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PEA

Diseases

AGRICULTURE HANDBOOK NO. 228

AGRICULTURAL RESEARCH SEI^VICE

U.S. DEPARTMENT OF AGRICULTURE
CONTENTS

Page

The jjcii industry 1

How pea diseases are disseminated 2

Insects 2

Infected seed 2

Drainage water 2

Refuse and stable manure 2

Farm animals and implements ;i

Wind 3

Control practices . 3

Using resistant varieties 3

Using disease-free seed 3

Seed treatment 4

Crop rotation 4

Diseases 5

AsGOchyta blight 5

Bacterial blight 7

Fusarium wilt 9

Near-wilt 13

Root rots 13

Root knot 17

Septoria blight 17

Powdery mildew 10

An thracnose 21

Downy mildew^ 21

Pea virus diseases 21

Nutritional disorders 27

Marsh spot 27

Seed cavitation 28

Molybdenum deficiency 28

Physiological spotting of seed 29

This haiulbook contains information of the type formerly

carried in Farmers' Bulletin No. 1735, Pea Diseases and

Their Control.

Washington, D.C. Issued October 1962
PEA DISEASES

By W. J. ZAUMEYEK, principal patlioloi/iat, Crops Research Division, Agriciiltural

Research Service

Peas are subject to a number of diseases, several of which may cause

serious injury or loss. Annual losses from diseases vary from year

to year, depending often on local weather conditions. If the soil is

wet from excessive rains and the weather is cool, heavy losses may be

expected from root rots and from such diseases as ascochyta and bacterial blights. If the soil is badly infested with root-rotting organisms, total loss may result. This bulletin describes the diseases briefly,

gives their causes, outlines known methods for their prevention, and

malses recommendations for their control.

THE PEA INDUSTRY

Peas are important in the diet of a large percentage of the population of the United States. They are grown to some extent in almost

every part of the country—for home use, for canning, for freezing,

for seed, and for market.

The processing of green peas for canning and for freezing is a

very important industry in a number of States. About 335,000 acres

were grown for this purpose in the United States in 1960. The States

growing the largest acreages were Wisconsin, Washington, Oregon,

Minnesota, Illinois, Idaho, and New York. Other States with smaller

acreages were California, Maryland, Delaware, Michigan, and

Pennsylvania.

The seed-growing pliase of the industry is highly specialized and is

centered chiefly in Washington and Idaho. States growing smaller

acreages of field peas for this purpose are Oregon, North Dakota,

Colorado, and Minnesota. The total acreage grown for seed in the

United States in 1960 was about 28.5,000 acres. Seed and field-pea

production often overlap, because peas grown for seed but not sold

for this purpose may be sold as dry field peas.

A comparatively small number of commercial companies produce

most of the seed, although a considerable number of farmers grow

the seed for them. Many of the dry field peas are used for split-pea

soup, and as seed for forage, green-manure, and cover crops.

Over 100,000 acres of green peas were grown for fresh market m

the United States 20 years ago, but at present this industry is of

minor importance. In 1960 about 6,000 acres were grown for this

purpose, with California growing 75 percent. Other States growing

small acreages were New York and Colorado.

1
HOW PEA DISEASES ARE DISSEMINATED

Tlie fungi and bacteria causing diseases of peas may be carried

f 1(1111 one place or plant to another by insects, infected seed, drainage

water, refuse and stable manure, farm animals and implements, and

wind.

Insects

There is no way to determine to what extent insects carry diseases

from one plant to another. Insects such as aphids or plant lice,

thrips, and leafhoppers are common inhabitants of plants and migrate

frequently from one to another. Wounds made by insects feeding on

the plant oft'er fa\-orable places for infection, insects \asiting the

lesions caused by fungi and bacteria may carry the genus on their

bodies and deposit them on noninfected plants and perhaps in the

wounds made by the insects; in this way, if conditions are favorable,

they start new infection centers.

Vims diseases such as the various pea mosaics and streak are transmitted by aphids from infected plants to healthy ones or from infected

legumes such as clovers to peas.

Infected Seed

Some disease-producing fungi and bacteria are carried by the pea

seed. If infected seed is planted, heavy los.ses may be expected if

weather conditions are favorable to the develoj^ment of the diseaseproducing organisms. If clean seed is planted, a crop can usually be

grown without much loss from diseases that are commonly seedbome.

Drainage Water

Surface drainage water running from one field to another may

spread disease germs to a field wliere peas have not been grown before. If seed is planted on high ground, gei-ms left in the soil may be

carried by the water to low-lying fields. In rotating their crops to

control diseases, growers may plant their seed in a low-lying field only

to find the disease already present in the soil.

Refuse and Stable Manure

Disposing of pea vines after the peas are shelled depends considerably on local conditions and farm needs. In some localities the vines

are returned to the soil after the peas are hulled. In other localities

they are stacked and used as feed and bedding for livestock. In still

other localities the vines are made into silage to be fed later to livestock.

If diseased \-ines are returned directly to the soil, the causal organisms will probalily infect the next crop of peas. Just what happens

to the disease germs on lines put into the silo is not definitely Icnown.

They are probably destroyed, and, if so, the vines made into silage will

not infect succeeding crops. On the other hand, returning those

vines to the field, using them for feed, or mixing them with stable

manure may spread the gei-ms to all parts of the farm where the

mixture is scattered.
In irrigated sections of the West where peas are grown commercially,

the threshed vines are sometimes used to dam irrigation ditches to

divert the flow of water into other ditches or to the fields. This

practice should be avoided, because any disease-producing organisms

on the vines will be carried by the water to all parts of the fields.

