

to turnip and rutabaga where these

crops are grown extensively for

stock feed—^^as in northern Europe

and in New Zealand. In vegetable

gardens and in truck-crop districts

the disease is very common on cabbage, cauliflower, broccoli, brussels

sprouts, kohlrabi, and Chinese cabbage, as well as on radish, turnip,

and rutabaga. The causal organism infects belowground parts of

many species of crucifers, wild

plants and weeds of that family

(such as mustard), and cultivated

ornamentals (such as wallflower

and stock). The losses due to clubroot are sometimes very heavy, and

the economic importance of the disease is increased by the fact that

soil once infested commonly remains so for an indefinite period

even in the absence of susceptible

host plants.
AGR. HANDBOOK 144, U. S. DEPT. OF AGRICULTURE

Characteristics

The outstanding symptom of

clubroot is the abnormal enlargement of the roots (fig. 1, ^). These

enlargements may occur on the very

small roots, the secondary roots, the

taproot, or the underground part of

the stem. The root clubs are often

thickest at the center, tapering

spindlelike toward either end. The

normal processes of the roots are

disturbed by this malformation.

Clubbed roots commonly decay before the end of the season, because

they are not adequately protected

from secondary soil organisms.

In plants in which fleshy roots

are normally formed—turnip, rutabaga, and radish—infection is commonest on the secondary roots. ^ On

the fleshy roots it occurs chiefly

where the side roots emerge.

Therefore, in fleshy-rooted species

differences in reaction to clubroot

correspond to the forms of the roots.

In globe varieties secondary roots

arise chiefly from the nonfleshy

taproot and the edible roots are not

commonly infected by the clubroot

organism. However, in the long

and half-long varieties, in which

the secondary-root zones extend approximately half the length of the

fleshy root, the fleshy root is commonly invaded by the clubroot organism and deformed by club

formation.

The eventual effect of the root

disturbance is to stunt the plant.

This stunting does not always occur

promptly, however. In a plantbed,

for instance, the aboveground parts

of the plants may show no evidence

of disease, but when the plants are

pulled they may be found to have

fair-sized root clubs. Likewise, infection occurring in the field may

easily escape notice, because the

stunting of the plants is often

very slow. Mildly affected cabbage

or cauliflower plants may form fairsized heads. If the environmental

conditions favor rapid development

of the disease, however, the stunting may be sudden and pronounced

and the plants may wilt during the

middle of bright days (fig. 1, A),

Permanent plant wilting may accompany advanced decay of the enlarged roots.

Causal Organism

The parasite associated with

clubroot is a minute organism

(Plasmodiophora hrassicae Wor.),

the spores of which remain in the

soil for long periods. With favorable soil temperature and moisture,

some of the spores germinate and

each produces a small motile body

that penetrates the underground

parts of the host plant. Once

within the host, it enlarges, probably divides, and progresses slowly

through the tissues. The presence

of the parasite stimulates abnormal

growth of the affected parts, and the

normal development of the waterand food-conducting vessels is inhibited. The clubbed roots therefore do not function properly, and

their abnormal growth draws the

sugar made in the leaves and diverts

it from its normal storage place,

such as the cabbage head. Later,

the clubroot organism divides into

innumerable very small spores that

are able to withstand long periods

of unfavorable soil conditions and

weather. These spores are returned

to the soil when the cabbage roots

rot.

Conditions Favoring Clubroot

Even though a susceptible cruciferous plant is growing upon soil

infested with the clubroot organism, it does not necessarily follow

that the plant becomes diseased.

The surrounding soil conditions

must be such that host and parasite

react together to develop the disease. Clubroot develops over nearly

as wide a range of temperature as
DISEASES OF CABBAGE

BN-61S4X

FIGURE 1.—Two phases of clubroot on cabbage : A, Wilting of tialf-grown plants that

have severely clubbed roots ; B, malformed, spindlelike roots.
6 AGR. HANDBOOK 144, U. S. DEPT. OF AGRICULTURE

that at which the host will ordinarily grow ; thus, the disease is not

restricted to any region on this

account. In very dry soil the

spores of the parasite fail to germinate, and it is possible to prevent

infection by keeping the soil moisture constantly at a low level.

However, this is of little practical

value, since a rise in soil moisture

for as short a time as 18 hours is

sufficient to permit spore germination and root infection; after that

period a return to low soil moisture

does not prevent club formation.

Acidity or alkalinity of the soil

has a great effect on clubroot. The

clubroot organism is sensitive to an

alkaline condition. If infectious

soil is made alkaline (pH=^ 7.2 or

above) and kept uniformly moist,

little or no infection occurs and the

plants grow normally and remain

healthy. This fact is the basis of

the long-known beneficial effects of

lime as a specific remedy for clubroot. However, the application of

lime sufficient to increase the soil

pH to 7.2 or higher does not always

lead to disease control. This is explained by the fact that, although

the bulk of the soil moisture is alkaline, the soil-moisture film immediately around the rootlets may be

acid, because carbon dioxide is continually being given off by the rootlets. It appears that in moist soil

there is sufficient movement of alkaline particles to neutralize the acid

promptly, while in drier soil this

may not hold. Thus, in dry soil

the favorable condition for germination of spores of the clubroot

organism prevails in the soil immediately around the roots.

The facts just mentioned are extremely important in the control

of the disease. They explain why

^The "pH value" is the most common

term used to express the degree of

acidity or alkalinity. A neutral soil has

a pH value of 7; values above 7 denote

alkalinity and those below 7 acidity.

liming is sometimes successful. Its

effectiveness varies with the soil

type, the season, and the locality,

depending upon how completely the

external environment protects the

root system from the clubroot parasite.

Control

Soil contaminated with the clubroot organism usually remains in-;

fectious for many years. Crop rotation systems with long intervals

between cruciferous crops are not

very effective in reducing soil infestation. If possible, it is better to

abandon badly infested soil for

growing crucifers for an indefinite

period.

The clubroot organism is not carried with the seed. It is carried

readily, however, with infested

soil, crop refuse, and infected

plants from locality to locality and

from field to field. Badly infested

areas having been abandoned, it is

most important to prevent the contamination of clean fields. Infested

soil is carried readily on farming

implements, by man and animals,

by surface floodwaters, by transplants from infested beds, and by

any propagative plant parts such as

potato seed tubers and bulbs. Manure from stock fed on diseased root

crops is infectious. By these

various means the clubroot organism is continually being transported, and such transfers must be

checked in order to prevent spread

of this disease.

The benefits and limitations of

liming have already been discussed.

Where crucifers are a dominant

crop an alkaline soil is desirable.

When favorable soil-moisture

levels prevail, reduction of clubroot

may be expected; furthermore,

new contamination of such soils is

less likely to occur. However, neutral or slightly alkaline soils are

particularly favorable to common
DISEASES OF CABBAGE

scab of potato. In areas where

high acidity of the soil is essential

for scab control, obviously it will

not be possible to bring about clubroot control of crucifers through

adjustment of soil reaction.

The use of resistant varieties to

control clubroot has limited possibilities at present. No varieties of

cabbage, cauliflower, broccoli, kohlrabi, brussels sprouts, or coUards

that show any practically important resistance to this disease are

known. Garden kales are very susceptible, but some varieties of stock

kale are highly resistant. ^ Hybridization now under way with a naturally resistant kale-cabbage hybrid

will form the basis of commercially

acceptable resistant cabbage. For

many years plant breeders in northern Europe have sought to develop

rutabagas and turnips that are resistant to clubroot. The Danish

Bangholm variety of rutabaga is

reported to be resistant in Denmark and Great Britain, and the

Bruce turnip is resistant in Scotland and New Zealand. However,

neither of these is satisfactorily resistant on very infectious soil in

Sweden ; a highly resistant turnip,

Immuna, has been developed there.

Certain varieties of marrow kale

have been found to be resistant in

England.

The fact that certain varieties of

turnip and rutabaga are resistant

in some localities indicates that different strains of the clubroot organism occur in different localities.

Apparently most of the rutabaga

varieties grown in the United States

are highly resistant to the parasite

as it occurs here; many varieties

of turnip are also resistant. Shogoin, an introduced Japanese

variety of turnip, is very susceptible in this country, however, and

should be avoided where soil is

known to be infested with the clubroot organism.

468203"—58 2

The important measures for control of clubroot may be summarized as follows :

(1) Discontinue the growing of

cruciferous crops on badly infested

areas if possible.

(2) Avoid infestation of new

areas by selecting clean soil for

plantbeds ; avoid transfer of infectious material from infested areas

by implements, farm animals,

plants, and surface drainage water.

(3) If compatible with the wellbeing of other crops in the rotation, keep the soil alkaline.

(4) Use resistant varieties of

turnip and rutabaga.

( 5 ) Use pentachloronitrobenzene

(PCNB) in transplanting water

to reduce the disease. It is especially recommended for cauliflower, brussels sprouts, and broccoli, in which the cost involved is

not objectionable.

Diseases Caused by Nematodes

Cabbage and related plants are

attacked by a variety of plant parasitic nematodes, of which the best

known are root-knot nematodes of

the genus Meloidogyne and cystforming nematodes of the genus

Heterodera,

Characteristics

Root knot is most common on

crops grown on the light sandy

soils in the South ; in the Northern

States it occurs chiefly in greenhouses. It affects a large number

of crop plants and weeds. Although the most distinctive symptoms are root malformations (fig.

2), the indirect effect is a stunted,

sickly appearance of the aboveground parts of the host plant.

In an attempt to distinguish root

knot and clubroot some confusion

is likely to result. Although the

organisms associated with the two

diseases are quite different, the effects produced on the roots bear
8 AGR. HANDBOOK 144, XJ. S. DEPT. OF AGRICULTURE

BN-8186X

FIGURE 2.—Root knot of cabbage.

some points of resemblance. (Compare figs. 1, B, and 2.) Eoot knot

is generally characterized by

smaller swellings than clubroot,

and infection as a rule is more uniformly distributed on the lateral

feeding roots. If, when the swellings on the roots are broken open,

pearly-white bodies about the size

of a pinhead are found, root knot

is to be suspected. These white

specks within the swellings are the

enlarged egg-bearing female nematodes, or eelworms {Meloidogyne),

that indicate the disease. The interior mass of clubroot is slightly

pinkish or brick colored.

Two cyst-forming nematodes are

known to attack crucifers in the

United States. These are the sugar

beet nematode, Heterodera schaehtii, and the cabbage cyst nematode,

H. cruciferae. H. schaohtii is

found in nearly all the sugar beet

producing regions of the United

States; H. cruciferae has been

found only in San Mateo County,

Calif. Either species will cause

severe reductions in the growth of

plantings of members of the cabbage tribe. Field symptoms usually

appear as patches of poor growth.

