1

v

ON lOISI

disease

L \ B R, A R Y

N0V141961

a 5 »"*.i«îiïii Of »e-iC'JiíiíK

BELTSViLLE BRANCH

and

their

oontrol

Agriculture Handbook No. 208

Agricultural Research Service

UNITED STATES DEPARTMENT OF AGRICULTURE
o-ÍNION DISEASES in the field vary in kind and in intensity with

the climate, soil, and other environmental factors in the various sections where onions are grown. Onion diseases in transit and in storage

are atïected by the variety of onions, farm practices, and conditions

of storage.

Onion smut, which remains alive in the soil for indefinite periods

and is found in the Northern States from coast to coast, can be controlled with a dust containing thiram, anti-carie, or captan applied to

seed coated with Methocel sticker. Most of the other diseases

described are not readily controlled with chemicals. To supply consumers with adequate quantities of sound onions, reliance must be

placed on crop rotation: timely cultivation; fertile soil; the use of

resistant varieties and disease-free bulbs, sets, and seed : sanitation ; and

proper methods of handling, curing, and storage, as suggested in this

handbook.
CONTENTS

Page

PRODUCTION AREAS 1

DESCRIPTIVE KEY TO ONION

DISEASES 4

DISEASES PRIMARILY IMPORTANT IN THE FIELD

Smut 5

Damping-off 7

Downy mildew 7

Tip and leaf blight 10

Leaf mold 10

Purple blotch 10

Pink root 12

Fusarium basal rot 13

Rusts 14

White rot 14

Yellow dwarf 15

Aster yellows 17

Dodder injury 17

FARM AND HANDLING

PRACTICES RELATED TO

STORAGE DISEASES 19

Sanitation 19

Harvesting 19

Page

FARM AND HANDLING

PRACTICES, etc.—Con.

Removal of diseased bulbs 20

Curing and storing 20

RELATION OF VARIETIES TO

STORAGE DISEASES 21

DISEASES PRIMARILY IMPORTANT IN STORAGE

AND TRANSIT 21

Botrytis neck rot 21

Bacterial soft rot 22

Black mold 23

Smudge 24

Diplodia rot 25

Aspergillus rot 25

Blue mold rot 23

NONPARASITIC BLEMISHES

AND MALADIES 26

Freezing injury 26

Sunscald 26

Physiological breakdowns 26

Chemical injuries 27

This publication contains information formerly presented in

Farmers' Bulletin 1060 of the same title.

Washington, D.C. Issued October 1961

UNITED STATES DEPARTMENT OF AGRICULTURE

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

Washington 25, D.C. - Price Li cents
ONION

di

and their

control

By J. C. WALKER, collaborator, and R. H. LARSON,' pathologist, Crops Research Division

Agricultural Research Service, and professors of plant pathology, University of

Wisconsin

THE ANNUAL ONION CROP

in the United States averaged

slightly more than 45 million 50-

pound sacks grown on about 117,000

acres from 1949 to 1959, inclusive.

Culture of the onion (AUhnn cepa

L.) is intensive; the growing of the

crop entails heavy investments in

land, fertilizer, and labor. Onions

are one of the few crops in which

repeated cropping or very short

rotations are commonly practiced.

Individual growing areas are therefore relatively small and concentrated. However, they are not

limited geographically, for intensive onion-growing districts are to

be found in most States. The 10

States leading in the total production of onions are Texas, New

York, Michigan, ("olorado, California, Oregon. Minnesota, Wisconsin,

Idaho, and Illinois.

PRODUCTION AREAS

Sections producing the crop of

table onions may be divided into

tliree groups: (1) Northern States

from Massachusetts to the Pacific

coast, including northern California, where about 75 percent of the

total is grown as a late-summer or

fall crop; (2) central California,

south-centi-al Washington, and New

Mexico, Iowa, Oklahoma, Kentucky, Virginia, and New Jersey,

wliere about 5 percent is grown as

an early-summer crop; and (3)

Southern States extending from

southern California to Georgia,

where about 20 percent is grown as

a spring crop.

In the northern sections most of

the crop is grown from seed sown

directly in the field in early spring

at the rate of 3 to 4 pounds per acre.

No tliinning of the stand is made.

Both upland soils and muck are

used, but in recent years the acreage

of the crop on muck has increased

so rapidly that it now comprises

the greater acreage. The older

onion varieties are Southport. Yellow Globe, Early Yellow Globe,

Michigan Yellow Globe, Southport

White Globe, Southport. Red Globe,

Mountain Danvers, and Yellow

Globe Danvers, which are grown in

the East and Midwest ; additional

' Deceased, August 29, 1961.
2 AGRICULTURE HANDBOOK NO. 2 08, U.S. DEPT. OF AGRICULTURE

varieties such as Australian Brown

are grown in California. Several

newer liybrid varieties are now in

use.

In the Rocky Mountain sections

the Sweet Spanish varieties are

grown extensively on irrigated alluvial soils. In tiie early-summerproclucing sections some of the

acreage is grown from sets. Flat

varieties such as Ebenezer, Red

Wethersfield, and White Portugal

and the globe-shaped variety, Early

Yellow Globe, are so grown. Most

of the northern crop is carried in

storage for various periods and is

placed on the consuming market

throughout the country up to March

and April.

Most of the spring crop and some

of the early-summer crop is the flat,

mild-flavored Bermuda type—

chiefly Yellow Bermuda, Crystal

Wax, and several new hybrid varieties. Globe varieties of the Denia

class are grown less extensively.

The seed is sown in late summer and

early fall in seedbeds, whenc« plants

are set during the mid-winter

months. Much of the acreage is

under irrigation. The crop matures

in April and May. These southern

crops thus supply the consuming

markets throughout the country

during April, May, June, and July.

The Bermuda type is not adapted to

long storage, so that the marketing

season of this crop is much shorter

than that of the northern crop.

Onion sets are chiefly of the Red

Wethersfield, Ebenezer, and White

Portugal varieties. They are grown

in several Northern States, always

on upland or alluvial soil. The majority are produced in the Chicago

section, extending from northwestem Indiana through northeastern

Illinois and southeastern Wisconsin,

and less extensively in the Greeley,

Colo., district. The seed is sown in

early spring, as are table onions,

but at the rate of 60 to 70 pounds of

seed per acre.

Although some of the crop is

shipped south immediately after

liarvest for fall planting, most of

it is stored and shipped during the

winter and througliout the spring.

Onion sets are widely distributed

and used generally for spring

bunching onions in the home garden

and on the commercial truck farm.

Onion seed is grown chiefly under

irrigation in the Western States

where dry summer weather facilitates pollination, harvesting, and

curing of the seed.

Several close relatives of the common bulb onion are mentioned here

because some of them are affected

with the usual onion diseases. The

Egyptian onion (AUium. cepa var.

viviparum Metz.) belongs to the

same species. It is hardy in the

Northern States. Its culture is confined largely to home gardens,

where it provides an early supply of

green onions. Being hardy, it continues as a perennial crop. Edible

bulbs are not formed, but during the

summer stems similar to seed stems

are produced and at the top of

them, bulblets, known as top sets.

The top sets are used for starting

new beds. The relatively uncommon multiplier onion {A. cepa var.

soljininmn Alef.) has dividing

bulbs. Garlic (A. sativum L.),

propagated from bulbs, is produced

commercially in California, Texas,

and Louisiana. Ijeek (.1. pomim

L.) is a flat-leaved relative, propagated from seed. Shallots {A.

ascaJoni-cum L.), propagated from

small bulbs, are grown commercially

as a winter crop in Louisiana, where

they are marketed as bunched green

onions. The Welsh onion (A.

fistulosum L.) is propagated from

seed, and each plant produces a

group of several scallions. It does

not produce a distinct bulb but may

be used as a source of bunched

onions. Chive (A. schoenopra-sum

L.) is an onion relative, which is

propagated vegetatively in green-
ONION DISEASES AND THEIR CONTROL

houses or outdoors for use as ii

garnish.