Farm Animals and Implements

Fai-m animals are potential agents for distributing disease germs

from one field to another. Some livestock movement is unavoidable in farm operations, but it sliould be limited when there has been

serious loss from diseases. Horses, cattle, or other livestock feeding on diseased refuse after the crop is harvested are likely to scatter

the disease-producing organisms. Farm implements may likewise

transport germs from one place to another, but thorough cleaning of

the implements eliminates the danger of spreading diseases in this

way. Reasonable care should be taken to keep infested soil from

being carried from one field to another by roaming animals or by

farm machinery.

Wind

The wind may be an important factor in disseminating certain

pea diseases. In localities where strong winds are prevalent, dry

weather is common, and soil is light, the spores of disease-producing

organisms may be carried long distances. The refuse from a diseased

crop may also be blown some distance by strong winds.

CONTROL PRACTICES

Using Resistant Varieties

The most satisfactory way of controlling pea diseases is to grow

known resistant varieties. The use of such varieties is about the only

way to control diseases caused by organisms that live almost indefinitely in the soil and are slightly or not at all influenced by crop

rotation and deep plowing. In the discussions of individua! pea

diseases that follow, varieties known to be resistant are listed.

Using Disease-Free Seed

Because some of the most serious pea diseases are carried by seed,

it is important to use disease-free seed whenever possible. Some seedborne diseases do not develop in regions that have low rainfall and

high temperatures during the growing season. Pea seed grown m

the semiarid intermountain area of the West and along the Pacific

coast is more nearly free from seedborne diseases tlian seed grown m

the East. For this reason, only seed from the western area should

be planted.

To economize, growers sometimes save seed from their own crops

to plant the following year. This practice is not recommended because, if the seed is infected, heavy crop losses may result. Seed

growing is an industry requiring particular knowledge of the type

and quality of the varieties grown. Special care is needed to maintain the purity and trueness to type of the stock. Seed-growing
compiinies employ experts to improve and maintain the high quality

of their stock. The piirc-liase of disease-free seed from reputable

iirms is reoonniieuded.

Seed Treatment

Disinfectiiifi pea seed with chemicals helps to prevent stand failures

caused by seed decay and increases the vigor and productivity of

plants. However, disinfecting destroys only surface germs; tho.se

beneath the seedcoat cannot be reached without injuring the seed itself.

Applications of chloranil (sold as Spergon), thiram (sold as Arasan), dicldoiie (sold as l-'bygon)/ ancl captan at the rate of II/2 to

2 ounces per bushel have proved beneficial in preventing decay of

seed planted in cool, wet soils. Addition of 1 ounce of graphite

to any of these except chloranil is recommended as a lubricant.

Most treatments are now applied as a slurry, and many contain a

dye that colors the seeds orange, red, green, or black.

CAUTION

The chemicals mentioned are injurious in varying degrees

to man and animals when taken internally; therefore, care

should be taken in handling them. When large quantities

of seeds are treated, a dust mask should be worn, but this is

not necessary when small quantities of seeds are treated

outdoors or in a well-ventilated room. Poisonous powders

and tablets should be kept in a safe place out of reach of

children.

Crop Rotation

The germs of several pea diseases live 2 or o years in the soil on the

debris of previous crops; the germs of others live much longer. If

jieas are repeatedly grown on the same soil, the disease-producing

germs may continue to multiply year after year. Lack of information

on the time required to star\e out all the disease organisms attacking

peas makes specific recommendations for control impossible. However, a crop rotation including peas planted not offener than once in

3 years is good practice. Where farm operations and practices permit, a longer rotation is desirable. Xo vetch should be grown in a pea

rotation, because some of the same diseases attack both crops.

Tlie life of some of the germs is shortened if they are buried deep

in the soil. Because of this, the refuse remaining on the field and the

vines returned to it after threshing or hulling sliould be plowed under

innnediatelv after harvest.

' The nieution of pruin-ietary iJroduct.si does not imply tlieir endorsement by

the U.S. Department of Agriculture over similar products not named.
DISEASES

Ascochyta Blight

Description

Ascochyta blight is cliaracterized in part by the formation of black

to purplisli streaks on the stem. Tlie lesions are more conspicuous at

the nodes and enlarge into brown or purplisli irregularly shaped areas

scattered indiscriminately from tlie roots to 10 inclies or more up tlie

stem.

Tlie leaves may sliow various characteristic symptoms (figs. 1 and

2) of the disease. The spots may be very small, purplish, and very

irregular in shape and size, or they may be fairly large and more or

less circular. The size and number of spots and the amount of damage caused depend somewliat on the age of the plants and on weather

conditions. Under favorable conditions, the entire leaf of a very

susceptible variety may be so badly injured that it slirivels and dries

up. Concentric circles sometimes form in the spots on the leaves.

Some of the lesions have ashen-gray centers.

Tlie symptoms on the pods (tig. 1) are similar to those on the leaves,

except that the spots on the pods are sunken and have no concentric

circles. The causal organism frequently grows through the pods and

penetrates the seed. If such seed is used to plant the next year's

crop, it serves as a source of infection. A few small, black or almost

black, slightly raised, pimplelike protuberances about the size of a

pinpoint may be found scattered in the lesions of the stems, leaves,

FIGURE l.^Ascochyta blight on a pea pod awl a leaf. Small, dark fi-uitmg bodies

about the size oí a pinpoint occur in the more or less circular spots. The spores

are enclosed in these receptacles and, on escaping, may fall on other plants

and serve as sources of new infections.
FIGURE 2.—Ascochyta blight (in a pea plaut. Note the small, purplish (dark)

spots about the size of a piuheail. (Courtesy, "Wiscousin Agricultural Experiment Station.)
''im île?' ^^'"^^ ^^'"'^'^ "^ *'"^ ''"^"'^' ^""^"^ '" '' P '1"<^«1 i" t'«se little

When infected seed is planted, the infection of the seedling may be

noted at the point where the seed is attached to the stem. A foot or

root rot may extend up the stem and down to the taproot. Tlie laro-er

roots may also become infected. Sometimes a stem lesion becomes an

miportant source of spread of the fungus.