Heavily infected plants show stunting and temporary wilting, particularly during the warmest part

of the day.

Control

Cabbage and related plants are

highly susceptible to all the different species of root-knot nematodes

that are widely distributed in the

United States. These can be controlled by crop rotation, but because

the kinds of root-knot nematodes

differ in various regions of the

country, general directions for crop

rotations cannot be given. Information on rotations can be obtained

from county agents or from State

experiment stations. Obviously,

cabbage or related plants should not

be planted in fields where crops

have been seriously injured by rootknot nematodes recently. Unless

special measures have been taken to

eliminate the nematodes, such fields

will almost invariably be heavily

infested.

Farmers' Bulletin No. 2054, Control of the Sugar Beet Nematode,

discusses rotation methods for control of this pest; these methods are

equally applicable to control for

growing cabbage. Suggested rotation crops are alfalfa, sweetclover,

beans, peas, potatoes, wheat, barley,

oats, corn, tomatoes, or truck and

garden crops with the exception of

cauliñower, brussels sprouts, broccoli, table beets, mangel-wurzel,

turnips, rutabagas, and radishes.

Rotation with these crops would

serve equally well for control of the

cabbage cyst nematode, since it

multiplies mostly on plants of the

cabbage family.

Injury due to root-knot or other

nematodes can usually be controlled

or substantially reduced by the use
DISEASES OF CABBAGE 9

of soil nematocides. These are

chemicals sold especially for this

purpose ; since many are fumigants,

these chemicals are sometimes

called soil fumigants. In the field,

they are applied by injection into

the soil at a depth of 6 to 8 inches

at least 2 weeks before the crop is

planted. The whole area of the

field may be fumigated by applications spaced at 10- to 12-inch intervals, the chemical being applied

by means of a special applicator attached to a tractor. These applicators deliver the chemical in continuous streams behind shanks

pulled through the soil. Good results can be obtained with a considerable saving of chemical by the

use of "row fumigation;" that is,

by the application of a single stream

of fumigant directly under the row.

Nematocides suitable for field use

contain dichloropropene, dibromochloropropane, or ethylene dibromide as the active ingredient.

Formulations vary and specific

recommendations for use cannot be

given. The user should follow the

manufacturer's recommendations

exactly and should also seek advice

from State experiment stations or

the U. S. Department of Agriculture.

Control of nematodes in the

plantbed is also very important, as

plants infected with nematodes in

the plantbed will not grow properly

in the field and, in addition, may

introduce nematodes into the field

soil. The nematocides mentioned

above may be used in the plantbed,

but there are also various mixtures

and special chemicals for this purpose. Some of these have the advantage of controlling other soil

organisms and weed seeds in the

plantbed. The materials in general

use at present contain methyl bromide, chloropicrin, or sodium

methyl dithiocarbamate.

Methyl bromide is applied by releasing the chemical under a gastight cover, which is placed over the

plantbed with the edges buried.

Other nematocides are applied as

drenches or by injection. As with

the materials for use in the field, the

manufacturer's directions should be

followed exactly and information

on use under local conditions obtained from State experiment stations or the U. S. Department of

Agriculture.

Yellows

The yellows disease is confined to

members of the cabbage tribe (cabbage, cauliflower, broccoli, brussels

sprouts, kohlrabi, kale, and collards). It is especially destructive

to cabbage and constitutes a serious

menace to this crop from Long

Island to Colorado, including the

southern parts of New York, Michigan, Wisconsin, and Minnesota,

and southward as far as cabbage

is grown as a summer crop. It is

most prevalent in warm weather

and does little or no damage in the

extreme northern sections or along

the northern Pacific coast. It is less

severe on winter-grown cabbage in

the Southern States. In the latitude of New Jersey and Maryland

westward through Ohio, Indiana,

Illinois, southern Wisconsin, and

Iowa, yellows remained the greatest

hazard to cabbage growing until

resistant varieties were developed.

In thoroughly infested soils it is

not uncommon for susceptible varieties to be destroyed completely.

Characteristics

Plants infected with yellows

usually show the characteristic

symptoms 2 to 4 weeks after being

transplanted, but the disease may

appear in the plantbed. The first

sign is the lifeless, yellowish-green

color of the foliage. Sometimes the

yellowing is uniform; often it is

more intense on one side, and a

lateral warping or curling of the
10 AGR. HANDBOOK 144, U. S. DEPT. OF AGRICULTURE

BN-6192X

FiGTJBE 3.—A partly grown cabbage plant, showing distortion of the lower leaves

caused by yellows.

stem and the leaves occurs (ñg. 3).

The lower leaves yellow first, and

the appearance of symptoms progresses upward. As the yellowed

tissue ages, it turns brown and

becomes dead and brittle. Affected leaves drop prematurely,

and normal growth is distinctly retarded. The water vessels in stems

and leaves of diseased plants become yellow to dark brown. In

many respects the disease resembles

black rot (p. 17) and is often confused with it. One fairly reliable

point of distinction is that in black

rot the veins become black rather

than brown and the smaller veins

of diseased leaves are much more

generally discolored than in the

case of yellows.

The rapidity with which yellows

progresses depends upon the degree

of susceptibility of the host plant

and the favorableness of the environment. In the latitude where

yellows is most severe the conditions optimum for the development

of the disease usually prevail during the first 3 or 4 weeks after the

transplanting of the main crop of

cabbage. The disease may then

progress so rapidly that the most

susceptible plants die within 2

weeks. Others may continue a

sickly existence for a month or

more. Some live through the summer, but head imperfectly. Eela-
DISEASES OF CABBAGE 11

tively highly resistant plants or

susceptible plants growing under

cool climatic conditions may show

the disease on one or two lower

leaves that eventually drop; the

plants may recover and produce

normal heads. The conditions favorable for yellows in early cabbage

may not appear until the crop is

approaching maturity ; therefore,

even though early varieties are very

susceptible, they often escape the

disease. The leaf symptoms on

other members of the cabbage tribe

closely resemble those on cabbage.

Causal Organism

The yellows organism is a fungus

{Fusarium oxysporum f. conclutinans (Wr.) Snyder and Hansen)

that is closely related to, but distinct from, those causing wilts of

cotton, tomato, watermelon, cowpea, garden pea, and China-aster.

It produces microscopic threads

(mycelium), which grow in the soil

and on dead-plant refuse. The

fungus also produces two types of

microscopic "seeds," or spores, one

type being thin-walled and shortlived (conidia) and the other

heavy-walled and capable of withstanding long periods of unfavorable temperature or drought.

Many plant parasites do not persist long in the soil in the absence

of their hosts, but the yellows

organism, like that of clubroot, remains viable in the soil for many

years. In fact, there is reason to

believe that the yellows fungus and

the related wilt fungi not only persist in the soil, but actually grow

and increase in many soils quite

independently of their respective

host plants.

The yellows organism enters the

plant almost entirely through the

region of the young rootlets. However, it may enter through wounds

in older roots incurred at the time

of transplanting. It penetrates the

young root tissue and migrates directly to the water-conducting vessels. It then progresses up the root

and stem into the leaves, confining

itself to the vessels until part or all

of the plant dies. Then the mycelium may permeate the dead tissue and produce conidia on the

surface. Occasionally conidia are

produced within the water vessels.

The characteristic browning of the

vessels is associated with fungus invasion. Migration is largely upward, and little lateral spread from

vessel to vessel occurs. The onesided development of leaf symptoms and of those on the plant as

a whole is thus associated with the

invasion of certain vessels by the

fungus and the freedom of others

from it. The browning of vessels

and the yellowing of leaves commonly occur well in advance of the

penetration of the fungus.

Conditions Favoring Yellows

As already pointed out, yellows

is a warm-weather disease. Itis almost completely checked when the

average soil temperature is below

60° F. Its appearance is hastened

and its severity is increased as the

air- and soil-temperature averages

rise above that point. When the

average temperature is 90° or above,

yellows development is retarded;

but since this condition does not

prevail for long periods in sections

where cabbage is profitably grown,

there is no practical check of the

disease by higher temperatures.

Soil moisture and soil reaction

have little effect on yellows, and

the fungus establishes itself readily

in a wide range of soil types.

Host Resistance to the Yellows

Fungus

Varieties of cauliflower, broccoli,

and brussels sprouts naturally have

a high degree of resistance to yellows. Varieties of cabbage, with

the exception of those developed for
12 AGR. HANDBOOK 144, U. S. DEPT. OF AGRICULTURE

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FiGUBE 4.—Resistance to cabbage yellows as shown in strains grown on naturally

infested soil. At the right center, two rows of Penn State Ballhead, a very

susceptible variety. At the left, two rows of Wisconsin Ballhead, which is entirely

free from yellows, since all the plants carry type A resistance. At the right,

Wisconsin Hollander, which contains only type B resistance, is a much better

stand that that of the Penn State Ballhead, although a large percentage of the

plants show mild to moderately severe stages of yellows.

yellows resistance, kohlrabi, and

kale are generally very susceptible

to the disease.

In most cabbage varieties, however, at least a small percentage of

the plants survive on thoroughly infested soil under optimum conditions for disease development.

These resistant survivors have become the basis of selection for resistant varieties of cabbage, many of

which are now in commercial use.

In the course of breeding for yellows resistance two genetic types of

resistance are recognized. Since

they are important in determining

the value of a given yellows-resistant variety, their main points of

distinction are indicated. They are

referred to as type A and type B

Since type A is controlled by a

single hereditary factor, it is relatively simple to obtain lines that run

true for this type of resistance; in

such lines the resistance remains

fixed if outcrossing with other varieties is not permitted. Plants that

have type A resistance develop no

typical symptoms of yellows even at

high soil temperatures.

Type B resistance is controlled by

a complex set of hereditary factors;

it is very difficult, if not impossible,

to fix this character in a variety so

that it will run true in successive

generations. Moreover, plants containing type B resistance differ

from one another in the "dosage" of

resistance they contain. As the

average soil temperature rises, this

type of resistance is ])rogressively

less effective until, at 77° F. (constant soil temperature) or above,

type-B-resistant plants succumb

with typical yellows on infested soil,
DISEASES OF CABBAGE 13

whereas type-A-resistant plants remain healthy.

In the field, therefore, type-Aresistant varieties remain free from

yellows even during very warm

summers. Type-B-resistant varieties are commercially successful in

moderately warm seasons, but as the

temperature increases, a large percentage of individual plants show

varying degrees of disease development. Heavy losses may result if a

protracted high-temperature period

prevails.

Therefore, type-A-resistant varieties are most valuable, and these

are being developed to meet the

varied needs as rapidly as time and

facilities permit. It is usually difficult to distinguish type-A- and

type-B-resistant plants in the field,

since they can be separated with

certainty only if grown in the

greenhouse on infested soil held at

îi constant temperature of 77° F.