It may be seen from this brief

survey that the crops of onions and

of onion relatives are grown under

a variety of climatic conditions and

on a wide range of soils. As environmental factors have a direct

tearing upon disease, the pi-eeminent diseases vary with geography.

In the discussion of the diseases,

reference will be made repeatedly

to specific growing sections and

cultural practices.
DESCRIPTIVE KEY TO ONION DISEASES

The following key to the outstanding symptoms of onion diseases

will aid in their recognition. Growing sections where the disease is liKely

to be most common or severe are included to help m the identihcation.

i. DISEASES IMPOETAXT IN THE FIELD :

1 Dark pustules within the leaves or scales, sometimes splitting open later and

exposing black powdery masses: principally on young seedlings (flgs._l

and 2). (Northern States) iiaiut ( p. c.)

2 Rapid death of very young seedlings in circular patches in the

fleid Damping-ofE (p. o

3 The leaves beginning at the tips, turn pale green and later yellow, becoming

covered with a violet furrv growth, and finally collapsing ; most .serious

in moist weather in midseason or later (figs. .3 and 4). ( Jlost onion-growing

States: severe in California, Louisiana, New York, Oregon, and llichi-^anl- Downy mildew (p. 7)

4 Dying back of leaf tips, often followed by blight of the leaves Tip or leaf

blight (p, 10)

5 Black moldy growth on leaf tips or .seed stems, often following downy mildew

or purple blotch Leaf mold (p. 10)

6. Large purplish lesions, sometimes showing zonation. eventually girdling

leaves and .seed stems Iflgs. 5 and 6) Purple blotch (p. 10)

7 Rapid dving back from the tit>s of the leaves, accompanied by a rot starting

at the' base of the bulb (figs. 8. 9 and 10). (Northern States; Rocky

Mountain irrigated valleys) Fusarlum basal rot I p. 131

8. Dying oí the tops rather similar to that in fusarium basal rot. but differing

"from It in that round black bodies about the size of poppy seeds appear in

the diseased bulb ifig. 11). (Southern States and Oregon) White rot

( p. 14)

9 Boots turn pink, shrivel, and die: new roots attacked and a marked stunting

of the plant results (fig. 7) Pink root (p. 12)

10. Orange or golden-yellow pustules on leaves or seed stems, especially on those

of the Egyi:>tian onion; rare Rusts (p. 14)

11. Pronounced stunting, accompanied by various degrees of yellow streaking of

the leaves (flg. 11) Yellow dwarf (p. 1-5)

12. Flower cluster of seed plants develop abnormally long pedicels, to which

flowers are attached : flower parts are distorted and flowers sterile (fig.

13). (California, Idaho, Wisconsin, and i)erhaps elsewhere) Aster

yellows (p. 17)

B. DISEASES IMPORTANT IN STORAGE AND TRANSIT :

1. Rot usually begins at the neck of the bulb and progresses downward : tissues

shrink and collapse : a gray to brown moldy growth and hard black kernels

appear later on the surface of affected scales (figs, l.i and 1(5) Botrytis

neck rot ( p. 21)

2. Rot appears at harvesttime or later, but differs from neck rot in being softer

and more watery, usuallv with a verv offensive odor Bacterial soft rot

(p. 22)

3. A semiwatery rot advancing from the base of the scales upward (fig.

10) Fusarium basal rot (p. 13)

4. Black powdery masses on the outer .surface of the scales or between them (fig.

17), as in smut Black mold (p. 23)

5. lämudgy. superficial, dark-green to black spots made up of fine dots, but with

no powdery masses, appearing shortly before harvesttime on the outer

.scales: primarily on white varieties (fig. IS and front cover Smudge

IP. 24)

6. Semiwatery de<-ay. at first deep yellow, then wine red, and finally black,

attaching the neck or wounds in the scales, which dry down to a papery

texture Purple blotch (p. 10)

7. Silvery-gray to black discoloration of dry outer scales Diplodia rot (p. 2-5)

8. Dry decay of garlic, with brown kernellike bodies in and on decayed

tissue Aspergillus rot (p. 2.Ö)

9. Light-yelknv lesions on fleshy scales of onions : later with a fine, white

surface mold ; conversion of garlic cloves to greenish-tan or gray powdery

ma.ss Blue mold rot (p. 25)

4
ONION DISEASES AND THEIR CONTROL

DISEASES PRIMARILY IMPORTANT IN

THE FIELD

Smut

Smut is a disease caused by a

fungus that remains viable in both

uph^nd and muck soils for many

years. Smut-infested soil is tlierefore likely to remain infested indeñnitely and is found usually in

areas where onions have been grown

intensively for a long time. This

disease is to be found in many

onion-growing sections in the

Northern States from coast to coast.

It is not a factor in the wintergrowing southern sections; its almost complete absence is due largely

to climatic conditions. Leek is also

affected.

Characteristics

The disease appears soon after the

seedlings emerge above ground.

Brown to black elongated blistere

form within the scales or leaves ; the

latter usually are slightly thickened

and often curved downward abnormally, as shown in figure 1. These

blisters often break open and expose

black" powdery masses made up of

the spores of the causal fungus.

Most of the infected seedlings die

within 3 to S weeks after germination. Such thinning of the stand

is thus an early and important feature of smut injury, although a

small percentage of infected seedlings outgrow the disease. Some

diseased plants survive until midseason or harvest, new leaves and

scales becoming infected as they develop. Figure 2 shows a halfgrown infected plant that survived

the seedling stage. Such plants

usually produce bulbs so small and

imperfect that they are thrown out

at harvest.

Occasionally, infected bulbs are

large enough and so slightly

/

\

FIGURE 1.—Young onion seedlings allowing dark, slightly thickened .smut lesions.

affected as to escape notice and

reach the warehouse and market.

Such bulbs are characterized by

slightly raised brown to black

pustules most prevalent near the

base of the outer fleshy scale and

occuring as deeply as the third or

fourth scale. Smut does not cause

a rot during storage, but smutted

bulbs shrink more I'apidly and are

more subject to the attack of other

organisms than healthy ones.

Causal organism

The fungus (Urocystis cefndae

Frost) associated with onion smut

infests only the onion and certain
AGRICULTURE HANDBOOK NO. 208, U.S. DEPT. OF AGRICULTURE

FIGURE 2.—A half-gruwn oni(jn plant

.'showing in the sr-ale.s and leaves hrown

to black unbroken blisters, or pustules,

caused by smut.

closely related species of plants,

such as leek. Most varieties of the

Welsh onion are resistant to this

disease. The black powdery masses

of spores, or seed bodies, are the

means of propagating the fungus.

These spores are liighly resistant to

environmental changes; as they

become incorporated in the soil,

they remain viable for many years.

Consequently, continuous cropping

is favoraljle to the accumulation of

the fungus, and soil once infested

remains so indefinitely.

The spores germinate by sending

out fungus tlireads, wliich permeate

the soil and often break up into

many secondary spores. These

stages of the fungus are sensitive

to cold and drought and survive for

short periods only. The threads

penetrate young onion seedings,

take nourisliment from the plant,

and form tlie characteristic blisters

within the tissue : finally, more dark

spores develop.