Cause

Ascochyta blight is caused by any of the three parasitic funoi—

Ascorhyta pisi Lib., A. pinodella L. K. Jones, and MycoKplutrella

pinodes (Berk. & Blox.) Vest. Although the symptoms produced by

the three fungi may differ somewliat, they have so many characteristics

in common that it seems desirable to treat the diseases as one rather

than to ditferentiate them by fine points of distinction. Ascochyta

bhght occurs in all States east of the Mississippi River, but it is not

nearly so common in the semiarid seed^producing sections of the West.

It may cause lieavy losses during seasons of abundant rainfall. The

causal organisms are carried by tlie seed and infect the seedlings when

they emerge. Kains after germination splash the spores to surrounding plants, and eventually many of the plants in a field may become

infected.

Confro/

The fungi causing ascochyta bhght are seedborne. Tliey may also

live from one season to the next on refuse in the field. New infections

may st,art from this refuse and from use of infected seed. To control

this disease, only clean seed should be planted. As stated earlier,

western-grown seed should be used if possible. Crop refuse in the

field should be plowed under deep as soon as the crop is harvested. At

least a 3-year crop rotation should be practiced, and diseased pea vines

should not be scattered on land to be used for the next year's crop.

Treating infected seed with chemicals is of no value in controlling

the disease.

No varieties of peas have been found that are highly tolerant to

ascochyta blight. Complete control of this disease is probably not

possible until definitely resistant varieties have been developed by

breeding and selection.

Bacterial Blight

Description

Bacterial blight of peas is found on all parts of tlie plant above the

ground. If the infection starts from the seed or if the plants are not

more than 3 inches high, the vines may die without producing a crop.

Infection established later may reduce the yield considerably. The

extent of the injury depends largely on weather conditions.

Bacterial blight causes water-soaked lesions on the pods (fig. 3, ^4),

stems, leaves (fig. 3, ¿?), and stipules. These lesions enlarge under

humid conditions, and sometimes spots of considerable size appear.

A white to cream-colored, slimy ooze may also collect on the surface

of the lesions. On the other hand, if the weather turns dry, the infection maj' dry up. The infected tissue of the leaves and stipules

eventually turns brown and becomes papery in texture.
FIGURE 3.—-1, Bacterial blight ou pea potls. The water-soaked, irregularly shaped

spots are slightly sunken. The seed may be invaded by the parasite and serve

as a source of infection for the new crop. Such seed should not be planted.

B, Bacterial blight on pea leaves. These spots start as small, water-soaked

areas which gradually kill part of the leaf. Sometimes the injury is so extensive that the plant dies.
Extensive infection of tlie pods reduces tlieir market value considerably because of their unsightly appearance. The germs may

spread through the pods and infect the seed. Thus, the infection

would be transmitted to a new crop if the infected seed were later

planted.

Bacterial blight has been reported from many States but is more

commonly found east of the Continental Divide than west of it. The

extent of loss from the disease is not known, but at times the estimated damage has been 25 to 30 percent or more in some fields. In

recent years, losses this great liave not been reported. The disease

is most severe when humidity is high.

Cause

Bacterial blight is caused by a parasite (Psetidomonas pisi Sackett)

tliat lives over winter in tlie seed. It is not known if the parasite

lives on tlie refuse left in tlie field from harvesting. If tlie germs

do not survive the winters in the field, tlie seed is practically the

only source of infection.

Control

No effective control measures are known. Clean seed offers the best

possibility of avoiding infection. Although the disease has been reported from several Western States, it is of minor importance and is

seldom observed in those States wliere pea seed is produced, as in

southern Idaho and central Washington, and in the Palouse area,

which comprises parts of eastern Washington and northern Idaho.

Growers of peas for processing and market-garden purposes should

request seedsmen to supply seed grown in these areas.

No permanent control under all conditions can be expected until

varieties resistant to the disease are developed by breeding and selection.

Fusarium Wilt

Description

When fusarium wilt first appears in a field, only a few plants show

any signs of disease. If susceptible varieties are planted several years

in succession on the same field, the infested areas (fig. 4) gradually

enlarge, become more numerous, and eventually merge with others.

Finally, all the soil in the field may be infested.

Attention is first attracted to wilt by the yellowing of the lower

leaves and the stunted growth of the plant. A more careful examination shows a definite downward curling of the margins of the leaves

(fig. 5). If infection occurs when the plants are fairly small, they

may die without producing any peas. If the plants are older when

infected, a few poorly filled pods may develop. At or near the soil

level, the stem is sometimes slightly swollen.

Infection takes place from the soil through the roots. The causal

organism follows the water-conducting canals up the stem, often

well into the upper branches, and thus hinders tlie passage of water

to the stem and leaves. If the lower part of the stem or the mam

root is cut open, a lemon to orange-brown discoloration of the canals

may be seen. Sometimes, however, no discoloration is produced even
Fi( I i!E 4—P irt of a ijeifidd infested with fus u-ium wilt.

though the Avilt organism is present. These symptoms must not be

confused with tlie much brigliter brick-red discoloration of tlie water

canals characteristic of near-wilt. If wilt-infected plants are pulled

and examined several days after injury is apparent, root-rottnig or

other disease-producing germs in the soil may have so damaged the

roots that it is almost impossible to determine what organism was

responsible for the injury.

Cause

Fusarium wilt of peas is caused by a fungus parasite (Funnr'nim

o.Tyspiiiinii Schlecht, f. plsi (Linf.) race 1 Sn3-d. & Hans.) that is

very similar to organisms causing wilt in a number of other cultivated plants, including tomato, sweetpotato, cotton, cowpea, watermelon, and cabbage.