This method is used in breeding

new varieties and can be used also

to determine what percentage of

seeds in a commercial resistant variety carry type A resistance.

Control Through Use of Resistant Varieties

The yellows organism is not

seedborne; but, like the clubroot

organism, it is readily transported

by man, animals, implements,

plants, bulbs, seed tubers, surface

drainage water, and plant refuse.

Since the organism persists indefinitely in the soil, crop rotation is

ineffective. The disease has been

successfully controlled by the development of yellows-resistant varieties. As a wide range of resistant

types now exists, they should be

used exclusively in those regions

where climatic conditions permit

the development of the disease.

The breeding of new cabbage varieties to meet the changing market

requirements continues.

Eesistant varieties now in commercial use range from early- to

late-maturing types. The days

given in the varietal descriptions

are the average number elapsing

from the time of transplanting to

the maturing of the heads; the

plants of the first eight varieties

listed were set out at Madison,

Wis., in the middle of May and

those of the other varieties in the

second half of June. This period

will, of course, vary with the season, locality, and method of propagation. It is useful here chiefly in

giving a basis for comparing the

rates of maturing of the several

varieties described herein.

1. Wisconsin Golden Acre,—60

days. Developed by the Department of Agriculture and the Wisconsin Agricultural Experiment

Station. Introduced in 1940. Selected from Golden Acre (a name

applied generally to the early-maturing strains of Copenhagen Market). Heads globular. Plants

conforming closely to Golden Acre

in type. Maturing 2 to 5 days earlier than most strains of Golden

Acre. Authentic seed stocks testing close to 100 percent type A

resistance.

2, Resistant Detroit,—67 days.

Introduced in 1936 by Ferry-Morse

Seed Co. Selected from Golden

Acre. Heads globular (fig. 5, 5).

Plant type conforming closely to

that of acceptable stocks of Golden

Acre. Maturing a little later than

the earliest strains of Golden Acre

and about a week later than Wisconsin Golden Acre. Original

stocks testing about 80 percent

type A resistance and new stocks

now coming into use close to 100

percent.

S. Badger Marhet,—67 days.

Developed by the Department of

Agriculture and the Wisconsin

Experiment Station. Introduced

in 1953. A roundhead secondearly type bred to produce small,
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FiGUEB r..-Head types of yellows-resistant cabbage varieties: A, Jersey Queen; B, Resistant Detroit ; 0 Marion Market; D, Globe;

J'J, All Head Select ; F, Improved Wisconsin All Seasons ; G, Improved Wisconsin Ballhead ; H, Wisconsin Hollander.

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DISEASES OF CABBAGE 15

compact heads. Authentic seed lots

testing close to 100 percent type A

resistance.

^. Jersey Queen,—67 days. Developed by the Department of

Agriculture and the Wisconsin

Agricultural Experiment Station.

Introduced in 1932. Selected from

Jersey Wakefield and practically

identical with it in type and season.

Heads pointed (fig. 5, J.). Plants

small, permitting close planting,

and conforming to the smaller

headed strains of Jersey Wakefield.

Authentic seed lots testing close to

100 percent type A resistance.

6, YR Charleston Wakefield.—

80 days. Developed by the Department of Agriculture and the Wisconsin Experiment Station. Very

close to Charleston Wakefield in

type and season. Authentic seed

lots testing close to 100 percent

type A resistance.

6, Racine Market,—80 days. Developed by the Department of

Agriculture and the Wisconsin

Agricultural Experiment Station.

Introduced in 1935. Selected from

Copenhagen Market. Heads globular. Plant type similar to that of

the parent variety except for leaf

and head color, which is blue green

rather than yellow green. Heads

maturing more slowly and more

satisfactorily than those of Copenhagen Market and remaining

standing without bursting. Authentic stocks testing close to 100

percent type A resistance.

7. Marion Market.—85 days.

Developed by the Department of

Agriculture and the Wisconsin

Agricultural Experiment Station

with the aid of the National Kraut

Packers' Association. Introduced

in 1927. Selected from Copenhagen Market but distinct from

that variety in its later maturity,

greater size of plant and head (fig.

5, (7), and bluer cast of foliage.

Widely used as a midseason cabbage for market and sauerkraut in

468203«—58^ 3

the North and as a winter shipping

cabbage in the South. Authentic

stocks testing 95 to 100 percent

type A resistance.

8. Glohe.—88 days. Developed

under the same auspices as Marion

Market. Introduced in 1927. Selected from Glory of Enkhuizen.

Midseason, roundhead type (fig. 5,

Z>), requiring a few days longer to

mature than Marion Market but

suited to the same market requirements. Core relatively shorter

than that of other roundhead varieties. Authentic stocks testing 95

to 100 percent type A resistance.

9. All Head Select.—85 days.

Developed under the same auspices

as Marion Market. Introduced in

1927. A flathead type (fig. 5, E),

selected from All Head Early.

Midseason cabbage used for the

southern shipping market and for

early sauerkraut manufacture in the

North. Authentic stocks testing 95

to 100 percent type A resistance.

10. Wisconsin Ballhead.—91

days. Developed by the Department of Agriculture and the Wisconsin Agricultural Experiment

Station. Introduced in 1935. Selected from Danish Ballhead.

Practically identical with the latter

in type and season; roundhead;

short stem; blue foliage. Suitable

in the North for early-fall shipping ; when planted so as to mature

late enough, suitable for storage.

Plants not so large as those of Wisconsin Hollander, permitting closer

planting; heads also smaller. Authentic stocks testing close to 100

percent type A resistance.

11. Wisconsin All Seasons.—92

days. Developed by the Wisconsin

Agricultural Experiment Station.

Introduced in 1921. Late and very

irregularly maturing, drumhead

variety similar to All Seasons.

Suitable for late-fall market and

used somewhat for the southern

winter crop; used most widely in

the North for the late pack of sauer-
16 AGR. HANDBOOK 144, U. S. DEPT. OF AGRICULTURE

kraut. Most of the individual

plants containing type B resistance; up to 20 percent of them containing type A resistance; under

very high summer temperatures

sometimes showing a considerable

percentage of yellows.

12, Improved Wisconsin All Seasons,—92 days. An improved selection of Wisconsin All Seasons. Eeleased by the Department of Agriculture and the Wisconsin Agricultural Experiment Station for increase in 1947. Similar in maturity

and general plant characters to

Wisconsin AH Seasons, but plants

of this variety are very uniformly

like those of the most desirable type

of the mother variety. Original

stock testing 100 percent type A

resistance (fig. 5, F),

13, Badger Ballhead.—97 days.

Developed by the Department of

Agriculture and the Wisconsin

Agricultural Experiment Station.

Introduced in 1955. This conforms

closely in type with Improved Wisconsin Ballhead except that it produces a smaller head at maturity.

Authentic stocks testing close to

100 percent type A resistance.

14, Improved Wisconsin Ballhead,—97 days; maturing slightly

later than Wisconsin Ballhead.

From a cross between Wisconsin

Ballhead and Wisconsin Hollander.

Keleased for increase in 1946. Authentic stocks testing close to 100

percent type A resistance (fig. 5,

G),

15, Empire Danish,—97 days.

Developed at Cornell University

from a cross between Wisconsin

Ballhead and Donk's Danish. Introduced in 1955. It is reported to

yield more heavily in New York

than Wisconsin Ballhead and heads

more tapered at the base. Authentic stocks test close to 100 percent

type A resistance.

16, Pennvalley,—100 days. Developed at Pennsylvania State University from a cross between Penn

State Ballhead and Wisconsin Ballhead. It resembles the former in

type. Introduced in 1952. Authentic stocks test close to 100 percent

type A resistance.

17, Wisconsin Hollander,—113

days. Developed by the Wisconsin

Agricultural Experiment Station.

Introduced in 1916. Selected from

Danish Ballhead; 10 to 14 days

longer in season than the latter and

producing larger, flatter heads (fig.

5, H). Excellent storage cabbage,

used widely in the northern part of

the yellow zone ; type B resistance.

18, Red Hollander,—113 days.

Developed by W. J. Hansche, of

Eacine, Wis. Roundhead, late, red

variety. Suitable for winter storage. Originally containing largely

type B resistance, but improved by

commercial breeders so that it now

contains a high percentage of typeA-resistant plants.

19, Resistant Flat Dutch,-—11^

days. Developed by Wisconsin

Cabbage Seed Co. Introduced in

1942. Selected from Late Flat

Dutch ; type and time of maturity

practically identical with those of

the latter variety. Suitable for very

late home use and sauerkraut production. Most individuals in authentic stock containing type B

resistance ; up to 30 percent containing type A resistance.

20, Bugner,—120 days. One of

the first resistant varieties developed in North America ; selected by

a Mr. Bugner, a gardener in the

vegetable-growing district near

Chicago, 111., where it has been

used extensively for more than 30

years. Ballhead or Hollander type,

somewhat longer in season than

Wisconsin Hollander. Heads very

large, flattened, of excellent keeping quality. In extremely warm

seasons often showing high percentage of yellows. Resistance all

of type B ; degree of resistance different in different strains.
DISEASES OF CABBAGE 17

Black Rot

Black rot, a bacterial disease,

occurs on all the cultivated crucifers and may affect some of the

wild species. Since the causal organism is seedborne, it has been

distributed to many parts of the

world. As the organism does not

persist indefinitely in the soil and

depends upon the seed for distribution, the disease is decidedly sporadic. When it becomes established

early in the growing season and favorable environment prevails, it

may become extremely destructive,

rendering a large percentage of cabbage plants incapable of producing

marketable heads and making the

remainder of the crop unsafe for

storage or shipment. Cauliflower

is one of the most susceptible hosts,

and it requires the greatest care to

grow disease-free plants.

Characteristics

Plants may be affected with black

rot at any stage of their growth.

Black rot is primarily a disease of

the aboveground organs. The

belowground storage organs of

fleshy-rooted crucifers (turnip,

rutabaga, radish, and horseradish)

may be affected, however, and a

dry rot may occur. This is commonly followed by bacterial soft

rot (p. 24).

Plant infection takes place primarily through water pores at the

margin of the leaf, and the first

signs of the disease often appear

at this point. The progress of the

disease from this point can frequently be traced through the veins

of the leaf (fig. 6) by the blackening of the bundles. Marginal infection is followed by browning

and drying of the infected areas of

the leaf. Invasion also occurs

through wounds made on the leaf

by chewing insects. The organism

progresses down the leaf to the

main stem, where it may advance

up or down. As it goes up the stem

younger leaves are invaded. The

affected bundles in leaf or stem are

easily recognized in cross section by

their blackened appearance, and

often they can be traced through

individual leaves of the cabbage

head. If plants are infected while

still young, dwarfing or one-sided

growth commonly occurs. Affected

leaves drop prematurely. In extreme cases heading is prevented.