Important to the understanding

of onion smut and its control is the

fact that the fungus can invade the

onion plant only in tlie early seedling stage. If the outer seedling

leaf (cotyledon) escapes infection

until it is mature (about 3 weeks

from germination), no further invasion occurs even in heavily

infested soil. Smut is therefore not

a disease factor when the crop is

started from sets or transplants,

even thougli they are set in smutty

land. Wlien the mean soil temperature is S4° F. or alx)ve during

the period of the seedlings" susceptibility, no infection occurs, because

the fungus is inactive at this higli

temperature. At slightly lower

temperatures the seedlings tend to

outgrow the disease more etfectively

than in cool soil. These temperature relations explain the absence of

the disease on the southern onion

crops, where the seed is sown in late

summei'. The seedlings are thus

started olf in very warm soil, which

is unfavorable to smut. In the
ONION DISEASES AND THEIR CONTROL

main crop of the Northern States

the seedlings come up in cool soil,

and the smut fungus, if present, is

not inhibited.

Smut spores are spread locally

by farm implements, by the feet of

men and animals, by surface drainage water, and by airborne soil.

The transfer from locality to locality is rarely made through the

medium of seed, but commonly by

means of onion sets and plants

grown on smutty soil.

Control

As just stated, the onion crop

started with sets or transplants is

not affected by smut. In the principal crop of the North, started from

seed in the field, the disease is controlled with a fungicide dust applied to the seed. Thiram is the

most satisfactory material. It is

used on many types of seeds to prevent their decay by soil organisms

and thus insure a good crop. When

it is applied to onion seed in the

amount usually advocated for seed

protection ( 1 ounce to 10 pounds of

seed), it has little or no effect upon

smut. For set production, however,

1 pound of thiram to 10 pounds of

onion seed has given satisfactory results, even though all tlie dust does

not adhere to the seed.

To protect seed for bulb production, better distribution of the dust

is obtained by applying Methocel ^

sticker to the seed first, even though

more dust is needed. One ounce of

4-percent Methocel solution (2

ounces of Methocel to 1 quart of hot

water) is applied to 8 pounds of

seed and mixed thorouglily ; 6

pounds of thiram (Arasan Y5 percent) is then added. The seeds are

agitated in a butter churn or similar

device until they are well and uniformly pelleted with the dust. The

- The mention of proprietary product.s

does not imply tlieir endorsement by the

U.S. Department of Agriculture over

similar products not named.

S97985 0—61 2

rate of seeding should be adjusted

to allow for the greater size of pelleted seeds.

Practical smut control in bulb

onion plantings has also been obtained when seed is pelleted with

equal weight of anti-carie (80 percent liexachlorobenzene) or captan

75 percent and a 4-percent Methocel

sticker.

Damping-Off

Damping-off is a disease common

to seedlings of many plants. One

or more of several soil fimgi may

be involved. When seedlings are

growing rapidly they are subject to

attack by these fungi, whereas at

other times they are resistant. The

plants are attacked at the soil line

or slightly below, and the tissue

shrinks rapidly while the still turgid above-ground parts topple over.

Damping-off in onions does not ordinarily kill plants here and there,

but more often it kills practically

all plants in roughly circular areas

of various sizes.

There is no specific remedy for

this disease. The best cultural

practices, such as frequent cultivation, good drainage, and good fertility, all discourage damping-off

by providing the most favorable

growing conditions for the onion

plant. Treatment of the seed with

a dust containing thiram or captan,

according to directions given by the

manufacturer, may be helpful but

not necessarily so.

Downy Mildew

Downy mildew has been reported

from most States where onions are

grown. Its destructiveness varies

widely with locality and season.

Cool, moist weather is most favorable for its development. Losses

are most severe on the bulb crop in

New York, Oregon, and Michigan.

In California and Louisiana losses

in the seed crop are sometimes

heavy during the winter and spring,
AGRICULTURE HANDBOOK NO. 2 08, U.S. DEPT. OF AGRICULTURE

since the foggy weather is especially favorable to the mildew. It

attacks the .seed stems and causes

them to fall over before the seed is

mature. Losses also sometimes

occur in Oregon and Idaho.

Characteristics

Tlie first symptoms of downy

mildew are found most readily by

examining leaves closely in the

early morning wliile the clew is still

present. A violet furry growth on

the surface of the leaf or seed stem

is characteristic of the disease (fig.

3). Atïected leaves gradually become pale green and later yellow,

and the diseased parts collapse.

The furry fungus growth becomes

more widespread if the humidity

remains high, but the advance and

spread are dependent on favorable

weather.

The disease commonly starts in

spots in the field and spreads to

surroimding areas. If the weather

remains dry after an outbreak,

plants send out new leaves and partially recover, but on the return of

humid conditions the fungus revives and the new growth becomes

blighted. Repeated killing of

leaves in this manner reduces

growth. The bulbs remain small,

and the necks are unduly succulent

and subject to attack by storagedecay organisms, as shown in figure

3. Lesions on seed stems are circular or elongate, often affecting

only one side of the stalk. They

weaken the stalk so that it breaks

over from the weight of the seed

umbel, thereby causing the seed to

shrivel. The ftingus may also infect the flower parts. There is circumstantial evidenc« that it is

carried with the seed.

Causal organism

The causal organism (Peronospora destructor (Berk.) Casp.) is

closely related to the ftmgi associated with potato late blight, grape

FIGURE 3.—Onion plant affected with

downy mildew. The furry growth on

the dying, older leaves consists of

fruiting branches of the causal fungus

and uiany thin-walled, light, microscopic si)ores, which are readily disseminated by the wind.

downy mildew, and cucumber

downy mildew. The onion fungus

is confined largely to onion, Welsh

onion, and chives. Garlic and leek

have been reported as hosts, but it is

not certain that they are subject to
ONION DISEASES AND THEIR CONTROL 9

infection by the same strain of the

fungus as tliat which affects the

onion. The furry masses (fig. 4)

on the affected parts of the plant

are made up of brandies of the

fungus tliat extend above tlie surface and bear numerous microscopic

spores. Tliese spores are thinwalled and light, and are easily

if]

FIGURE 4.—Violet lesions on older leaves.

Downy mildew affected leaves become

pale green and yellow.

detached and transported by air

currents—the chief means wliereby

the fungus is disseminated locally.

They are short-lived, however, and

very sensitive to drying. If moist

weather, favorable to infection,

does not prevail, most of them die

witliout causing infection.

When damp weather prevails,

tlie spores germinate promptly. If

they germinate on the leaf surface,

the germ tube growing into a

threacl enters through a breathing

pore (stomate) into the internal

tissue. As the fungus absorbs food

from the onion, the cells of the

latter gradually succumb; as a result, the leaves turn yellow, shrivel,

and die. Tlie fungus sends fruiting branches to the surface, where

they produce more spores.

Another type of spore, the resting spore (oospore), is formed in

the dying plant tissues. Because

tliis spore has a thick wall and a

concentrated food supply, it is

suited to withstand an unfavorable

environment. The internal threads

(mycelium) also survive for long

periods, especially when they get

into the bulb before it matures.

Oospores and mycelium may be

carried with the seed, but there is

little evidence that seed is an important carrier of the organism. Thus,

the fungus lives over unfavorable

periods chiefly as oospores or as

mycelium in bulbs and sets and may

be transported long distances in

this way. In some places, as in New

York, the perennial Egyptian winter onion in home gardens serves

as an important overwintering

source of the fungus.

Control

Although many other downy

mildews, such as grape downy mildew and potato late blight, are

successfully controlled with sprays

containing basic coppers or Bordeaux mixture, this treatment has

not been effective against the onion
10 AGRICULTURE HANDBOOK NO. 208, U.S. DEPT. OF AGRICULTURE

downy mildew. One reason is tlie

great difficulty in obtaining

tliorougli coverage of the waxy

onion foliage. A second reason is

the continual emergence of new

leaves from the necli, necessitating

frequent spray applications for

complete protection. Wlien used

with a spreader-sticker in a 5-day

spray schedule, maneb, nabam plus

zinc sulfate, or zineb has given

satisfactory control of downy mildew of onion.