Control

Fusarium pea wilt can be controlled only by growing resistant

varieties, because the parasite lives indefinitely in the soil and cannot be desti-oyed by crop rotation. A large number of resistant

strains, especially of the canning and freezing varieties, are now on

the market. Resistant strains of the choice market-garden varieties

are also available.

Table 1 contains a list of common varieties of peas and shows if

they are resistant or susceptible to fusarium wilt. Eesistant varieties

should be selected for planting on soils that are taiown to be infested

with the wilt organism. Investigations have shown that strains of

peas that are resistant to the wilt organism in one infested locality

are resistant to it in other infested localities, no matter how distant.

Not all infected plants are killed before they produce seed; infected

plants tliat survive usually mature earlier than do noninfected ones

and pass the canning stage before the bulk of the crop is ready to

harvest. These prematurely ripened peas reduce the quality of the

pack.

10
FIGURE 5.—Fusarium wilt on a pea plant. Xiite the wilting and dryini- cf the

lower leaves and the downward curling of the upper ones.

11
TABLE l.—React'/on of common varieties of peas to fusarhim. v)flt

Coinmereial USP,

season, and variety

Market-garden varieties:

Early:

Alaska ' (numerous strains).

Hundredfold

Laxton Progress. _

Laxton Superb -

Little Marvel

Progress No. 9

Thomas Laxton ^. _

World Record

Midseason:

Alderman ^

Giant Stride {No.

40, leer No. 95).

Morse Market

Rondo

Canning varieties:

Early:

Alaska (numerous

strains).

Alsweet No. 4683.

Early Harvest

Earlv Sweet

Early Sweet No. 11

Laurel

Lolo

Pixie

Surpass

Surprise

Winner

Wisconsin Early

Sweet.

Yukon

Midseason:

Ace

Bridger

Cascade

Early Perfection^

Eureka.

Reaction to

fusarium

wilt

Resistant,

Susceptible.

Do.

Do.

Resistant

and susceptible

strains.

Resistant

Resistant

and susceptible

strains.

Resistant.

Do.

Do.

Do.

Susceptible.

Resistant.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Resistant

and susceptible

strains.

Susceptible.

Resistant.

Do.

Do.

Do.

Do.

Resistant

and susceptible

strains.

Resistant.

Commercial use,

season, and variety

Canning varieties—Con

Midseason—Con.

Famous

Hardy

Midway

New Era

Nome

Perfected Wales. __

Perfection (numerous strains).

Pride

Profusion

Ranger

Shoshone

Signal

Small Late Canner,

Wasatch

Wisconsin

Perfection.

Freezing varieties:

Early:

Early Freezer

Freezonian

Freezer 37

Frosty

Glacier

Jade

Laxton 7

Thomas Laxton

Midseason :

Alderman

Dark Skin

Perfection.

Freezer 626

Freezer 640

Hyalite

Midfreezer

Oracle

Perfected Freezer. _

Pluperfect '

Small Sieve

Freezer.

Victory Freezer

Viking

Edible podded

varieties:

Dwarf Gra}'

Sugar. I

Mammoth Melting

Reaction to

fusarium

wilt

Resistant.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Resistant

and susceptible

strains.

Resistant.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Susceptible.

Resistant.

Do.

Do.

Do.

Do.

Í Also used as a canning variety.

2 Also used as a freezing variety.

12
Near-Wilt

Description

In the past 20 years a disease of peas called near-wilt, which has

much in common with wilt, has been observed. The leaflets of

hifected plants curve downward and become pale yellow green, and

the growth of the plant is stunted. The stem near the ground

becomes swollen and brittle, the leaves wither, and tlie plant eventually dies. The near-wilt organism may attack the plant at a later

stage of growth, sometimes after the crop is nearly mature. For this

reason the loss from near-wilt is much less than that from wilt,

because the wilt fungus usually enters the roots of very small plants

and kills them while they are fairly young. Near-wilt may be readily

overlooked ; and the sickly appearance of the jjlants may be attributed

to root rots, lack of soil fertility, or other causes.

Near-wilt has early symptoms similar to those of wilt, but with

these diiferences; (1) The water-conducting vessels become brick red

instead of orange brown, and the discoloration runs up the entire

plant; (2) near-wilt is usually not found in circular spots in the

field; only scattered plants may be affected; (3) near-wilt develops

more slowly and when temperatures are higlier.

Near-wilt difl'ers from root rots and may be distinguished from

them by at least two characteristics. Plants afl'ected with root rots

usually pull from the soil much more easily than do near-wilt- or wiltinfected plants. Near-wilt-infected plants may not show any signs

of root decay, whereas plants affected with root rots do.

Cause

Near-wilt is caused by a fungus (Fusarimn oxyaporum Schlecht.

f. fisi (Snyd.) race 2 Snyd. & Hans.) closely related to that of fiisarium wilt that enters the water canals of the taproot and stem, just

as does the wilt fungus.

Confrof

When near-wilt was first discovered, it was found to affect all

wilt-resistant and wilt-susceptible varieties, although it develops more

slowly in some than in others. The only control measure is the use

of the disease-resistant varieties Commando, New Era, New Season,

New Wales, and New Pride. These varieties have not been generally

accepted by canners, but they are used primarily as breeding lines.

Roof Rots

Description

Eoot rots are caused by several different parasites that produce

symptoms so much alike that it is not always easy to distinguish them

by a casual examination.

Root rots, as the name indicates, occur on the roots or on all the

underground parts of the plants and sometimes extend a short distance above the surface of the soil. Most of the lesions are gi-ayish

brown or almost black, but occasionally some are reddish; and definite streaks form on the taproot or on the stem near the soil line.