Black rot does not cause a soft rot

of affected cabbage or cauliflower

heads, but it opens the way for soft

rot bacteria (p. 24) ; consequently,

soft rot symptoms are common in

association with black rot. For

this reason it is dangerous to store

cabbage heads or turnips or rutabaga roots from fields where black

rot occurs.

There are some points of resemblance between yellows and black

rot that often lead to confusion.

Both cause discoloration of the

bundles, and both may bring on

one-sided development of the leaves

or of the whole plant. The bundles

affected with black rot are black in

the main, and in the later stages

they are commonly surrounded by

cavities due to the breakdown of

surrounding cells. V-shaped lesions at the leaf margins are not so

distinct in yellows as in black rot.

With yellows the bundles are brown

and no cavities occur.

Causal Organism

The black rot organism is one of

the bacteria {Xanthomonas campestris (Pam.) Dows.). These minute

organisms are carried by insects,

spattering rain, surface drainage

water, and possibly by wind, or

with windborne particles of dust.

They are able to swim about in

liquid ; and, when they come in contact with drops of liquid that collect on the margins of leaves or

about insect wounds, they enter the

water pores or wounded tissue.
18 AGR. HANDBOOK 14 4, U. S. DEPT. OF AGRICULTURE

BN-6195X

FIGURE 6.—Symptoms of black rot of cabbage that result from invasion of the

marginal water pores of the leaf by the causal organism and its migration downward through the water vessels.

They are able to penetrate the tissue until they reach the water-conducting vessels, and from then on

they multiply in this channel and

progress chiefly through it. They

may be carried over from year to

year in or on seeds from infected

host plants or in diseased-plant

refuse.

Conditions Favoring Black Rot

The initial spread of the parasite

usually occurs in young plants

started in plantbeds. Occasional

plants become diseased from seed or

plant debris in the soil, and there

the germs multiply and become

available for spread. Spattering

water from rainstorms or artificial
DISEASES OF CABBAGE 19

sprinklers is the chief means of dissemination. The amount of such

spread determines how widely the

plants are infected in the plantbed

before they are transplanted. Infected lower leaves usually drop off,

and plants commonly show little or

no external sign of the disease,

though internally infected when

transplanted. Infected plants may

grow normally with little or no external sign of disease for 3 to 6

weeks after they are transplanted ;

after that period the lower leaves

gradually develop symptoms, owing to the migration of the organism from the main stem. Thus,

fields of cabbage or cauliflower may

rather suddenly show large percentages of plants with black rot in

midseason. The epidemic is caused

by favorable conditions for early

spread in the plantbed and the slow

subsequent development of the parasite in the plants.

Black rot is practically absent in

the Pacific coast cabbage-seedgrowing sections. Its retardation

is attributed to the lack of heavy

spattering rains during the period

in which cabbage plants are in the

plantbed and the consequent unfavorable conditions for the spread of

the parasite. Cabbage seed from

those sections is generally free from

the black rot organism.

Control

The major points in the control

of black rot are as follows :

(1) Before planting use the hotwater treatment on the seed (p. 2).

Liquid or dust treatment of seed is

not effective,

(2) Use clean plantbeds. The

black rot organism lives in the

plant refuse in the soil during one

and possibly two northern winters.

It is therefore essential to grow

noncruciferous crops for two seasons on soil used for crucifers or to

sterilize the soil between cruciferous crops. In Southern States

avoid making plantbeds in proximity to oversummering crucifers such

as coUards, kale, turnip, and rutabaga.

(3) If plantbeds are watered artificially, avoid sprinkling the foliage, since to do so aids in disseminating the black rot organism. It

is preferable to irrigate in furrows

between the rows.

(4) Practice rotation in the field

so that at least 2 years, and preferably 3, elapse between cruciferous

crops. Rotation is of no avail if

infected plant debris is allowed to

become a source of inoculum spread

by means of wind or surface drainage water.

Blackleg

Blackleg, or dry rot, is important

on members of the cabbage tribe

and on turnip and rutabaga. A

strain of the blackleg organism attacks turnip and rutabaga in Europe and in New Zealand, where for

many years it has caused a serious

dry rot in storage. This phase has

now been reported in the United

States and Canada. Blackleg is

found most commonly on cabbage,

cauliflower, broccoli, and brussels

sprouts and occasionally on other

members of the cabbage tribe.

When the disease gains a start in

the plantbed it may be very destructive in the field.

Characteristics

The earliest conspicuous symptoms occur in the plantbed 2 or 3

weeks before transplanting time.

Spots occur on leaves and stems.

The leaf spots appear as inconspicuous, indefinite, pallid areas that

gradually become well-defined spots

with ashen-gray centers in which

innumerable black dots much

smaller than a pinhead are scattered irregularly (fig. 7). These

are the fruiting bodies (pycnidia)

of the fungus. Similar spots occur
20 AGE. HANDBOOK 144, U. S. DEPT. OF AGRICULTURE

FIGURE 7.—Blackleg of cabbage.

in the stem, where they are sunken

and are often surrounded by a

purplish border. The stem lesions

gradually enlarge after the plants

are removed to the field, extending

to the underground parts. In

fact, incipient, invisible infections

at transplanting time gradually

develop into subterranean stem and

root lesions. The fibrous root system may be largely destroyed, although new roots sent out above the

lesions serve to keep the plant alive.

Often badly affected plants survive

until fair-sized heads form, when

they topple over as the increasing

top weight overbalances the gradually decaying root anchorage.

Many plants wilt abruptly and die.

The wilting leaves tend to remain

attached to the stem instead of

dropping as in black rot and yellows. As root corrosion progresses,

the still-turgid plants often show a

reddish color of the outer leaves,

particularly near the margins.

Other types of root injury may

cause similar discoloration, however. Pycnidia are not common to

subterranean parts until the plants

are dead and the succulent tissue

has completely decayed.

Ijeaf spots continue to appear,

their number depending on favor-
DISEASES OF CABBAGE 21

ableness of the environment. The

blackleg fungus may invade heads

in storage, but it does not cause

serious decay. It attacks the

branches, leaves, and pods of seed

plants and invades the seeds.

Many seeds when attacked in their

early formative period become

shriveled and nongerminable ;

others, only slightly affected, carry

dormant mycelium in the seedcoat

and remain viable. Young seedlings are attacked either on the

cotyledon, which commonly carries

the infected seedcoat aboveground,

or at the base of the stem (hypocotyl) if the seedcoat remains

below the ground. In the plantbed

or field, seedlings attacked early in

the season die promptly and furnish

an early crop of pycnidia, from

which the spores may spread to surrounding plants.

When the turnip dry rot strain

of the blackleg fungus affects turnip and rutabaga, leaf and stem

lesions similar to those on cabbage

develop but the typical blackleg

cankers at the ground line may or

may not occur. Root cankers may

appear before harvest. Contamination may occur at harvest and as

invasion continues in storage a dry

rot follows.

Causal Organism

The blackleg organism is a parasitic fungus {Phoma Ungarn (Fr.)

Desm.). It is carried in the seed

and persists on plant refuselfor 1 or

2 years. Damage to the host is done

by the mycelium that penetrates the

tissue, killing as it goes. The mycelium in the dead tissue produces

the dark pycnidia, which are flaskshaped, with openings to the exterior of the leaf or stem. Myriads of thin-walled spores are

formed within the pycnidia, where

they remain embedded in a gelatinous matrix during dry weather.

Moisture on the lesion in the form

of rain or dew is absorbed by the

matrix, which expands rapidly,

forcing its way through the opening of the pycnidiuih to the exterior

and carrying spores (pycnospores)

with it.

The spores germinate within a

few hours and penetrate the host if

moisture is present. If the plant

surface dries off quickly the spores

die. It is thus evident that a moist

period of several hours is essential

for spore expulsion and reinfection,

and spattering rain is essential to

carry the viable spores from diseased to healthy plants.

Conditions Favoring Blackleg

Initial infection in the plantbed

starts with occasional plants infected from the seed or the soil.

The blackleg fungus usually kills

them promptly and fruits. Further

spread is dependent on moist periods and spattering rain. Epidemics are usually to be traced to

rapid spread in the plantbed. This

often occurs just before transplanting, and the new infections, being

invisible, appear later in the infected field. The Pacific coast cabbage-seed-growing sections fortunately have a climate during the

plantbed period that tends to check

the spread of blackleg from any

initial centers; consequently, the

cabbage-seed crop in this section is

usually free from blackleg, but the

organism has been found recently

on several lots of seed from those

sections. The turnip- and rutabaga-seed crops from this section

also appear to be relatively free

from the dry rot organism.

Control

The measures of control outlined

for black rot (p. 17) are effective in

the control of blackleg.

Rhizoctonîa Disease

The fungus that causes the rhizoctonia disease is widely distrib-
22 AGR. HANDBOOK 144, U. S. DEPT. OF AGRICULTURE

uted in the soil. It attacks a great

many species of plants and causes

a variety of pathological effects.

Probably there are numerous

strains, which vary in the hosts that

they attack. The strains that attack the potato, for instance, do

not ordinarily affect crucifers and

vice versa. With its many effects

the rhizoctonia disease is of major

importance on cruciferous crops.

Characteristics

As cruciferous plants are attacked at different stages by the

rhizoctonia disease, the symptoms

are grouped accordingly.

Damping-off,—Rhizoctonia is one

of the more common fungi that

attack young seedlings, invading

them at the base of the young stem

(hypocotyl) or atthe soil line. The

tissues become water-soaked and

rapidly collapse, and the tender

young plants topple over and die.

A number of other soil fungi affect

young seedlings in a similar manner.

Wire stem,—Young seedlings

that are attacked less vigorously or

when they are somewhat older have

a much greater chance of warding

off the fungus. Later attacks are

less serious, because the young stem

has begun to thicken and grow in

diameter and to slough off its outer

cells. At this stage plants previously attacked by Rhizoctonia

are brownish to black just above

and below the soil line; the stems

are usually somewhat smaller than

normal but tough and woody. This

type of disease is known as wire

stem. Affected plants may recover

and grow normally, but under some

conditions the fungus may continue

to retard their growth after they

are transplanted.

Bottom rot,—Bottom rot occurs

in midseason, either as a carryover from wire stem or from new

infection. The lower leaves droop,

decay, and turn dark but do not

drop. Plants may recover and

produce normally. In cabbage,

bottom rot may develop into head

rot.

Head rot,—Head rot of cabbage

develops between early head formation and maturity as a dark firm

decay at the bases of outer leaves

and heads (fig. 8). The outer

leaves of the head become wilted

and pallid and brown to black near

the main stem. Mycelium becomes

conspicuous on decayed tissue and

between the head leaves, on which

it produces dark, sunken spots.