Tip and Leaf Blight

A dying bacli of the tips of older

onion leaves occurs during periods

of heavy precipitation, high humidity, and cool cloudy weatTier in the

Rio Grande Valley and in muck

areas of ííorthern States. The

affected parts turn tan to light

brown and die. This browning

may be limited to a tipbum or it

may extend down the leaves to cause

a destructive blight. Sometimes

separate grayish-white lesions are

scattered over the leaves. This

disease is caused by one of the fungi

associated with Botrytis neck rot

(page 21). It lives over in the soil

and on plant refuse as small black

bodies (sclerotia) which in cool

moist weather give rise to airborne

spores. They germinate and penetrate leaf tips. This disease has

been held in check by the sprays

used for mildew.

Leaf Mold

In midseason or later when the

leaves are dying back from the tip

because of mildew, pui-ple blotch,

or tip blight, a prominent black

mold commonly appears on the

dead tissues. This condition is

brought about by a secondary fungus, Btemphylium hotryomm

Wallr., which is a low-grade parasite but grows profusely on dying

tissue and produces a mass of black

spores. It may also appear on se«d

stems on lesions produced by other

organisms. Sometimes the spores

of Btemphyl'mra are so abundant

they form a black mold on the surface, which obscures the spores of

one or another of the other fungi

that may be the real cause of the

diseased condition.

Purple Blotch

Purple blotch is a disease of

leaves, seed stems, and bulbs. It

has been confused with leaf mold,

because the leaf mold organism

commonly follows the purple blotch

fungus as a secondary invader.

However, unlike the leaf mold organism, the purple blotch fungus is

capable of invacling the onion plant

quite independently of any forerunner. The disease is of some importance on the seed crop and

sometimes causes appreciable losses

as a bulb rot. The causal organism appears to be rather widely

distributed.

Characteristics

The disease appears first as small,

whitish, sunken lesions with purple

centers that rapidly enlarge and

eventually girdle the leaf or seed

stem. About 2 or 3 weeks after its

first appearance, darkened zones,

consisting of superficial masses of

fungus spores, appear on the lesion.

as shown in figures 5 and 6. Usually the affected leaves or stems fall

over and die within 3 or 4 weeks

after the lesions appear, if the environment is favorable for disease

development. Purple blotch requires less humid weather than

downy mildew : thus, it is much less

limited in geographic distribution.

The bulbs are attacked at harvesttime. The fungus invades most

commonly through the ne<-k, but it

may invade wounds on the fleshy

scales. The decay is at first semiwatery and is especially conspicuous

because of the color associated with

it. The parasite secretes in abun-
ONION DISEASES AND THEIR CONTROL 11

comes dark brown to black. As the

tissue becomes desiccated, the diseased scales eventually dry down to

a papery texture. Often only one

or two outer scales are aiïected; in

otlier instances, notably witli white

bottom sets, the entire bulb may be

destroyed.

FIGURE 5.—Conspicuous purple blotch on

neck area of plant seen as a wine-red,

slightly sunken lesion.

dance a pigment that diffuses

through the scale tissues somewhat

in advanc« of the fungns threads.

The affected tissue is deep yellow at

first, but it turns gradually to a wine

red. With the profuse development

of dark-colored fungus threads, the

older decayed tissue eventually beFiGUEE 6.—Purple blotch stem lesion

showing masses of fungus spores and

light and dark zonation.
12 AGRICITLTURE HANDBOOK NO. 2 08, U.S. DEPT. OF AGRICULTURE

Causal organism

The causal fungus of purple

blotch {Alterna rla porn (Ell.)

Cif.) is closely related to that associated with early blight of potato

and tomato. However, it is confined to onion and possibly such

closely related plants as leek and

chive. The spores, which are produced on short fruiting branches on

the surface of the decayed leaves,

stems, and scales, are many-celled,

dark-colored, and capiable of withstanding unfavorable environment :

thus, they carry the fungus through

northern winters or hot southern

summers. They germinate to produce fungus threads (mycelium)

that invade the stomates of leaves

and stems and wounds on the bulbs.

Control

No satisfactory measures for the

control of the disease on leaves and

stems have been devised. The practices described for the control of

Botrytis neck rot ( p. '22) are recommended for the bulb rot.

Pink Root

Pink root of onions is serious in

the Delta district of the lower Sacramento and San Joaquin Valleys

of California, in the Rio Grande

Valley of Texas, and in muck districts of New York, Ohio, Indiana,

and Michigan. It occurs also on

upland soils in Idaho, Iowa, New

lork, Michigan, and Wisconsin.

Affected onion roots turn yellow,

shrivel, and die, meanwhile "taking

on a distinctly pink color. As the

plant sends out new roots, they in

turn eventually become diseased

and functionless ( fig. T). This procedure continues throughout the

growing season, and although the

affected plants are not commonly

Irilled by the disea.se, the reduced

food supply results in the formation

of mere scallions, or small bulbs.

During the growing season there are

often few outward symptoms of the

disease. It becomes most apparent

at harvesttime in the small size of

tiie bulbs; the size varies with the

severity of the attack. The disease

is confined to the roots and not to

the bulb.

The fungus that causes pink root

(Pyrenochaeta fet-resfrií (Hansen)

Gorenz, .1. C. Walker, and Larson )

lives and multiplies in the soil.

Conseciuently, the longer the onions

are grown in the same field the more

destructive the disease becomes. It

is disseminated on diseased green

or bottom sets, on tools, and by natural agencies, such as surface

drainage water. It attacks all

varieties of onions, as well as shallot

v^'M \M

FIGURE 7.—f)nion seed bulb affected

with pink root. This soil-borne fungus

causes the roots to turn pink, shrivel,

and die.
ONION DISEASES AND THEIE CONTROL 13

and garlic, and a large number of

unrelated economic plants.

Scientists in tlie U.S. Department

of Agriculture and in several State

agricultural experiment stations

have developed varieties of onions

that are resistant to pink root.

Several are now available for use

in winter onion-growing areas in

the South. They include Excel, L36, and Granex, which are of the

Yellow Bermuda type; and Eclipse,

1^365, Early Crystal 281, and

White Granex, of the Crystal Wax

type.

Fusarium Basal Rot

' '{

Fusarium basal bulb rot is a widespread disease, but it seldom causes

severe losses. However, it sometimes does reach serious proportions

in the irrigated valleys of the Rocky

Mountains.

A wilting and rapid dying back

of leaves from the tips as the plants

approach maturity is the first evidence of trouble (fig. 8). Most of

the roots eventually rot ort', and in

their place a mass of white moldy

growth is produced. The bulbs become soft, and when cut a semiwatery decay is found advancing from

the base of the scales upward. The

rot progresses somewhat slowly,

and many beginning infections are

unnoticed at harvesttime. Thus,

the disease becomes a factor in

transit and in storage, where the

decay may continue until the bulbs

are entirely destroyed (figs. 9 and

10).

The causal fungi (Fu.iarlum

spp.) live in the soil. Infection of

the plants is sometimes correlated

with maggot injury. Invasion occurs througli wounds or in the vicinity of old root scars at tlie base

of the bulb. The disease is favored

by high soil temperatures, which

is probably the chief reason for its

appearance in the Northern States

after midseason and for its more

common destructiveness in the westFiGUEE 8.—Onion plant affected with

FiLsarium basal rot. This rot begins

at the ba.se of the scales and causes

the leaves to die back from the tii«.

ern valleys, where high temperatures are not unusual.

Careful sorting at harvest is recommended. Rotation is necessary
14 AGRICITLTURE HANDBOOK NO. 208, U.S. DEPT. OF AGRICULTURE

y,j

FIGURE 9.—Base of mature onion bulb

with white moldy growth of Fusarium

basal rot.

when this disease becomes troublesome.