Root rot may begin when the plant is in the seedling stage, or before

it comes through the ground. Death of the plant soon follows such

13
r

^\

FiüUEE 0.—Root rot on pea seedlings, which may be caused by several diftereut organisms.

r-arh^ infections. Root decay often begins at tlie tips of the small

feeding roots and progresses graduallj' upward to the main root. In

some instances, all the roots are destro3'ed, leaving nothing, or only

shreds, below the attacliment of the seed (fig. 6). Sometimes, however, tlie main root is the first to be affected. Eoot rots cause severe

crop losses.

In the root rot caused by the parasite Aphanomyces euteicTies

Dreehs., the initial infection takes place either through the small

feeding roots or directly on the taproot. The final result is that the

surface becomes soft and slimy and dark gray or brownish black. The

slimy condition of the surface is greater in wet than in dry soils. In

typical examples of root rot caused by this ¡parasite, the outer portion

of the taproot can be slipped readily from the central cylinder, which

may often become somewliat water-soaked and soft. The decay may

extend a short distance above the surface of the soil.

The root rot just described should not be confused with another very

widespread root rot caused by the parasite Fiisafhim nohmi (Mart.)

Appel & Wr. f. pisi ( F. E. Jones) Snyd. & Hans. This fungus usually

attacks the lower part of the stem where the seed is attached (fig. 7)

and grows in both directions but more conspicuously up the stem. The

infected region turns dark brown to brick red. The same parasite

may be found on the feeding roots, but it is less common there than on

the main root. This root rot maj' be identified with more certainty

bj- the deep-red discoloration of the central c\dinder. By splitting

open the stem where the seed is attached, or just above, the red color14
FIGURE 7.—Root ret on a yea plant. Note the typical darkened area above and

below the point of attachment to the seed.

15
ing caused by the ])reseiice of I lie fiinous iii:iy l)e seen lo extend a

little distance upward.

It is not possible to describe symptoms of other root rots in sufficient

detail to distinguish tliem from root diseases caused by otlier parasites.

In advanced stages, the symptoms for all are very much alike. The

root diseases of a single plant are not always caused by just one parasite; several parasites may damage the roots at tlie same time.

JS^o particular skill is required to recognize root rots. Observant

fai-iners can recognize them if they examine the roots. It makes little

difference what organism or group of organisms cause them. Even

skilled observers are frequently unable to identify the causal organism

without laboratory study. If the plants in the field are stunted or

tlie leaves are pale yellow or appear unhealthy in other ways, it is a

good plan to dig up some of the plants, carefully wash off the soil and

examine the roots for decay. '

Root rots occur wherever peas are grown, although the loss in on©

field may sometmies be small. Taking the country as a whole more

loss IS believed to be caused by root rots than by any other sino-le

di-sease and possiljly more than by all other diseases combined E^ot

rots and wilt frequently occur together in the same fields. When this

Jiappens, it is difficult to determine wliich causes the greater loss.

Cause

Se\-eral organisms are known to cause root rots, and others may be

responsible. The fungi generally associated with root decay are:

Fumrmm solani f. pisi. Aphanomyces eufeiches, RMzoctonki solani

Ivuehn, Pytluum ulfhnum Trow, Ascockyta pinodeUa, Mycosphaerella

pmodes, Sclerofinia sclerotiomm. (Lib.) DBy., TMelaviopsu hasicola

(Bei-k. and Rr.) Ferr., and Srlerothim ro!f.<<ä Sacc. Tlie last-named

organism is restricted mostly to the Southern States. Several of

these organisms cause root rot of crops other than peas, and probably

all of tliem can live indefinitely in the soil.

Conffof

The root rots of peas, like those of many other crops, are hard to

control. Because they are caused by organisms that live indefinitely

in the soil, there is no practical way to eradicate them. Control

measures for all root rots are about the same.

Root rots are usually more se\'ere during seasons of heai'y rainfall

than during dry seasons and on low-lying fields that are poorly drained

tlian on well-drained fields. Excessive soil moisture, regardless of

tlie location of the field, favors root rots. Where root rots have been

troublesome, it is advisable to select well-drained fields and to prepare

the soil thoroughly before planting. Plants that start growth poorly,

as they do on impoverished soils, are mucli more subject to attack of

root rots than are plants grown on rich soils where a good, vigorous

growth is maintained from the beginning. Thus a fertilizer should

be added to tlie soil where needed.

Crop rotation is recommended as a good practice, but it is of no

particular \-alue in controlling pea diseases except in the case of those

organisms that attack only peas. However, most of the fungi are

about as parasitic on other crops that might be grown in rotation

as they are on peas.

At least a fair crop often can be grown in spite of root rots.

T6
Root Knot

Descr/p/ion

Root knot whicli occurs tlii-oughout the United States but is most

severe in the light, sandy soils of tlie South and in similar districts in

I aJitoniia, is characterized by enlarged, irregularly shaped, defomied,

fleshy galls (fig. 8) distributed on the root system. If the galls are

broken open, pearl-wlute bodies about the size of a pinhead may often

be seen. These are tlie female worms that cause tlie galls.

The root knot galls may sometimes be confused with the nodules

that normally develop on the roots of garden and field peas, beans,

and other legumes; but they are quite difièrent. The nodules are

usually smaller than the root knot galls, they are more nearly round,

and they are attached somewhat loosely to the roots ; whereas the root

knot galls are enlar-gements of the roots tliemselves. Plants bearing

nodules generally grow well and look healthy; whereas those with

root knot are usually stunted, yellow, and sickly looking and may even

be killed by the disease.

Cause

Root knot is caused by parasitic nematodes, or eelworms, of several

species of the genus Meloidogyne. These nematodes are so small they

are almost invisible to the unaided eye. They enter the roots and

cause them to form irregular swellings, or galls. These galls retard

the growth, reduce the yield, and often kill the plants. To a considerable extent, they cut oiï the transport of food material from the

roots to the rest of the plant. The nematodes travel very slowly in

the soil, probably not more than 1 or 2 yards a year; and they overwinter in the soil.