Firm, persistent, dark decay continues in storage and transit.

Root rot,—Root rot of turnip,

rutabaga, and radish occurs both

before harvest and in storage. In

greenhouse culture of radishes a

rather soft decay of partly grown

roots is common; gray surface

mycelium and dark-brown kernellike bodies (sclerotia) form on the

affected tissue. A similar brown

and spongy decay occurs on turnip

and rutabaga roots, particularly in

storage. The cobwebby mycelium

and brown sclerotia help to distinguish the disease from root rots

caused by other organisms.

Causal Organism

The causal fungus (sterile form,

Rhizoctonia solani Kuehn; sporeforming stage, Pellucularia filamentosa (Pat.) Eogers) is common

in soil. Its occurrence as a parasite depends largely on the susceptibility of the host and the favorableness of the environment. It attacks

chieñy young succulent tissue and

dormant storage tissue. The mycelium is long-lived; as the food

in the decaying tissue becomes exhausted it rounds up into dense,

brown, kernellike bodies (sclerotia), which may resist unfavorable
DISEASES OF CABBAGE 23

FiGXJBE 8.—Rhizoctonla head rot of cabbage.

conditions for an indefinite period.

Spores are seldom formed. Occasionally a superficial white mycelial

weft on the lower side of leaves near

the soil bears thin-walled, shortlived spores. As pointed out previously (p. 22), the fungus has a

number or strains that are specialized on certain groups of plants.

Control

The important measures for the

control of the rhizoctonia disease

follow :

(1) When plants are grown in

hotbeds or coldframes, treat the

soil with steam or formaldehyde.

Crucifers are very sensitive to formaldehyde ; therefore, allow the soil

treated with it to stand 2 or 3 weeks

before planting crucifers in it.

(2) Drench mercuric chloride

(1:2,000) at the base of plants in

the plantbed, as used to kill maggot eggs ; this treatment is helpful

in reducing wire stem.

(3) Sort out plants affected with

wire stem when transplanting.

(4) Do not plant crucifers in

short rotations on sites where bottom rot, head rot, and root rot have

occurred.

Formaldehyde solution is irritating to the skin. Hands should he

protected with gloves. Surplus

solution should he carefully disposed of. Vessels and clothing

should he thoroughly cleaned.

Mercuric chloride is a deadly poison; great care therefore must he

taken in mixing and handling the

solution to prevent any contact with

the mouth, eyes, or nostrils. Unused solutions should he huried in

the ground at least 1 foot deep. All

vessels and clothing must he thoroughly cleaned hefore they are used

again. Any surplus chemical

should he plainly labeled and stored

out of the reach of children and

animals.
24 AGR. HANDBOOK 144, U. S. DEPT. OF AGRICULTURE

*- ^j

FIGURE 9.—Bacterial soft rot of cabbage.

Bacterial Soft Rot

Characteristics

The loss from bacterial soft rot

alone or in combination with other

rots is considerable both in storage

and in transit (p. 40). It is occasionally destructive in the held,

especially if it follows black rot.

Freezing injury is commonly followed by soft rot.

Bacterial soft rot of cabbage is

characterized by a soft, mushy, almost slimy decay, which after entering, generally at the surface or

base of the head (fig. 9), spreads

rapidly through the whole plant.

The soft rot bacteria as a class are

marked for their ability to destroy

plants very quickly under favorable

temperature and moisture conditions. They seldom aiïect uninjured plants, as they require a

wound or other injury in which to

gain a foothold or they appear in

conjunction with black rot or black

leaf spot. Infection takes place in

the field where considerable damage may result, but the greatest destruction to cabbage is caused in

storage houses or in transit (p. 40).

Causal Organism

Soft rot in cabbage and related

crops is caused by bacteria belonging to a group usually referred to

as the soft rot bacteria, which may

attack carrot, turnip, celery, and

other vegetables. Erwinia carotovora (L. R. Jones) Holland, which

causes bacterial soft rot of cabbage,

is a common example of these

bacteria.

Although these bacteria are universally present, they require special conditions to gain a foothold in

plant tissue. Not only are wounds

in the tissue necessary, but the conditions must be such that the

wounds are not corked over before

soft rot sets in. The gnawing action of maggots keeps the tissue

freshly wounded and predisposed

to soft rot. Maggot larvae ingest

the bacteria, which remain in their

bodies until the adult flies are

formed; the flies carry the bacteria

to new locations and lay contam-
DISEASES OF CABBAGE 25

mated eggs on healthy plants.

Newly hatched maggots thus are

provided with soft rot bacteria as

they begin to feed. In fact, the bacteria aid the larvae by making the

raw plant tissue more digestible.

Cabbage plants approaching the

heading stage are often attacked by

maggot larvae on the main stem at

the base of the head. Such insect

injuries may become the forerunner

of soft rotin the field.

Control

Various measures can be used to

avoid soft rot. Cabbage and other

cruciferous crops should be carefully selected for storage or shipment and handled so that they will

be injured as little as possible.

All surface moisture should be allowed to evaporate. Temperatures

slightly above freezing and ventilation adequate to prevent high relative humidity are advisable in storage. The same precautions should

be observed in long-distance

transit.

Watery Soft Rot and White

Blight

Characteristics

Watery soft rot is most common

in the gulf coast region and in the

humid coastal valleys of the Pacific coast, where it occurs on crucifers and many other vegetable

crops, especially lettuce, celery, cucumber, and carrot. The earliest

symptoms of this fungus disease,

also known as drop, are indicated

by water-soaked areas over the

stem and lower leaves. The wilting

of the lower leaves is followed finally by the collapsing of the whole

plant into a shapeless mass. The

plant may succumb to the disease

in a few days, or it may live 2

weeks or more. In and about the

decayed area a dense white cottony mass of mycelium accumulates. In the later stages of the

disease irregularly shaped, hard,

black bodies, the size of a mustard

seed or larger, can be found scattered among this cottony mass.

These bodies (sclerotia) are almost

sure evidence of the disease (fig.

10).

Decay of the harvested crop may

continue; in fact, new infections

may show up later in plant parts

that appear healthy at harvesttime.

Usually these result merely from

the progress of incipient infections

to visible size. The fungus causes

a soft rot of storage organs such

as cabbage heads and turnip roots.

This disease may be distinguished

from bacterial soft rot of crucifers

by the lack of sulfurons odor, the

presence of cottony mycelium in the

decayed tissue, and the prompt

drainage of water from the rotting

tissue. In contrast the water is retained in a slimy mass when the rot

is caused by the bacterial soft rot

organism. For that reason on the

market bacterial soft rot is often

called slimy soft rot to distinguish

it from watery soft rot.

The watery soft rot fungus

causes the disease of seed plants of

cabbage, turnip, and rutabaga

known as white blight. In the second growing season, when these

plants send up seedstalks, the spores

(below) discharged from fruiting

bodies at the soil surface find the

points of attachment of leaves and

the main stem particularly favorable for germination. Water from

dew or rain tends to accumulate

there, and fallen petals deposited

there decay and provide nutriment

for the spores. Infection of the

stem follows. A light-colored

lesion develops, extends above and

below the leaf axil, and eventually

girdles the stem. The fungus progresses even farther in the pith of

the stem, and sclerotia are formed

abundantly there. The affected

stem withers and becomes unfruit-
26 AGE. HANDBOOK 144, TJ. S. DEPT. OF AGRICULTURE

FIGURE 10.—Watery soft rot of cabbage.

ful. Therefore, white blight sometimes is very destructive to seed

crops of crucifers.

Causal Organism

The watery soft rot fungus

{Selerotinia sclerotiontm, (Lib.)

DBy.) forms a coarse, white growth

in and about the decayed region

of the plant that it attacks; later

the hard, black bodies (sclerotia)

develop from the mycelium and

keep the fungus alive during

periods of unfavorable environment. In the spring they send up

small mushroomlike bodies (apothecia) bearing an abundance of

microscopic spores (ascospores),

which are discharged into the air;

when these spores come in contact

with the moist surface of the host

plant, they germinate and cause

infection.

Control

Control of watery soft rot is difficult. Replanting should be

avoided and general sanitation followed. Rotation with crops not

attacked by the fungus, such as

com, cotton, crotalaria, peanuts,

and sweetpotatoes, is advisable.

Black Leaf Spot

Characteristics

Black leaf spot, sometimes called

black mold or brown rot, occurs as

a leaf spot of cabbage, cauliflower,

collards, and a number of other

crucifers. Stems and pods of seed

plants of crucifers are commonly

aiïected. It causes a mold of cabbage heads in storage and a brown

rot of cauliflower heads in transit.

As a field disease it is ordinarily of

minor importance, but in storage

or transit the organism may be

very destructive.

In the field it appears on the

lower or outer leaves of the maturing plants as distinct, roundish

black spots commonly marked with

concentric brown zones. These
DISEASES OF CABBAGE 27

spots range from one-fourth to

more than one-half inch in diameter. On stems and pods the spots

are irregular in size and shape.

They are distinguished from blackleg spots (fig. 7) and ring spots

(see fig. 11, Ä) by the absence of

the numerous dots (pj^cnidia) in

the diseased area. In storage these

spots may blend together on cabbage heads until the outer leaves

are covered and entirely blackened

by the moldy development. On

the curd of cauliflower the disease

appears as brown spots that turn

olive green with age.

Causal Organism

Black leaf spot is caused by a

fungus^ that overwinters on cabbage refuse or on the seed. The

black mold that develops on the

leaf spots or on cabbage or cauliflower heads consists largely of the

dark-colored sports of the organism. These are readily disseminated by wind or water, and they

germinate in water, thus invading

healthy plants and causing new

infections.

Control

Black leaf spot ordinarily is not

sufficiently important in the field to

warrant the practice of specific

remedial measures. The mercuric

chloride treatment is not effective;

and the hot-water treatment recommended for blackleg (p. 2) is necessary to rid the seed of the fungus.

Because the disease is most destructive in storage and transit, care

should be taken to handle the crop

so as to minimize the trouble.

Heads should be handled carefully

to avoid bruising, and surface moisture should be allowed to evaporate

^The organism commonly associated

witli the disease is Alternaría hrassicae

(Berk.) Sacc. A closely related species

may also cause similar symptoms.

before storage. The storage house

should be kept at 33° to 34° F., and

ample ventilation should be provided to reduce humidity.

Ring Spot, or Black Blight

Characteristics

In North America ring spot is

most prevalent on the Pacific coast,

where it affects cauliflower, cabbage

(including seed plants), kale, and

certain other crucifers. It appears

in the early stages on all aboveground parts as dark-purple spots,

which gradually enlarge and often

become an inch or more in diameter

on the leaves (fig. 11, Z). The

older zonate spots are dark brown

with light-green borders. In advanced stages minute black specks

(fruiting bodies) appear in the dead

parts of the spot, resembling very

much those of blackleg. They are to

be distinguished from blackleg by

smaller and more numerous specks

and thus more closely crowded. The

disease appears on the stems (fig.