Rusts

Two distinct rusts have been

noted in North America upon the

the Egj'ptian onion. One, caused

by the fungus Pu-ccinia pom

(Sow.) Wint., has been found in

several areas in Connecticut. It

appears in midseason on the leaves

and seed stems as subcircular or

elongated spots that split lengthwise and expose dusty orange-yellow spore masses.

The other rust is caused by the

common asparagus rust fungus

{Puccinia a-spamgi DC), which

occasionally goes over to the Egyptian onion when the latter is grown

close to an infected asparagus

patch. The disease appears before

midseason on leaves and seed stems

as light-yellow, roughly circular

to oblong lesions, in each of which

are numerous spore cups, or aecia,

that eventually split the skin and

expose the golden-yellow rust

spores. A secondary disease is

caused by a species' of Botrytis,

similar to or identical with one of

those causing neck rot of onion

(p. 21), which commonly invades

the seed stem tlirough the asparagus rust lesions and causes girdling and lodging of the stem.

Tlie first-named rust is apparently not of serious economic importance. The last-named one may

be avoided by planting Egyptian

onions at a considerable distance

from asparagus.

White Rot

^\liite rot, a serious fungus disease on onion, garlic, shallot, and

leek, has l>een reported in northeastern Oregon : near Xorfoik, Va. ;

near Louisville, Ky. ; and in Louisiana. It is widespread in Europe,

where it is destructive on onion and

leek in tlie British Isles and on

garlic in Italy and Spain.

Usually, white rot appears first

during the cool, moist weather in

the spring and fall. The first signs

are yellowing and wilting of the

leaves, followed by a total collapse

FIGURE 10.—Semiwatery decay of the

base of mature bulb affected with Fusarium basal rot.
ONION DISEASES AND THEIR CONTROL 15

of the top. The causal fungus

(Sclerotium cepivoriim Berk.),

which inhabits tlie soil, actually invades the roots and tlie basal parts

of tlie bulb scales (fig. 11). If

diseased plants are pulled gently,

FIGURE 11.—White rot on an onion plant

grown in infested soil. Note that the

entire root system has been destroyed

and that the bases of the scales have

also been affected. The small, spherical, black, kernellike masses, or sclerotia, which develop upon the diseased

tissue are characteristic.

they will come up readily, because

the roots have been almost entirely

destroyed. The diseased bulb is

commonly covered with a white

fluffy mass of fungus threads.

Somewhat later this mass takes on

the appearance of a closely fitting

weft in which are embedded numerous black spherical bodies, or sclerotia, about the size of poppy seeds.

The fungus continues to invade the

bulb until it becomes shrunken and

dried up.

In the early stages white rot

might easilj' be confused with

fusarium basal rot, but in the

fusarium rot no black sclerotia are

ever formed. In its later stages

white rot may be mistaken for neck

rot, in which black sclerotia are commonly formed. The neck rot sclerotia, however, are several times as

large as tliose of the white rot

fungus. Moreover, a gray mold is

commonly associated with neck rot,

w'hereas the mold associated with

white rot is distinctly white. Neck

rot almost always appears after harvest and generally starts at the neck.

AVhite rot commonly appears on the

growing crop, with the infection in

the roots and bases of the scales.

The fungus that causes whit« rot

is disseminated chiefly by bulbs that

have come from diseased soil. Once

established, the fungus may persist

in the soil indefinitely and is known

to survive northern winters. Bulbs

from a diseased field should never

be used for propagation.

In Europe the disease is controlled by coating onion seed with

calomel before planting. In the

southern United States where the

disease is severe only on set-propagated shallots, the only economical

control devised so far is early planting of tlie crop.

Yellow D\varf

Yellow dwarf was first recognized in a destructive form in 1928

in the Pleasant Valley district of
16 AGRICULTURE HANDBOOK NO. 208, U.S. DEPT. OF AGRICULTURE

Iowa, where it is said to have caused

a ¿.5-percent loss. It occurred to

some extent in tliat district in 1927

and since then has been found in

a few other States. It is most

serious on shallots, on crops grown

from onion sets, and on seed

plants. The outstanding symptoms

are severe stunting of the plants

and in seed plants, dwarfing and

twisting of the seed stems. The

atfected leaves and stems change

from their normal green to various

degrees of yellow, extending from

a few streaks to almost complete

yellow. Leaves tend to flatten and

are crinkled, and, as a result, bend

over. The crinkled and drooping

habit gives the plant a dwarfecl appearance, although the leaves are

not shortened to any great extent

(fig. 12).

Yellow dwarf is a virus disease,

which means that it is transmissible

and infectious, but the contagious

entity, or virus, is ultramicroscopic

and not a bacterium or fungus. The

virus persists in bulbs of infected

plants. "When such bulbs or sets

are planted, the plants are diseased

the next year and contain the virus

throughout the leaves. The virus

is then carried to surroimding

healthy plants by plant lice

(aphicls), and thus the contagion

may be spread rapidly during'the

growing season. Yellow dwarf is

not serious where the crop is grown

from seed only, since the virus is

not carried in the seed. Wliere

bulbs are carried over as sets, or for

the propagation of seed in proximity to onions from seed, there is

an opportunity for continual carryover and propagation of the virus.

It is under such conditions that

yellow dwarf has become very

serious.

Onion sets and bulbs to be used

for seed production shoidd not be

grown near a bulb crop grown from

sets of the previous year or near a

seed crop. In this way carryover

FIGURE 12.—Onion set plant affected with

yellow dwarf. This virus causes the

leaves to crinkle and to become flattened. The affected leaves show

longitudinal j-ellow streaks extending

the length of the leaves and flower

stalks.

of the virus is limited. Onion sets

can be indexed in the greenhouse

during the winter months for the

presence of the yellow dwarf virus.

It is now recognized that there

are several strains of the yellow

dwarf virus, and that they may differ in their virulence on onion and

related plants. The Sweet Spanish

variety of onion is highly resistant

to the commonly occurring strain.
ONION DISEASES AND THEIR CONTROL 17

Aster YelloAvs

Aster yellows is a virus disease

transmitted by various leaflioppers.

This virus affects many wild and

cultivated plants, including such

economically important ones as carrot, lettuce, celery, and aster. "Wlien

leaflioppers carrying the virus feed

upon first-year onion plants, yellowing starts on the basal portion of

the youngest leaves and spreads upward. When infected bulbs are

used in seed production, the leaves

are yellow and tlie flower umbel

takes on an abnormal appearance.

The pedicels, to which the flowers

are attached, become much longer

than normal, and the flower is distorted and usually' sterile, with a

corresponding seed loss (fig. 13).

FicuEE 13.—Aster yellows affecting the

flower cluster of an onion plant (left).

In this diseased cluster many of the

pedicels, to which the flowers are attached, are abnormally long (center),

and the parts of such flowers are distorted. In the cluster at the right the

flowers are developing and opening

normally.

This disease has been noted in California, Idaho, and Wisconsin and

will probably be found elsewhere,

since the aster yellows virus is widespread. Whether the damage that

aster yellows causes will raise it to

a major disease in some sections remains to be determined. In any

case, it is well to avoid planting

onion seed fields in areas adjacent

to crops susceptible to aster yellows

such as carrot and lettuce.

Dodder Injury

Dodder (Cuscuta sp.) belongs to

the higher plants and differs from

the previously mentioned parasites

in that it produces true flowers and

true seeds. The seeds live over winter in the soil and germinate in

the spring, as do the onion seedlings.

The young dodder plant has no

leaves, and its tendrillike stem soon

winds around the onion leaves and

forms suckers, or haustoria, which

invade the host tissue (fig. 14).

After having thus become established as a parasite, the dodder gets

most of its food from the onion and

grackially sends out its tendrils to

attack plants nearby. It finally

kills the onion leaves prematurely

and thus prevents normal bulb development.

The dodder continues to spread

from original centers, and by the

end of the season there may be

i-oughly circular areas of considerable size in which the onion tops

have been completely killed. It is

not confined to the onion, but may

attack a wide range of plants.