Contre/

The root knot nematodes are parasitic on a large number of cultivated plants and on many weeds. However, the various species or

kinds of nematodes differ from one another regarding tlie plants on

which they develop. A number of crops are immune from many of

the species, or at least highly resistant to them. Iron and Brabham

cowpeas, Laredo soybean, peanuts, velvetbeans, barley, rye, redtop,

sorghum, timothy, wlieat, and winter oats show varying degrees of

resistance to most species.

The most satisfactory method of controlling (he disease is to practice rotation with immune crops for 3 or more years to starve out the

nematodes. Clean cultivation should be practiced to prevent the

growth of susceptible w-eeds on which nematodes feed. Growers

should guard against carrying nematodes from one field to another

in tubers, bulbs, or roots of growing plants; on implements; in drainage water ; by roaming farm animals ; or by other means.

Septoria Blight

Septoria blight, caused by a fungus parasite {Septoria pisi West.),

is one of the minor diseases of peas. It occurs mostly in the Northern

States during cool, wet weather. Septoria blight is sometimes confused with ascochyta blight; but, when the two diseases are carefully

compared, they are easily distinguished. The spots caused by Sep17
FIGURE 8.—Root kuot ()f pea itlants. t-aused by a small parasitic neuiatode. or

eelworm, that invades the roots. The galls may form at any place on the

roots.

18
FIGURE 9.—Septoria blight on pea leaves. Note the small, round, black, slightly

raised pimples in whiob the spores of the fungus parasite are produced. These

spores escape and start new infections on other leaves, stems, and pods.

foria are somewhat indistinct, whereas those caused by Ascochyta are

distinct. Ascochyta blight is in part characterized by lesions with

ashen-gray centers. With septoria blight, however, the infection begins at the edge of the leaf, causing yellowish indefinite areas that

gradually darken and enlarge until the entire leaf is invaded (fig. 9).

The casual fungus spreads from the leaves to the nodes, which are

likewise yellowed and often shrunken.

Pycnidia, or spore receptacles, more or less indefinitely arranged,

develop in the lesions on the leaves, and especially on the nodes and

lower part of the stem. Numerous spores are formed in the pycnidia,

emerge from them, and are disseminated almost exclusively by spattering rain to other plants, where new infections take place. By this

means, an entire field may become infected within a couple of weeks.

If infection takes place when the plants are young, tliat is, 3 to 8

inches high, they will probably die before a crop is produced. If, on

the other hand, no infection occurs until the plants are nearly mature,

at least a partial crop can be expected.

Septoria blight occurs in epidemic form so infrequently that no

chemical control measures have been worked out. Rotation of peas

with other crops is suggested if the disease becomes serious.

Powdery Mildew

Powdery mildew of peas is caused by a fungus parasite (Erysiphe

folygoni DC.) that occurs on beans and other legumes. The organ19
^m

FiciUKK 10.—INiwdery mildew (in pea IKMIS, showing irregularly shaped tu own

(dark) spurs and blotches. I'od infection reduces the market \alue

ism ciiusiiifi powdeiy mildew of peas is probably as widely distributed

as is the crop. Like the powdery mildews of other crops, it develops

most abundantly under cool conditions. Consequently, it is more

prevalent on fall crops or on crops that mature in the late summer

and where the nights are cool.

Powdery mildew is characterized by the formation of a white,

powdery, dustlike coating on the surface of the leaves and less frequently on the petioles of the leaves, stems, and pods. The leaves are

3'ellowed, dwarfed, and sometimes consiclerably malformed. Only

in extreme cases, however, are the vines killed. In severe cases of the

disease, the powdery mildew fungus causes small, brown spots (fig.

10) or streaks on the pods. Damaged pods are not marketable, although the food A-alne of the peas is not affected.

Powdery mildew causes considerable damage in some sections of

the country when conditions are favorable for the growth of the

causal fungus. Its spores are produced on the surface of the diseased

tissues and are disseminated to other plants by air currents and wind.

Splashing rain tends to injure the spore-bearing organs and reduces

formation of spores. For this reason, the disease is never as destructi\-e in sections of high rainfall or during seasons when rainfall is

heavy.

In tlie pea-growing sections of the States along the Pacific coast and

occasionally in other States, control measures are required every year

to produce a crop. Dusting the plants with sulfur has given good

results. The first dusting should be done as soon as there is any sign

of the disease. Another application should be made a week or 10 days

later. Sometimes as many as six or seven dustings are necessary to

insure a crop. Crop rotation and the immediate turning under of the

refuse left in tlie field after harvest by deep plowing are practices

growers should follow. The disease is not seedborne.

20
Anthracnose

Aiithnicnose of peas is caused by a fungus parasite {OoUetotriehum

pisz 1 at.) tliat IS uiipoftaiU only in Wisconsin in some years. It has

also been reijorted from sexeral other States.

On all parts of the plant aboveground, the fungus causing anthracnose produces lesions that resemble tliose caused by tlie ascocliyta

blight fungi (fig. 11). On the leaves it causes irregularly shaped,

gray to brown spots ; on the pods the spots are more nearly circular.

The spots on the stem are elongated and ashen.

The spores are foniied in large numliers in tlie lesions and may be

distrdnited by rain or carried from one plant to anotlier by insects,

on farm implements, oi' by other means. Wlien it is moist, the spores

quickly germinate and cause new infections. How the organism

survives the winter is not known, but it may live from one season to

the next in and on the seed and in the refuse left on the field from a

previous crop.

No control measures have been worked out. Crop rotation, plowing

under of refuse left on the field, and use of disease-free seed may help

to hold the disease in check.

Downy Mildew

Downy mildew is caused by a fungus {Permionpora pid Syd.).