11, ^) and pods of the seed plants as

small spots or as long purple

streaks. Itis sometimes destructive

to the cabbage-seed crop in the

Puget Sound section, where it is

commonly known as black blight.

The chief damage is done to cauliflower, on which ring spot causes

defoliation and also losses in transit

when necrosis on the leaves of the

curds occurs.

Causal Organism

The ring spot fungus {Mycosphaerella 'brassideola (Fr.) Lindan) produces on the lesion bodies

that appear as black specks to the

unaided eye. These are known as

spermagonia ; within these are produced myriads of microscopic bodies

known as spermatia. The spermagonia resemble the pycnidia of the

blackleg fungus (p. 19) and the

spermatia its pycnospores. The
2$ AGR. HANDBOOK 14 4, U. S. DEPT. OF AGRICULTURE

FiGUBE 11.—Symptoms of Myoosphaerella ring spot of cabbage : A, Dark zonate ring

lesions on seedling leaf; B, part of blighted plant showing localized neerotic

lesions, which are thickly studded with small, black fruiting bodies.

spermatia differ from the pycnospores of the blackleg fungus, however, in that they do not germinate

in water and do not serve as a

means of disseminating the fungus.

Later there appear in the lesions

in which the spermagonia were

formed, somewhat larger fruiting

bodies (perithecia) that contains

spores (ascospores). The ascospores are discharged into the air

when the perithecia become moist.

The ring spot fungus, then, depends

on moisture for ascospore discharge

and upon air currents for dissemination.

In the dry midsummer period on

the Pacific coast little development

and spread occur, but fall and

winter rains favor spore discharge

and infection. The ring spot fungus

becomes most destructive on winter

cauliflower in the coastal valleys of

California and on the seed plants of

cabbage in the sections along the

coast in California, Oregon, and

Washington. In seed-growing sections the greatest source of infection

is likely to be dry stalks and leaves

from threshed seed plants. Fall

rains provide favorable conditions

for ascospore discharge ; atthattime

the first-year plants of the next seed

crop are readily infected. The

causal fungus may then continue to

spread through late fall and the fol-
DISEASES OF CABBAGE 29

lowing spring. General infection of

seed pods results in reduced yields

and inferior seed.

Control

The chief means of controlling

ring spot is strict sanitation by destroying during the dry summer all

crop refuse that may provide ascospores with the onset of fall rains.

In seed-growing sections it is necessary to burn straw piles after

threshing the seed crop, to plow

seed fields soon after harvest, and to

isolate the plantbed from maturing

seed fields.

Powdery Mildew

Characteristics

Powdery mildew appears as a

white powdery fungus growth in

spots or more or less completely

covers the upper surfaces of leaves

and the stems of cruciferous plants.

It occurs more abundantly in semiarid regions or in more humid regions after a protracted dry spell.

Causal Organism

The powdery mildew fungus

{Erysiphe polygoni DC.) attacks

many plants; one or more of its numerous strains are probably confined to the crucifers. The powdery

material on the host consists of the

spores (conidia) and the branch of

the mycelium on which they are

borne. Later black bodies smaller

than a pinhead appear embedded in

the mycelium. They are fruiting

bodies (perithecia) containing another type of spores (ascospores).

Perithecia may survive periods of

unfavorable environment and upon

the return of favorable conditions

produce another crop of ascospores.

This stage of the fungus is the one

whereby it survives from one crop

season to the next.

Control

Ordinarily powdery mildew is of

no economic importance on crucifers. Should remedial measures become necessary, finely divided sulfur dust gives adequate control.

Downy Mildew

Characteristics

Downy mildew differs from powdery mildew in that the causal fungus is conspicuous on the lower

surfaces of leaves, on which, in a

humid climate, it forms a fluffy,

downy growth, usually in welldefined areas. Often the fungus

causes yellow-brown spots having a

somew^hat scorched appearance on

the upper surface above the mildew

growth on the lower surface. The

spots may be small, purplish, and

numerous when abundantinfections

have occurred, but enlargement of

the spots is checked by a hot, dry

environment. The exposed leaves

of maturing cabbage heads are

sometimes affected by many small

spots of mildew, which predispose

them to soft rot in transit. Stems

and seed pods are also affected, the

chief symptom being dark-purplish

spots irregular in size and shape.

The causal fungus thrives best in

a cool, moist environment and is

therefore found chiefly in humid

regions. In the gulf coast and Pacific coast sections it is commonly

destructive to cabbage seedlings in

plantbeds or coldframes. Damage

to cabbage in the field is not serious

except in the winter crops on the

gulf coast, where the fungus predisposes the outer leaves of the

heads to bacterial soft rot in transit.

Losses in marketable heads also result from discoloration due to infection (fig. 12).

Causal Organism

The downy mildew fungus {Peronospora parasítica (Fr.) Tul.) af-
30 AGR. HANDBOOK 14 4, V. S. DEPT. OF AGRICULTURE

FIGURE 12.—Discoloration of cabbage head induced by systemic invasion of the

downy mildew fungus through outer leaves. Head leaves and pith area of the

stump are discolored.

fects many cruciferous plants. It

is made up of many strains, each

restricted to certain groups of host

plants. The mycelium penetrates

the plant directly and grows between the host cells, gaining its nutriment from knoblike projections

(haustoria) into the cells. It produces spores by first sending out

special branches on the surface of

the lesion. These make up the cottony mildew growth visible on the

leaf surface. The thin-walled

spores (conidia) are readily carried

by air currents and are short-lived ;

but in relatively cool moisture they

germinate promptly and cause new

infections. Within the tissue dark,

heavy-walled resting spores

(oospores) form. These can carry

the fungus through long unfavorable periods. The fungus mycelium

persists in perennial roots and in

storage organs such as cabbage

heads and turnip roots. In storage

it causes in both roots and heads a

slow, dry decay that occasionally

becomes important (fig. 12). In

such cases the decay may be diagnosed by placing decayed parts in

a moist chamber, where the fungus

ordinarily fruits readily on the

surface.
DISEASES OF CABBAGE 31

Control

Strict sanitation and rotation are

the chief means of control. In open

plantbeds in sections where downy

mildew is destructive the plants

should be sprayed or dusted with

maneb, zineb, or nabam+ zinc sulfate. The directions supplied by

the manufacturers should be followed rigidly.

White Rust

Characteristics

White rust occurs on many wild

and cultivated crucifers and is

worldwide in its distribution. The

white blisterlike pustules on leaves,

stems, and seed pods break open and

expose a white powdery mass consisting of spores of the causal fungus. These isolated pustules are

seldom associated with any marked

distortion of the plants. When the

interior of young stems and flower

parts are invaded, they grow to excessive size and are variously distorted. Flower petals, stamens, and

ovaries grow to many times their

normal size and can be recognized

only by their relative positions.

White rust is common on radish,

but ordinarily it is not destructive

except in midwinter greenhouse

crops. Horseradish is perhaps more

seriously affected than any other

crop in this country, since when the

perennial roots become infected the

normal growth is impeded. The infection of radish-seed plants causes

occasional loss. White rust is not

known on cabbage in this country,

but it occurs on this crop in Europe,

where it also causes occasional losses

to the seed crop. It occurs on cauliflower-seed crops in California

where it may be of economic

importance.

Causal Organism

The white rust fungus {Albugo

candida (Pers. ex Chev.) Kuntze)

is closely related to the downy mildew organism. It also consists of

several distinct strains, each confined to certain hosts. Relatively

short-lived spores (conidia, or

sporangia) are borne in the white

pustules in chains on short branches

of the mycelium. These windborne

conidia germinate best when it is

cool ; after a few hours in water they

each produce several swimming

spores. The latter come to rest on

the crucifer leaf and send out tiny

threads, which enter the breathing

pores (stomata). The fungus mycelium then grows between the cells

and feeds by means of haustoria, as

the downy mildew fungus does.

The spore-bearing branches form

from the mycelium below the host

epidermis, which is gradually raised

as the pustules form. The latter

remain shiny white and blisterlike

as long as the host surface is unbroken and retains the spores intact,

but the epidermis eventually breaks

and releases the conidia. The mycelium in young branches and in the

flowers stimulates the host cells to

multiply rapidly and abnormally,

causing malformations. In this

tissue from the mycelium between

the cells dark, thick-walled resting

spores (oospores) are formed;

these carry the fungus through unfavorable periods.

Control

Effective control measures have

not been worked out for white rust.

Bacterial Leaf Spot of

Cauliflower

Bacterial leaf spot occurs primarily on cauliflower, but it is

occasionally found on cabbage. It

occurs commonly on Long Island,

N. Y., in the tidewater district of

Virginia, and in the bay district of

California ; it has been observed also

in various other parts of this coun-
32 AGR. HANDBOOK 144, U. S. DEPT. OF AGRICULTURE

try and is common in the Scandinavian countries and in Finland.

Small brownish to purplish-gray

spots, sometimes irregular in outline, occur on both surfaces of the

leaf. A puckering of the leaf results when the midrib and larger

veins are infected abundantly.

This disease is caused by a bacterium {Pseudomonas maculicola

(McCul.) F. L. Stevens). It has

been observed that the spot disease

is most severe during cool, damp

weather and is held in check when

the warm, sunny days of late spring

arrive.

Crop rotation and sanitation

should be practiced. Since the

causal organism is probably seedborne, the hot-water treatment of

seed should be used.

Black Root of Radish

The white icicle type of garden

radish is very commonly affected

with black root, a dark-brown to

black dry decay of the fleshy roots,

which makes them unsalable. This

disease is caused by a fungus

{Aphanomyces raphani Kendr.)

that normally inhabits the soil ; the

mycelium enters the fleshy root

through the temporary wounds that

are made when the small secondary

roots push their way out. Once in

the fleshy root, the fungus progresses through the tissue and kills

cells, which soon discolor. The

thick-walled resting spores of the

organism are formed in the tissue.

In the icicle radish the secondary

roots emerge in two opposite zones—

on the taproot and approximately

halfway up the fleshy root. In these

zones black root occurs. In globe

types of radish the fleshy part

formed is almost entirely from the

lower stem (hypocotyl), and secondary root zones do not occur

except at the base. As a result they

are not noticeably affected. It is

consequently necessary to avoid

black-root-infested soil for radishes

of the long-rooted icicle type.

Other cruciferous crops do not appear to be affected.

Mosaic

A number of viruses affect

cruciferous crops. All are transmitted by aphids, or plant lice, and

each has a distinct host range.