Laborers commonly pick parts of

dodder plants out of curiosity, carry

them a short distance, and drop

them. The parasite may then take

new root and start a new center of

infection. Therefore, when dodder

first appears in the field it should be

carefully removed, along with affected plants, and burned. Dissemi-
18 AGRICULTURE HANDBOOK NO. 2 08, U.S. DEPT. OF AGRICULTURE

iiixtion by laborers or by tools should

be avoided, and the dodder plants

should never be allowed to go to

seed. Dodder injury has been noted

on onions in California, Washington, Illinois, and Wisconsin, and,

doubtless, it is likely to occur in any

onion district.

;

FiaxjsE 14.—Onion plants attacked by dodder, a parasitic flowering plant. This

leafless parasite obtains its food by sending suckers into onion leaves and eventually killing them.
ONION DISEASES AND THEIR CONTROL 19

FARM AND HANDLING PRACTICES RELATED

TO STORAGE DISEASES

The interval of several weeks between harvest and storage or shipment of onions is a critical one in

relation to the development of diseases in the warehouse or in transit.

The care taken with the crop at this

period may mean the difference between success and failure in carrying it through storage or in placing

it on the market in good condition.

The plant has practically terminated its growth and, on going into

its dormant state, becomes more susceptible to the attack of storage rot

fungi and bacteria, which are continually present in the soil and on

dead refuse.

While becoming dormant, the

bulbs must be allowed to sweat, or

cure, preparatory to storage or shipment. For this purpose they are

ordinarily placed in crates and

stacked in the field or in open sheds,

where the sunshine and air currents

aid greatly in removing the moisture that is given off. Thus, the

growers have found that if the

weather remains clear and dry during harvesting and curing, the crop

will go through storage with small

losses due to decay. However, if

the prevailing weather is rainy,

losses will almost invariably be

heavy. The moisture is favorable

for the development of fungi and

bacteria and at the same time delays

harvest and prevents the proper maturing of the crop. Under such

conditions certain storage diseases,

such as Botrytis neck rot and bacterial soft rot, make considerable

progress before the bulbs are pulled.

On the other hand, high humidity

during the curing period causes the

moisture given off by the onions to

accumulate in the crates; such accumulation favors decay.

The control of storage diseases,

therefore, will consist largely of attention to cultural methods based on

the knowledge of these general facts.

In view of this, the following specific recommendations are made

with regard to the handling of the

onion crop.

Sanitation

The organisms causing storage

rots in general thrive on dead vegetable matter. Onion tops and diseased bulbs left on the field and

onion refuse from the warehouse

furnish excellent opportunities for

these fungi and bacteria to multiply. The spores of certain of these

organisms, especially those causing

neck rot, are readily carried long

distances by the wind. A pile of

rotting onions near the warehouse

may thus be a means of infecting

a crop a considerable distance away.

All onion refuse left on the field

should be raked and burned after

harvest, or the field should be

plowed promptly. Waste from the

warehouse should be dumped in a

remote place, or if spread on the

fields it should be confined to those

not used for growing onions.

Harvesting

As soon as the neck of the onion

bulb loses its stiffness sufficiently to

allow the top to drop over readily,

the onion is ready to be han'ested.

It is best to allow the tops to dry

out as much as circumstances will

permit before cutting or twisting

them, since this drying will help to

reduce the trouble from storage

rots. If the field matures unevenly,

it is well to start pulling the plants
20 AGRICULTURE HANDBOOK NO. 2 08, U.S. DEPT. OF AGRICULTURE

when most of them have reached

this stage. An unusual amount of

rainy weather just l»fore harvest

may postpone ripening and tend to

cause an overproduction of scullions, or stiiï necks. They should

not l)e placed in storage, but should

be sorted out and sold for immediate

consumption. Cutting tlie roots

with a wheel hoe will tend to liasten

the ripening of the tops. In clipping or twisting the tops, a neck

1 or 2 inclies long should be left to

avoid the exposure of the succulent

tissue of tlie fleshy scales of the

bulb. Care should be taken not to

bruise the bulbs and thus open the

way for the organisms that cause

decay. Milling of sets before storage causes a certain amount of

bruising and lowers the keeping

quality.

Removal ofDiseased Bulbs

It is essential tliat car'e be taken

at harvesttime to throw out all bulbs

that show any signs of disease or

insect injury. Although smut and

downy mildew fungi do not themselves cause decay, bulbs that have

been attacked by these fungi are

thereby made more susceptible to

the invasion of storage-rot organisms. Fusarium basal rot, on the

other hand, gains a start, in the field

and continues to injure the bulbs

in storage. In unusually damp

weather bacterial soft rot and neck

rot may start in the field, and it is

well to be on the watch for bulbs

with softened necks at harvesttime.

Curing and Storing

In northern sections where the

crop is likely to encounter moist

weather during the harvest period,

thorough and prompt curing of the

bulbs is extremely important.

Since onions in this area are commonly held in storage for distribution during the winter months, wellinsulated storage houses are essential. Modern storages are also provided with a forced ventilating system wherein air is introduced at

frecjuent intervals beneath bins in

which bulte are usually stored in

bulk. Provision is made whereby

air can be heated, if necessary.

With this system, the bulbs are

usually put into storage directly

from harvest and air circulation is

begun at once. The circulation is

continued for several days until the

outer scales and the neck tissue are

thoroughly dry. If this curing procedure is carried out promptly and

thoroughly, incipient infections of

neck rot and bacterial soft rot are

permanently checked and progress

of smudge on white varieties is

greatly reduced. Once the onions

are well cured, only infrequent air

circulation is necessary to maintain

proper temperature and humidity.

The )3est storage temperature is

from 32° to 3.5° F. and the best relative humidity is from TO to 75

percent.

^Yhen the facilities described

above are not available, the curing

is best carried out in slatted crates

tliat are piled in open sheds where

they are protected from rain but

arranged so as to take advantage

of drying air currents. Bulbs so

cured should not be stored in bulk

but should remain in crates so that

ventilation of the warehouse by

natural air currents is most effective. The warehouse is usually provided with trapdoors at frequent

intervals along the bases of the

sidewalls. The trapdoors may be

opened during periods of low outdoor humidity, so as to create air

currents that rise in the warehouse

and carry humid air out through

vents in the roof. Onion sets are

generally stored in this manner.
ONION DISEASES AND THEIR CONTROL 21

RELATION OF VARIETIES TO STORAGE

DISEASES

In the Northern States and on the

Pacific coast, where globe onions

are grown most extensively, colored

varieties are much less susceptible

to decay in storage and transit.

White varieties, on the other hand,

are very subject to storage diseases,

especially neck rot and smudge, and

handling them successfully requires

much more care during harvest and

curing. In the onion-set-growing

sections the same is true of the

White Portugal, as compared with

the Red Wethersfield and Yellow

Globe Danvers. Certain white varieties, such as Queen, Pearl, and

Barletta, are such poor keepers that

they are seldom held in storage for

any length of time.

In the onion districts of Texas,

southern California, and Louisiana

the Bermuda varieties are largely

dis)5osed of soon after harvest because of their poor keeping quality

and the lack of cold-storage facilities. In Louisiana the Red Creole

variety is a favorite, because it resists the attacks of fungi and bacteria in storage and in transit much

more effectively than tlie white and

yellow Bermuda varieties.

DISEASES PRIMARILY IMPORTANT IN

STORAGE AND TRANSIT

Botrytis Neck Rot

Botrytis neck rot is a destructive

and widespread disease of onions in

storage and in transit. Luring certain seasons many growers have lost

50 percent or more of their crop on

account of this disease. White varieties are especially susceptible, but

considerable loss is often sustained

with red and yellow varieties.