This disease, although widely distributed, is not considered of economic importance except in Washington and California and possibly

in one or two other Western States. It causes a water-soaked condition of the affected parts and is further characterized by the white,

downy, or cottony growth (fig. 12) that may be found on any of the

aboveground parts of the plants. This growth occurs on the under

sides of the leaves; later their upper sides become yellowed and turn

brown, and the leaves die. (^n the pods, ]5ale-green blotches up]>ear

first; later they darken to dark brown mottled with light green. The

causal organism grows into the pods and produces a white, felty, hairlike growth. Infected pods may be malformed, and seed from these

pods may be infected.

The spores of the causal fungus are spread readily by wind; they

may reinfect other plants when the weather is cool and moist. Dry,

hot weather checks infection.

No satisfactory control measures have been found for the disease

once it is established. A bordeaux spray mixed with Penetrol may

offer protection against infection if the fungicide is applied before

the disease is noted. In localities where climatic conditions are ideal

for development of the disease, a 2-year crop rotation is advised. It

is also wise to use seed from arid sections where the disease is not

prevalent.

Pea Virus Diseases

Description

Peas are subject to several virus diseases, some of which are readily

diagnosed by their symptoms. Others, however, can be identified

only by greenhouse tests. Brief descriptions of the most common

virus diseases of peas follow.

21
FiGUBK 11.—Anthrac-nose ou pea plauts. ^oîe that the siH>ts on the leaves and

stems are similar to those of asoochyta blight. They are, however, niore

distinct and tend to be tan-colored rather than black to purplish. (Courtesy,

AViscousin Agricultural Experiment Station.)

22
FIGURE 12.—Downy mildew on a pea plant. Note white, cottony growth on

leaves.

23
FnuíUE 13.—Musaic on a pea plant. Note tlie veins (yellow), which cau.se a

niDttled appearance nf the infected leaves.

COMMON PEA MOSAIC.—Leaves of plants affected with common pea

mosaic sliow an intermixing of liglit- and dark-green areas (fig. 13),

wliicli are characteristic of mosaics in general. The mottled areas are

irregular in outline and may follow the small veinlets. In later

stages, these green areas enlarge and become yellowish green, especially

along the veins. The leaf then has a netted appearance. "Wlien the

disease is severe, the leaves may become somewhat curled, twisted, and

dwarfed. The symptoms of common mosaic cannot be recognized on

the stems or seed; but the pods are frequently distorted, malformed,

and dwarfed.

PEA ENATION MOSAIC.—The symptoms of pea enation mosaic consist of mottling, crinkling, and malformation of the leaves and stipules.

Yellowish spots develo¡:) on the leaves and later become white and

somewhat transparent. On very susceptible varieties, necrotic spots

appear accompanied by proliferations or enations on the undersurfaces of the leaves that distinguish this virus from others affecting

peas. If the plant becomes infected before the pods develop, the pods

become very distorted. The pod wall is rough, ridged, and wrinkled

and may be so severely twisted or curled that the pods are hardlj'

recognizable. These pods remain severely stunted and pi'oduce no

seeds.

24
PEA bTUNT.—The most conspicuous symptoms of pea stunt are a

severe stunting of the plant and malformation of the apical foliage

into a tight rosette of leaves, stipules, and blossoms. The intemodes

are greatly reduced in length, and diseased plants are often only onetourth or one-half the size of normal plants. The leaves and stipules

are reduced m size, are twisted or curled, and fail to open properly.

No mottling of the leaves is noted.

Very few, if any, pods are formed; and those that do develop are

stunted and poorly filled. Diseased plants often die prematurely.

YELLOW BEAN MOSAIC—The virus of yellow bean mosaic causes a

distinct mottling of the foliage of peas. The symptoms of this disease are frequently ditficult to differentiate from those of common pea

mosaic virus, although they are somewhat milder. The first sign of

the disease is a faint mottling, which later becomes more intense.

Next, a pronounced veinclearing appears; and, adjacent to the larger

veins, the dark-green tissue often persists. Between the veins, the

leaves remain green ; but they are a lighter shade than those of nonnal

plants. Often the edges of the leaves are yellow. Leaves and stipules

of infected plants are smaller than normal. Infected plants are only

slightly distorted, which helps to differentiate this virus from common

and enation pea mosaics, wliere the distortion is often marked. Pods

from yellow bean mosaic-infected plants are somewhat malformed,

distorted, and reduced in size but not so severely as are those infected

with pea enation mosaic virus.

PEA STREAK.—The pea streak virus causes purpling and streaking of the stem, which may extend from the top to the bottom of

the plant. The leaves later become flaccid, the tip of the plant

begins to wilt, and the plant ultimately dies. A plant infected when

young often dies before attaining mucli growtli. If it does not die,

it is usually decidedly stunted, the leaves become curled and twisted,

and the veins become very pronounced. The pods that are foi'med

before the plant becomes seriously infected take on a dark pnrplishgray or brown color. They may also be spotted, pitted, and

decidedly malformed. Frequently they do not reach maturity

(fig. 14.).

Cause

Eacli of the several pea virus diseases is caused by a distinct virus.

The diseases are discussed together, although they produce different

symptoms. All tlie mosaics and streak are found on other legumes

such as red clover, alsike clover, white sweetclover, and alfalfa. The

viruses may overwinter on these hosts and be transmitted from them

to peas. Not all the viruses infect the same hosts, but they are transmitted to peas from one or more of these crops. In general, tliese

diseases are not seedborne, although common pea mosaic is sometimes

carried in very small amounts in the seed.

These diseases are gaining in importance in the United States,

and in certain pea-growing localities they cause considerable damage

in some seasons by reducing the yield and quality of the product.

The severity of tlie diseases is usually correlated with the size of the

pea aphid population. In years of large apliid populations, the virus

diseases are more severe than in years when there are fewer aphids.

25
FIGURE 14.—Pea streak, showing streaking of stem, necrosis of the leaf veins,

and malformation of a pod.