Turnip is a host to all crucifer

viruses so far described. Some affect hosts outside the crucifer family. In California cauliflower

mosaic is caused by one virus and

cabbage black ring by another. In

the Middle West and in the Pacific

Northwest the mosaic of cabbage is

the result of the combined effect of

two unrelated viruses ; one of these

is closely related to that associated

with cauliflower mosaic and the

other to turnip mosaic. Each may

affect cabbage independently of the

other.

On cabbage the leaves of plants

affected with mosaic show a mottle,

resulting from lighter yellow and

darker green areas than normal and

so arranged as to give a mosaic pattern which may be so inconspicuous

that it is readily overlooked. The

most serious injury is stunting of

the plant as a whole. Another

serious effect is killing of some tissue, which turns black or brown.

Necrotic areas occur as spots on the

outer leaves and often throughout

the mature head (fig. 13). Mosaicaffected seed plants may be severely

stunted.

The cabbage mosaic viruses are

carried from plant to plant by

aphids and are probably spread

chiefly by this means. The viruses

can be transferred from diseased to

healthy plants, however, by injecting extracted sap from the former.

The usual method of overwintering

is in cabbage-seed plants and in

cruciferous weeds such as shepherds-purse and pennycress.
DISEASES OF CABBAGE 33

"aBfS'.i". , ^;l.

»t^ ^:«Ki

k

BN-6185X

FiGUEE 13.—Internal spots of cabbage head associated with infection by the

mosaic viruses.

In the Pacific Northwest mosaic

has been very severe on the cabbageseed crop. When plantbeds commonly have been made in the same

locality as the maturing seed crop,

this disease has caused extensive

losses. Infective aphids move from

the maturing crop to the new plantbed. To break this continuous

chain of increasing virus concentration it is necessary to make the

plantbed in a locality free from seed

crops and infected crucifers. When

this isolation practice is followed,

the damage from mosiac is reduced

to a negligible amount. The viruses

are neither soilborne nor seedborne.

Cabbage varieties differ in their resistance to mosaic, some being less

affected.

Damping-OfF

Damping-off of seedlings of many

crops is common. It is associated

with infection by various fungi;

ordinarily, the effects of individual

species cannot be readily distinguished. Some fungi attack cruciferous and other plants only at the

seedling stage and only under certain environmental conditions. In

general, poor light, high moisture,

and other factors that promote the

rapid extension of seedlings aboveground rather than slow extension

and more rapid thickening bring on
34 AGR. HANDBOOK 144, U. S. DEPT. OF AGRICULTURE

conditions conducive to dampingoff. This effect has already been

described in connection with the

rhizoctonia disease (p. 21).

Damping-ofl may be caused by

the watery soft rot fungus (p. 25),

by the blackleg fungus (p. 19), and

by the fungus that causes pythium

head rot (p. 40) and its relatives.

Control of damping-off can be effected by the following practices :

(1) Use of soil of good texture.

(2) Sterilization of soil if it is

used repeatedly.

(3) Eotation of outdoor plantbeds.

(4) Seed treatment with fungicidal protectant dusts.

(5) Avoidance of too thick

sowing.

( 6 ) Carefulness in watering.

(7) General sanitation.

Leaf Spots

Leaf spot phases occur in a number of diseases already described.

Chief among these are black leaf

spot (p. 26), blackleg (p. 19), ring

spot (p. 27), white rust (p. 31),

downy mildew (p. 29), powdery

mildew (p. 29), cauliflower bacterial spot (p. 31), and rhizoctonia

disease (p. 21).

Other leaf spots on crucifers are

described in the following paragraphs.

Anthracnose,—Anthracnose is a

common disease of turnip, Chinese

cabbage, radish, and mustard

greens in the South. Storage decay

of turnip has been reported from

New York. It consists of relatively

small, brown-bordered spots with

grayish centers, which often coalesce. When heavy infection occurs,

it may cause considerable damage

to turnip and Chinese cabbage.

In moist weather the causal fungus {OolletotHchum higginsianwn

Sace.) fruits abundantly in the

center of the lesions and bears

short-lived conidia, which are dispersed chiefly by raindrops. Southern giant curled mustard is highly

resistant.

White spot,—^White spot is another disease common in the South

on the hosts on which anthracnose

occurs, but usually it is of little importance. It is distinguished by

white bleached spots. If these

spots become very numerous they

destroy the marketability of turnip

greens. The causal fungus {Oercosporella albomaculans Ell. and

Ev.) produces conidia on the surface of the leaf lesions. A black

dendritic leaf spot of cabbage incited by the same species has been

noted in the Pacific coast sections.

Other leaf^ spots,—A number of

other fungi, particularly Cercospora spp. and Ramularia spp.,

cause leaf spots on the minor

cruciferous crops, such as mustard,

horseradish, bunching turnip, and

watercress. These are generally of

minor importance and need not be

discussed in detail here.

NONPARASITIC MALADIES

Low-Potash Injury

Crucifers as a whole thrive best

on soils that are practically neutral

in reaction. Extreme acidity such

as prevails in some Atlantic seaboard soils results in abnormal

growth, particularly if the soil solution is more acid than pH 5.

Cabbage is a heavy user of potash ; when its supply falls below a

certain level, disease symptoms appear. These are characterized by

the yellowing of the foliage between

the veins and particularly around

the margins of the outer leaves,

where the tissue eventually dies and

becomes brittle and saprophytic
DISEASES OF CABBAGE 35

fungi develop. As plants in this

condition pass into the heading

stage, the heads remain soft indefinitely ; as a result a marketable crop

fails to develop.

This condition can be readily prevented by testing the soil before

planting ; there should be 250 to 400

pounds of available potash per acre

for a normal crop of cabbage. Even

though effects of malnutrition fail

to appear on the plants, a low-potash level results in cabbage of poor

quality for sauerkraut manufacture.

Magnesium-Deficiency

Chlorosis

In many soils in the Coastal Plain

of the Atlantic seaboard a low level

of available magnesium results in

characteristic disease symptoms on

cruciferous crops. Yellowish areas

develop in tissue between the veins

of the leaves, and a chlorotic condition not unlike mosaic results. As

the disease advances, the mottled

areas may change to light-colored

desiccated spots. The chief distinction from low-potash injury is that

the low-potash level causes chiefly

marginal yellowing and dying. The

yield of affected plants may be

greatly reduced. The condition can

ordinarily be corrected by the use

of fertilizer that is made slightly

basic with dolomitic limestone and

to which small amounts of soluble

magnesium salts have been added.

Boron-Deficiency Injury

Boron is one of the minor nutritive elements essential for plant

growth. In many soils, especially

those which are alkaline in reaction,

this element is deficient or it is in

a form unavailable to plants.

When such a deficiency prevails,

turnip, rutabaga, cauliflower, and

cabbage commonly show characteristic symptoms. In turnip and rutabaga dark-brown, water-soaked

areas appear in the central flesh of

the root, either in small scattered

spots or in large areas that may develop into a hollow center. In

cauliflower the small leaves around

the curd become deformed. The

curd shows a brown discoloration

that extends through the flesh, the

core splits transversely and vertically, and the stem eventually becomes hollow. In cabbage there are

sometimes mottling along the margins of the oldest leaves and crosshatching on the upper surface of

the petiole and midrib. As a rule,

however, no sign of the disease is

seen until the head is cut, when

water-soaked spots appear in the

fleshy pith of the core and stem.

These may be brown, and in extreme

cases hollow cavities with darkcolored linings occur in the lower

part of the core and upper part of

the stem. When the crops mentioned are grown on soil that tends

to produce plants showing symptoms of boron deficiency, 20 pounds

of borax per acre should be applied

broadcast and harrowed into the

soil before the crops are planted.

Whiptail of Cauliflower

Cauliflower plants sometimes

form abnormally narrow, ruffled

leaves with irregular margins, and

in extreme cases enough dwarfing

results to preclude heading. This

disease, known as whiptail, is primarily a malnutritional disorder.

It occurs chiefly along the Atlantic

seaboard on highly acid soils.

Whiptail is also common in Europe,

New Zealand, and Australia. This

disease is caused by the lack of one

of the trace elements, molybdenum.

Broadcast application of sodium or

ammonium molybdate (20 pounds

or less per acre) controls whiptail.

The application of lime to acid soils

reduces the disease by releasing sufficient amounts of the trace elements. Growers who have losses

from this disease are advised to con-
36 AGR. HANDBOOK 14 4, U. S. DEPT. OF AGRICULTURE

suit their county agents about the

most recent control recommendations.

Low-Temperature Injuries

Many crucifers grow well in cool

climates and in fact do their best

in such an environment. In the

Southern States, cabbage, cauliflower, turnip, and radish are favorite winter crops, since they harden

well and withstand light frosts.

Certain detrimental effects, however, can be recognized.

The bolting of cabbage without

its coming to a mature head often

occurs in the winter crop. This is

brought about by protracted cool

or frosty weather. Under such conditions cabbage goes through dormancy without heading. It is a

purely environmental effect and

cannot be attributed to poor seedstock, as is often done.

Multiple heading in the early

northern cabbage crop from southern-grown cabbage plants sometimes causes appreciable losses.

This is caused by injury done to the

main growing tip by frost when the

plants are young. After transplanting, several side buds grow instead of the main growing point and

several small unmarketable heads

result.

Freezing injury is most common

on the late-fall crop in the North.

This subject is discussed further in

the section on Transit and Storage

Diseases (p. 38).

Tipburn

The term "tipburn" refers to a

nonparasitic breakdown that affects

the margins or lap areas of one or

more leaves in the upper part of

the head. As plants approach maturity, these inner leaf margins turn

a dark brown and show up as desiccated, papery thin tissue (fig. 14).

The disease is not evident externally. When soft-rot bacteria invade the dead leaf tissue, a slimy

decay follows. Internal tipburn

heads are undesirable for the fresh

market. Also, when used in sauerkraut production, the discolored

tissue shows up as very conspicuous

darkened shreds in the kraut vat and

reduces the quality. The disease is

widespread on several soil types in

the northern cabbage areas, and

appears spasmodically. In field

tests, cabbage varieties have shown

marked differences in susceptibility

to tipburn, ranging from 0 to 70

percent. The causal factors that

bring about the disorder are not

fully understood.

Flooding Injury

The flooding of soil in cabbage

fields often causes the death of

plants when the soil remains saturated for several days. Sudden,

permanent wilt occurs, and the roots

decay rapidly. This is not the result of asphyxiation, as plants can

be grown in liquid culture. It is

more probably caused by the abnormal anaerobic fermentation set up

by the soil flora, which liberates

substances in the soil solution that

are toxic to the plants.