Characteristics

Usually thei'e is little or no evidence of Botrytis neck rot up to or

at the time of harvest, but after the

onions are topped and have laid in

crates for a few days the early signs

appear. The scales begin to soften

usually at the neck and more rarely

at the base or at a wound.

A definite margin appears between the healthy and diseased tissue, the latter taking on a sunken,

water-soaked appearance. A gray,

feltlike growth later forms on the

rotting scales, which may be accompaniecl by a gray to brownish mold

consisting of the spores (seeds) of

the causal fungus, and by brown to

bhick kernellike bodies (sclerotia)

one-eighth to one-fourth of an inch

in diameter (figs. 15 and 16). On

red and yellow onions the pigment

of the diseased parts is destroyed;

in the former the rotted tissues

sometimes assume a pinkish tint.

The disease may progress rather

slowly unless conditions are moist,

and several months often elapse before the entire bulb is destroyed.

Causal organisms

Botrytis neck rot is caused by one

or more species of fungi (Botrytis

spp.) closely related to the common

gray molds that attack lettuce, cabbage, and numerous other vegetables. These fungi are not vigorous

parasites and seldom seriously at-
22 AGRICULTURE HANDBOOK NO. 208, U.S. DEPT. OF AGRICULTURE

FIGURE l.j.—Onion bulb affected with

Botrytis neck rot, .showing softening

and shriveling of the scales which begin at the neck of the bulb. Note the

black kernellike masses, or selerotia.

on the surface.

tack actively growing onion plants.

Ordinarily they do not penetrate

the dry outer scale of tlie onion, for

they enter the plant tissues only

through wounds. The gray to

brown moldy growth on the rotted

scales consists chiefly of the spores

of the causal fungi, which are especially adapted to dissemination by

air currents. They are thus carried

to the healthy bulbs, where they

germinate and' send fungus threads

into the necks, either through the

dead tops or through wounds left

by the removal of the tops. These

threads then kill the tissue sliglitly

in advance of their progress

through the scale. The black selerotia are compact masses of fungus

threads, which, being resistant to

cold and drought, carry the organisms overwinter.

Control

As already pointed out, the handling of the crop at harvest is

particularly important in northern

sections of the United States where

rainy periods are likely to occur.

Botrytis neck rot is of little importance in tlie western areas where

the crop noi-mally matures in continuously dry climate. Since infection by the neck rot fungi takes

place in the field as the plant

matures, a large percentage of the

bulbs may be infected "without

showing signs of disease at harvest. Experiments have shown that

if the neck tissue is dried out

promptly at that time, the fungus

is permanently checked and no rot

will appear. This is one of the reasons why curing of bulbs by forced

ventilation in the warehouse has

proved to be so effective, and so

much more reliable than natural

curing. Since bulbs of white varieties are very susceptible to neck

rot, they should not be stored in

northern sections without artificial

Bacterial Soft Rot

Bacterial soft rot usually starts

at the neck of the bulb, and progresses down one or more scales.

FIGURE 16.—Longitudinal section of an

onion bulb affected with Botrytis neck

rot.
ONION DISEASES AND THEIR CONTROL 23

Sometimes bulbs in the field are

slightly affected before harvest.

At first the tissue is water-soaked ;

later, it disintegrates into a soft,

slimy mass. An offensive sulfurons odor is usually given off.

This characteristic distinguishes

bacterial soft rot from other bulb

rots. The decay does not spread

readily from scale to scale. It may

start at bruises or wounds on the

scales and commonly develops in

tissue injured by freezing. When

the rot affects only one oi' two inner

scales, as it often does, the only

external sign of the disease is the

lack of firmness of the bulb and

often the appearance of a watery

exúdate through the neck when

pressure is applied to the upper

part of the surface of the bulb.

Soft rot differs from neck rot in

tliat it is caused by a bacterial

organism (Erminia carotovora (L.

R. Jones) Holland) rather than a

fungus. The organism enters only

through wounds and in moist environment. Onion maggots (Hylemya antigua Knight) may carry

the bacteria and througli their

feeding on bulbs provide the necessary wounds for infection. The

bacteria may persist in the intestinal tract of the larvae and the

adult fly and thus be carried from

place to place. The eggs may be

contaminated when laid by the fly,

and thus the soft rot bacteria become identified closely with the life

history of the maggot, depending

upon the latter for its perpetuation

and dissemination.

The precavitionary measures already recommended regarding harvesting, curing, and storing should

be followed carefully.

Black Mold

Black mold occurs to some extent

in northern onion-growing districts,

but it is of slight economic importance there. In Louisiana, Texas,

and California, liowever, it is one

of the most economically important

diseases during storage and transit.

Because of its resemblance in appearance to onion smut, black mold

is often confused with it by growere

and dealers. It is chiefly characterized by the black powdery masses

of spores that are borne on the

exterior of the scales and can be

rubbed off' readily (fig. 17). It is

true that the disease is not confined

to the exterior of the bulb, but inasmuch as the inner scales are separated, the black powder will be

found on the exterior of the individual scales. Onion smut, on the

other hand, as seen in storage or

market, is characterized by oblong

or linear black lesions, found most

commonly near the base of the bulb

and as deep as the third or fourth

scale.

Black mold causes a slow shriveling of the affected scale, which

assimies a brittle texture. Moist

conditions favor the disease, while

a cool, dry environment seems to

check it. All varieties are susceptible, but Australian Brown is

V.

FIGURE 17.—Onion bulb affected with

black mold, showing irregular sooty

masses on the outside of the scales or

between them. The outer scales shrivel

about the neck of the bulb and assume

a brittle, papery texture. Compare

with onion smut.
24 AGRICULTURE HANDBOOK NO. 208, U.S. DEPT. OF AGRICULTURE

usually the most commonly and

seriously affected variety.

The causal organ-ism {Aspergilluis nigei' van Tiegh.) is a very common saprophyte living on almost

any dead or dying vegetable

matter. Where it is most serious

on onions, it undoubtedly grows

and multiplies tliroughout the year

in the soil or on dead refuse. It

is present to a slight extent on the

dead outer scales of the bulbs before harvest, but it is not noticeable

until tlie onions are pulled. Rainy

weather at this critical period will

result in widespread infection,

wliicli continues to develop in storage and transit.

To hold black mold in check,

general sanitary measures and protection from moisture after harvest

are essential. Bulbs should be

thoroughly dry before being

shipped, since moisture in the pack

favors rapid development of the

disease in transit. Dealers in

nortliern markets receiving infected

lots to be held for any considerable

length of time before consumption

should transfer them to cold storage in order to hold the disease in

check.

Smudge

Onion smudge is confined largely

to white varieties. It appears in

the field just before harvesttime and

continues to develop during the

storage period. It is characterized

by small, dark-green to black dots,

which appear on the outer scales.

These small dots may be grouped

together in various ways, often in

concentric rings, giving a smudgy,

unsightly appearance to the white

bulbs (fig. 18 and front cover).

The fungus ordinarily attacks the

fleshy scales only mildly and in such

cases does not cause any appreciable

shrinkage in storage. The chief

damage is the reduction of the

market cjuality of the crop. However, after rainy weather during

harvest, if the bulbs are crated and

stored without being dried and

cured thoroughly, the disease causes

considerable loss.

The causal fungus {CoIIetotnohum.- cirrlnans (Berk.) Vogl.)

lives over between seasons on onion

scales in the soil or on bulbs in the

warehouse, and, consequently, it

increases in amount where onions

are grown in the same fields year

after year. It is widely distributed

through the trade on white onion

sets and by this means is introduced into soil new to onions. Under favorable conditions the fungus

attacks the outer scales and forms

many small black dots made up of

myriads of minute spores (fig. 18

FiGUKE 18.—Onion bulb affected with

smudge. Note the blaclî smudgy spots

made up of small black dots on the

outer scales.
ONION DISEASES AND THEIE CONTROL 25

and front cover). These spores

may be carried away in drops of

water to other onion scales, wliere

they germinate within a few hours

and renew tlieir attack. Tlie fungus passes througli tlris entire life

cycle within a few days wlien tlie

weather is warm and moist. A

slight infestation in tlie field before

harvest will furnish spores enough

to spot the bulbs badly if a few

days of moist weather sliould come

during harvest or while the crates

are stacked in tlie field.