26
Con/ro/

No effective control measures are known to combat streak and

mosaic. 1 ea aphicls are responsible for much of the spread of these

diseases in the field, and controlling them may reduce disease spread.^

Because of the transmissibility of certain virus diseases of other

legumes to peas, peas should not be planted too close to other cultivated legumes, such as clovers and alfalfa. Also, eradicating legumes

that grow wild close to peafields may tend to partly reduce the

secondary spread of viruses to peas.

The relative susceptibility of the different pea varieties differs with

each virus. In general, canning or field varieties are more resistant

than are market-garden varieties. Three new canning and freezing

varieties resistant to pea enation mosaic have been developed by the

New York Agricultural Experiment Station. They are Thomas Laxton 60, Surprise 60, and Perfected Freezer 60. These varieties can

be grown wliere pea enation mosaic is a factor in production. In

general, the Perfection varieties are resistant to the several strains of

yellow bean mosaic.

NUTRITIONAL DISORDERS

Marsh Spot

Marsh spot, first reported from Holland and other European countries, shows no external symptoms on the plants or on the seeds of

peas. When the latter are split open, the flat inner surface of one

or both of the cotyledons exhibits a brown cavitylike lesion (fig. 15)

sometimes accompanied by a partial or entire necrosis of the plumule

that may letaid or prevent germination, depending on the size of the

lesion. The disorder is generally found on peas planted on alkaline

soils but not on those planted on acid soils.

The necrotic lesion enlarges when the seeds approach maturity. It

is not found in all seeds of a pod nor in every pod of a plant. Latematuring varieties with large seeds are more severely affected than

are early-maturing varieties with small seeds. Varieties with round

seeds (a characteristic usually associated with earliness) are generally free of this condition. Varieties with heavier seeds are more

severely damaged.

Marsh spot results from a partial deficiency of available manganese. The amount present may be sufficient for normal vegetative

requirement of the plant but not for healthy seed production. Complete absence of manganese produces a mottling of the younger leaves

and death of the growing tip, and the plant fails to reach the flowering stage.

Spraying the foliage with a 1-percent manganese sulfate solution

at flowering time and 2 or 3 weeks later controls the disorder. Applying heavy soil dressings of a soluble manganese salt at planting has

little effect on controlling marsh spot.

^ For current recommendations and precautions, see U.S. Dept. Agr., Agr.

Handb. 120, Insecticide Recommendations of tlie Entomology Kesearch Division

tor the Control of Insects Attacking Crops and Livestock for 1961. This publication is revised each year.

27
FIGURE 15.- -iíarsh spot of Dark Skin Perfection variety. Note brown (dark)

cavity in cot.vledons and necrosis of plumules.

A condition tlnit resembles marsh spot has been found in pea seed

from certain areas of Alberta, Canada. Wien the condition is serions, the stem tip or primary bud dies. This symptom is often accompanied by dark-brown spots on the inner face of cotyledons, but the

two symptoms are not always associated in individual seedlings.

Seed Cavitation

The condition known as cavitation, which produces a depressed

area in the inner face of the cotyledon that may turn dark, is somelimes widespread in the pea-seed-producing areas of the united

States. The cause of this disorder is not yet known but is believed to

be nutritional. No plumule abnormalities hare been observed, and

the symptoms are not necessarily associated with weak seedlings.

Seed cavitation has been found in seed grown in different areas in

different years, and it is generally more prevalent among the latematuring varieties. It has ne\er been observed in Alaska, an earlymaturing variety.

AAolybdenum Deficiency

¡Molybdenum deficiency, which is most severe in peas grown on

certain acid sandy soils, is expressed as a leaf chlorosis. Peas normally cany enough molybdenum within the seed to supply plant

needs. When grown several years on molybdenum-deficient soils, the

seeds no longer contain a sufficient amount of the mineral.

The symptoms are somewhat similar to those of nitrogen deficiency

in peas, since a lack of molybdenum decreases the availability of nitrogen in the soil. Also, peas suffering from this deficiency cannot form

nitrogen-fixing nodules on their roots. The plants may be dwarfed

and pale and may exhibit a sudden wilting of the leaves. Scorched

28
FIGURE 16.—Physiological spotting of Wisconsin Early Sweet pea seed.

leaf margins, suppression of flowering, and poor pod setting may

follow.

Peas respond readily to molybdenum fertilization. Two ounces to 1

pound of ammonium molybdate per acre corrects tlie deficiency.

Molybdenum comj>ounds can also be added to the seeds as a seed

treatment.

Physiological Spotting of Seed

A spotting of seed, apparently nonparasitic in origin, has been observed since 19.30 on peas grown in the States of Wisconsin, Idalio,

Montana, and California, and in Sinaloa, Mexico. This spotting has

been found only in the Surprise variety and in closely related types

such as Peerless, Wisconsin Early Sweet, and Early Canner, and in

crosses between Surprise types and other varieties. The spots occur

only on the seed, and the pods are normal in every respect. When

the seed reaches the canning stage, the spots appear as small, somewhat

circular, water-soaked areas that later enlarge slightly. In dry seed,

the discolored areas are very conspicuous—they are darker green than

normal and in some cases almost black (fig. 16).

29
Spotted seed is of normal size and germinates just as well as does

healthy seed. Not all the pods on a plant produce spotted seed ; nor

are all the seeds in a pod necessarily atïected. Sometimes only one

seed in a pod may show spotting; in other instances all the seeds may

be affected. Normal seed may under favorable environmental conditions produce spotted progeny. On the other hand, spotted seed

does not always produce spotting in the progeny but may do so if

conditions are favorable. Seedsmen have reported heavy handpickiug costs in areas wliere this disorder commonly occurs. Handpicking is not reconnnended as a method of control but only for improving

the appearance of the seed stock.

30

TINS OFFICE: 1962