Lightning Injury

When lightning strikes the earth

in cabbage fields, plants soon wilt

in roughly circular areas of various

sizes. Within a few days most of

the plants are dead. Such lightning

spots are usually not observed until

some weeks after the occurrence,

and there is thus commonly some

question as to the cause. Certain

characteristics help to distinguish

the injury (fig. 15).

Around the margin of the area

are to be found plants that are weakened or slowed up in growth. As

these are pulled, it is usually found

that the lightning scar is at the soil

line and is limited to the side facing the center of the area. Some-
DISEASES OF CABBAGE 37

FiGUKE 14.—Cabbage tipburn : A, Cross section of head showing darkened tissue ;

B, top view of thin, papery, desiccated lap leaves.

times this injury is slight and is

rendered conspicuous by being callused over. Greater injury leads to

a side stem growing out just below

this point or to adventitious roots

forming just above it. The electric

charge apparently passes directly to

the pith of the stem, in which it is

conducted more readily. The lightning injures the pith most readily

also ; some time after the injury the

pith is hollow and dark-colored, and

within it fibrous roots develop abnormally. The peculiar earmarks

of lighting injury are very useful

in diagnosing it several weeks later.

Intumescences

On leaves and stems of cabbage,

cauliflower, and other cruciferous

plants in the field, wartlike projections the size of a pinhead or larger

often occur. They may be scattered

in large numbers over both surfaces

of the leaf. These are the result of

injury to the tissue brought about
38 AGR. HANDBOOK 144, U. S. DEPT. OF AGRICULTURE

FIGURE 15.—Symptoms of lightning injury of cabbage after several weeks : A, Darkened callus tissue at ground level ; B, longitudinal section of stem shown in A,

showing hollow and blackened pith ; C, adventitious root formation above lesion

on injured stem.

by the blowing of soil particles

against the leaves. As a result of

the wound, the tissues respond by

building up callus, which takes this

form. The protuberances are of no

great importance, but owing to their

conspicuous nature they often attract attention.

TRANSIT AND STORAGE DISEASES

Various diseases that cause direct

loss or necessitate heavy trimming

of cabbage in storage are described

'Adapted from Miscellaneous Publication No. 292, "Market Diseases of Fruits

and Vegetables: Crucifers and Cucurbits." That publication may be consulted

in libraries in most large cities.

briefly so that they can be recognized and avoided.

Freezing Injury

Heavy losses of cabbage occur annually as a result of freezing, both in

the field (p. 36) and after harvest.

Cabbage tissue has one of the high-
DISEASES OF CABBAGE 39

est freezing points (31° F.) found

among vegetables. Sometimes frozen cabbage will thaw out without

injury and sometimes it will not.

In the former case the tissues are

merely frozen, while in the latter

they are frozen to death. However,

there is no way of telling by examination of a frozen head of cabbage

or of other vegetables that act

similarly whether they will show injury and breakdown when they

thaw out. Immediately after they

are thawed, the frozen areas look

water-soaked, because of suffusion

with water liberated in the intercellular spaces by the thawing ice.

Ifthe cells are still alive, some of the

water will go back into them; the

only effect on the tissue is a slight

wilting or shriveling due to the excessive loss of water by transpiration and evaporation. Tissues responding in this way become flabby,

pithy, or spongy and tough, and

lose their characteristic flavor.

Some of them can withstand the

freezing process several times before

the effect becomes pronounced;

others show it immediately. If the

cells are not alive, the water does not

reenter them ; it may be lost rapidly

into the air, with attendant drying

out of the tissues ; or it may remain

and the tissues become a leaking,

disorganized mass, which is attacked by saprophytic fungi and

bacteria.

Whether ice formation in the tissues is attended by permanent injury and death seems to depend

upon the degree to which the temperature falls, the rate at which it

falls, thé duration of the critical

temperature, and the condition of

the plant tissues themselves.

Ice in cabbage tissues renders

them rigid or brittle. Frozen leafy

tissues and even such storage tissues as in turnip and rutabaga lose

their natural luster and take on a

glassy appearance. Immediately

upon thawing, they become watersoaked. The leafy green tissues in

the water-soaked area also have a

dirty- or muddy-green color. In

colorless or fleshy parts there is at

first no discoloration. Later the

more sensitive tissues, especially the

water-conducting tissues, may become yellowish brown and finally

black.

In cabbage there is no such discoloration, except possibly in parts

of the stem. If the tissues have not

been killed, they soon lose the watersoaked appearance and look much

as they did before, except that they

may be more flabby. If the tissues

are killed, discolorations usually develop; these discolorations vary

with the different kinds of vegetables and tissues. The epidermis

of tender, leafy tissues, such as in

cabbage, Italian broccoli, kale, and

mustard, is often loosened in spots,

especially along the petioles, midrib, and smaller veins; it appears

blistered and can be removed easily.

Fleshy tissues such as in turnip,

radish, rutabaga, and horseradish

often show no discolorations except

in the water-conducting tissues;

these become yellow, brown, or

black.

In cabbage the outer leaves seem

to be more resistant to killing by

freezing than the inner leaves and

the stem. Generally they thaw out

without injury. After prolonged

exposure to freezing temperatures,

the inner tissues, especially the pith

of the stem, usually are killed ; soon

they become affected with bacterial

soft rot.

In dry air the killed tissues may

dry out. Leafy tissues usually become brittle and parchmentlike;

more fleshy ones become spongy and

pithy or hard and stony. In moist

air various rots develop, depending

upon the external conditions. Thin,

leafy tissues usually are affected

with bacterial soft rot, alternarla

rot (black leaf spot, p. 26), or gray

mold rot. More fleshy ones are at-
40 AGR. HANDBOOK 144, U. S. DEPT. OF AGRICULTURE

tacked by bacterial soft rot, rhizopus rot, or other rots.

Control is obtained by preventing

exposure of the products to critical

cold temperatures for a long enough

period to cause their death.

Bacterial Soft Rot

As stated previously (p. 24), the

diseases grouped as bacterial soft

rot are caused by bacteria of the

Erwinia carotovora group. Soft

rot probably causes more loss on the

market than any other disease of

cabbage and related crops. The reason is that practically all invasion

of tissue by soft rot organisms is

through wounds, the lesions of

other diseases, or tissues weakened

by mechanical agencies, freezing,

asphyxiation, or aging. These organisms will attack only injured or

dead tissues, and such tissues only

when they are wet. Breaks in the

tissues or moistened surfaces provide the water and food the organisms need to get started and to carry

on growth and reproduction.

The pith of stems or roots is very

often affected; in cabbage the resultant condition is often referred

to as "stump rot." Stump rot

usually follows infection during

cutting or after freezing. Unless

care is taken in cutting to avoid

heads affected with bacterial soft

rot, it is easy to carry bacteria on

the knife from diseased heads to the

freshly cut stems ; when moist these

stems provide ideal starting places

for decay. In turnip and other root

crops the lesions usually occur in

the pith, starting at the crown of

the plant. In cabbage the lesions

usually are on the surface or in the

pith, following bruises or wounds.

If the lesions are on the surface, the

outer decayed tissues can be removed and the progress of the disease arrested by drying the freshly

exposed surface. If the stem is invaded or the disease penetrates deep

into the head following black rot

(p. 17), the entire head is worthless.

In transit and storage soft rot

may be spread by contact or by the

oozing or dripping from the decayed tissues. There probably is

much less actual spread of this

decay in transit than of many fungus rots because bacteria do not

form mycelia, or fungus threads.

The soft rot organisms are particularly sensitive to drying and to

direct sunlight. Wet, warm weather favors the rot.

Pythium Head Rot

The pythium head rot fungus

travels rapidly along the midrib,

which is reduced to a soft, pulpy

consistency but is held together by

outer layers of tissue. Offensive

odor is not characteristic, and the

mycelium in the decaying tissue has

practically no cross walls. The

causal fungus {Pythium debaryanum Hesse) and its close relatives

are common causes of soft decays of

storage organs of fruits and vegetables. They also cause rootlet

decay and damping-off of many

plants (p. 33).

Rhizopus Soft Rot

Crucifers are often subject to infection by a fungus known as

Rhizopus, Spores of this fungus

are ever present in the air and soil,

and consequently there is always

more or less danger of infection if

conditions are favorable. This disease is of little or no consequence in

the field or on leafy crops such as

mustard and kale; but on the more

ñeshy vegetables such as cabbage,

cauliflower, turnip, and rutabaga,

consequent decay developing during

transit and marketing sometimes

causes considerable loss. The chief

predisposing factors are wounds

and high temperature. Rhizofus

does not affect cabbage, cauliflower,
DISEASES OF CABBAGE 41

or root crops in cold storage (32°

In succulent tissues the decay is

light brown, soft, and watery. In

roots the rot is moderately soft and

moist, but it lacks the characteristic

disagreeable odor of the bacterial

soft rot (p. 24) that sometimes presents a similar appearance. Usually in cjabbage, cauliflower, brussels sprouts, and turnip, the coarse,

stringy fungus threads bearing

white and black fruiting bodies

make the disagnosis of this disease

fairly easy.

Gray Mold Rot

In cabJDage, cauliflower, and turnip, as in most other vegetables,

gray mold rot, caused by a fungus

known as Botrytis cinérea Fr., occurs most commonly in aging tissues. Consequently, it is of most

importance in storage. There it is

sometimes the most important single factor of loss, since the causal

organism can grow at cold-storage

temperature. The rot is found

even in freshly harvested stock if

conditions are especially favorable

for the fungus. Chief among these

is high humidity. If this is associated wit^i moderate temperature,

the fungus attacks even the outer

aging leaves of growing plants.

Water-soaked, grayish-green areas

are produced, and usually a gray

mold is present on the surface of

affected tissues. In the more advanced stages of decay the grayish-brown vegetative threads and

spore clusters of the fungus are

characteristic.

Black Leaf Speck

Black leaf speck is characterized

by small, sharply sunken, brown or

black specks occurring anywhere on

the leaves of cabbage, cauliflower,

and Chinese cabbage from all sections of the country. At times,

these specks are found in great

abundance, especially on the leaf

blades. They may occur in a few

of the outer head leaves only, or

in leaves throughout the head. Frequently it is difficult to distinguish

them from the early stages of black

leaf spot (p. 26), which occurs in

California cabbage, and from ring

spot (p. 27) as it appears in Oregon

cauliflower. The spots render these

products unsightly, and thus affect

their marketability. They also tend

to lower their resistance to storage

rots.

Very little is known about the

factors that bring about black leaf

speck, except that it is not caused by

organisms. It occurs commonly in

transit and storage ; for this reason

it might be classed as a breakdown

associated with low temperature.

It also occurs sometimes in the field,

where it has been found, in some

instances at least, to have been associated with a decided drop in

temperature.

U. S. GOVERNMENT PRINTING OFFICE: 1958