Susceptibility to this disease and

to neck rot has caused a great reduction in the acreage of the white

varieties in northern districts.

Special care at harvesttime is necessary in handling them. The

crop must be harvested promptly,

exposure to rain sliould be avoided

if possible, and curing should be

rapid and thorough.

Diplodia Rot

So far, diplodia rot has been

found only on Texas-grown CiTStal Wax onions. Colored onions

from the same section do not show

the disease, and there is reason to

believe that they are highly resistant. The disease causes a silverygray to black discoloration of the

diy outer scales, usually on the

upper half of the bulb. Parts of

fleshy scales that are di-ying down

may be affected and become black

and leathery. Firm, succulent

scales are not attacked. The discoloration is due mainly to the

threads of the causal fungus

{Diplodia natalensk P. Evans)

on the surface of the outer scales

and between them and the next

inner scale. Black pin-point bodies

(pycnidia), which contain the

spores of the fungus, appear later.

The damage is chiefly that of a

blemish. The fungus attacks other

crops, including- citrus fruits.

Aspergillus Rot

Aspergillus rot is a decay of garlic bulbs, which lias been found

twice on imported stock. The scales

are I'educed to a rather di-y powdery condition, and brown kernels

(sclerotia) two or three times the

size of the head of a pin occur

throughout the mass. The causal

fungus {A.^pergillus alliucensTYiova

and Church) commonly fruits

on the surface as a lemon-yellow

mold similar to black mold except

in color. The fungus, though not

found naturally on the onion, can

readily decay onion bulbs if it

gains entrance through wounds.

The sclerotia are somewhat larger

and browner than those of the

white rot organism and smaller

than those of the neck rot fungi.

Attention is called to its occurrence, since it is likely to be found

on the onion at some future time.

Tlie parasite works only at somewhat high summer temperatures

and thus will not cause a serious

rot in cold storage.

Blue Mold Rot

Blue mold rot occasionally affects

onion bulbs but more often those of

"arlic. Light-yellowish lesions first

appear on the fleshy scales and as

the decay progresses a fine white

surface mold becomes visible. The

formation of bluish-green spores in

mass on the surface mold soon follows, giving the characteristic blue

mold symptoms. This disease may

proceed within the cloves of garlic

and convert the tissue to a greenishtan or gray powdery mass without

external evidence of decay other

than the decided loss in weight of

the bulb.
26 AGRICULTURE HANDBOOK NO. 2 08, U.S. DEPT. OF AGRICULTURE

NONPARASITIC BLEMISHES AND MALADIES

Freezing Injury

Occasionally onions are subjected

to freezing temperatures. Growing

plants may be killed, if the temperature is low enough and lasts long

enough. The Bermuda and Swe«t

Spanish varieties are much more

susceptible than the Globe varieties.

The first-named varieties are also

more likely to show a larger percentage of doubles and multipliers

among the plants that survive freezing in the field.

Bulbs exposed to freezing temperatures in the field, in storage, or in

transit will show freezing injury.

The average freezing temperature

for onions of the globe type is about

30° F. It is possible to cool them

below the freezing point without injuring them. If tliey are handled

or moved in the undercooled condition, ice forms in the tissues and

damage sets in. Undercooled bulbs

sulïer less damage if they are

thawed out at 50° or above than if

thawed slowly at 32°.

Freezing injury can be determined by cutting the bulb longitudinally or transversely. The

affected tissue is water-soaked, discolored, and more or less transparent, with indefinitely scattered

opaque areas. Freezing injuiw is

easily confused with physiological

breakdown. It is not uncommon to

find some scales injured and others

not, because the freezing points of

the tissues vary. Thus the outer

scale may be injured, the next one

sound, the third injured, the next

sound, and so on. Soft rot commonly follows the freezing injury.

If the latter is confined to the outer

scales, the bulbs can usually be salvaged by being spread in thin layers

and allowed to dry out, or subjected

to artificial curing.

Sunscald

When onions are harvested under

conditions of high temperature and

very bright sunlight, the tissue exposed to the sun may be scalded.

Immature uncured Inilbs are more

subject to injury, since the succulent tissue is more likely to be exposed. The damage tissue, usually

in areas up to 2 inches in diameter,

appears bleached and becomes soft

and slippery. This tissue loses

water rapidly and when it dries

down becomes a disfiguring blemish.

However, decay-producing organisms, principally the bacterial soft

rot organism, commonly gain entrance while the injured tissue is

still succulent and greater damage

follows. Sunscald can be prevented

in districts subject to the damage

by protecting the bulbs from direct

exposure to the sun during curing.

One method is to pull the onions

with tops intact ancl lay bunches in

windrows so that the tops of each

bunch will cover the bulbs of the

previous bunch.

Greening of bulbs by the formation of chlorophyll in the outer

fleshy scale may occur if onions are

allowed to cure too long in moderate

light. This sometimes causes an

unpalatable taste, but no shrinkage

or decay. It can be avoided by removing the stock to dark storage as

soon as suiRcient curing has proceeded in open sheds or outdoor

piles.

Physiological Breakdowns

Physiological breakdown of

onion, an injury not unlike freezing

injury in symptoms, sometimes develops after harvest in bulbs that

have not been exposed to low temperatures. The scale tissue is
ONION DISEASES AND THEIR CONTROL 27

discolored and water-soaked; and,

though more common on scales

near the surface, it may be found

on deeper ones. The nature and

cause of this injury are not known.

It may be found in the market and

in storage and may continue to develop during the storage period.

For this reason it is likely to be

confused Avith freezing injury, and

it is therefore advisable to know the

history of lots in question before

final diagnosis is made.

In garlic a physiological breakdown, commonly known as waxy

breakdown, sometimes occurs in the

outer cloves of garlic bulbs. Small,

slightly sunken, light-yellow areas

apipear in the clove tissue, turning

gradually to a deep-yellow amber

color throughout the clove. The

tissue thus becomes translucent and

sticky or waxy, but not soft. The

breakdown is usually obscured by

the unaffected, dry outer scale until

a late stage, when shrinkage and the

amber-brown color become visible

through the outer scale. The conditions which favor waxy breakdown are not known.

Chemical Injuries

Blemishes of market onions that

reduce their attractiveness are conimonly encountered, especially in

colored varieties. The color compounds in the scales are in the same

general class as the chemist's color

indicators, and therefore they respond accordingly to changes in

acidity or alkalinity. A number of

color blemishes are caused by contact with alkaline materials, one of

the most common of which is alkali

scorch, or bag print. Dark-brown

to black spots on yellow or red

onions occur at points at which the

bulbs come in contact with the jutebag containei-, particularly if the

bag has become moistened after being packed. Alkaline materials

infiltrated in some bags are responsible for this. They occur in some

lots of bags and not in others.

Another reaction of the same type

is sometimes encountered in cold

storage, where leakage of ammonia

fumes results in alkaline reaction

of moisture on the surface of

colored bulbs. Less than 1 percent

of ammonia in the storage-room

air will cause marked discoloration.

A similar effect has been noted on

stock covered with manure for protection from frost, in which case

ammonia fumes arise from the

manure. In all these cases the

strength of the material, the amount

of moisture on the bulb surface, and

the duration of the exposure determine the extent of the injury.

U.S. GOVERNMENT PRINTING OFFICE : 1961 O - 597985