Root Rots, Wilts, and Blights of Peas

W. T. Schroeder.

The pea (Pisum sativum) is subject to various types of diseases blights, root rots, and wilts. The formulation of control measures for them requires knowledge of their symptoms, causes, relations of host and parasite, and possible inherent resistance.

Blight refers to the discoloration, gradual drying, and eventual death of the affected plant parts. Bacteria and fungi cause blights, which usually are seed-borne and develop best during wet weather.

Only one major bacterial blight (Pseudomonas pisi) affects peas. A number of fungal blights, however, do occur on peas, among which are septoria blotch (Septoria pisi), downy mildew (Peronospora pisi), powdery mildew (Erysiphe polygoni), anthracnose (Colletotrichum pisi), ascochyta blight, and several minor ones.

Ascochyta blight is among the oldest and worst. Its nature and control are much like those of other fungal blights (except powdery mildew). Details about it therefore apply generally to the others.

ASCOCHYTA BLIGHT is a composite of three diseases resulting from infection, singly or collectively, by Ascochyta pisi, A. pinodella, or Mycosphaerella pinodes.

The nature of the disease was first determined in Europe in 1830. It was a constant threat in the United States until about 1915, when it declined with the shifting of the seed industry to areas in the West where the low rainfall before and during the harvest reduced the amount of infected seed. But in some years unseasonal rainfall during harvesttime in western seed-growing districts means contaminated seed, so that the disease remains something of a problem.

The symptoms the three parasites cause are almost alike. Most evident are the lesions that begin as small purplish specks on leaves and pods. When infection is caused by M. pinodes or A. pinodella, the specks may enlarge on the leaves into round, target-like spots. If numerous, they join to make irregular brownish-purple blotches on both leaves and pods. A. pisi produces relatively few, rather definite, sunken, tan or brown spots, which have dark-brown margins and are circular on leaves and pods. The pod lesions usually become sunken.

Stem lesions are elongated and purplish black. They originate as separate infections or as a continuation of petiole infection around the nodal area. They also may coalesce and girdle the entire stem, weakening it so that it is easily broken.

Affected leaves eventually shrivel and dry into a blighted condition, which resembles freshly cured clover hay. M. pinodes may also blight the blossoms and young pods and cause withering, distortion, and eventual dropping.

All three organisms can attack that part of the stem and root at the soil line and produce a bluish-black foot rot. It is severest when it is caused by A. pinodella. A. pisi seldom causes severe foot rot.

The three organisms responsible for the ascochyta blight complex are closely related, but each has marks that classify it as a distinct species. In pure culture on artificial media, the light-colored mycelium and the abundant carrot-red spores of A. pisi readily distinguish it from the darker-colored mycelium and relatively scarce light-buff spore exudate of M. pinodes and A. pinodella. All produce waterborne spores, the pycniospores, but those of A. pinodella are only half as large as the spores of the other two species. The incubation period of the disease caused by M. pinodes and A. pinodella is 2 to 4 days, compared with 6 to 8 days for A. pisi. M. pinodes is the only species that produces ascospores. Such spores can be carried for considerable distances by air currents and largely account for the more aggressive nature of that pathogen.

The three disease organisms can infect the seed. Such infection serves to overwinter the pathogens and is a means of transporting the disease from one region to another. The pathogens also overwinter in infected pea straw. In regions of extremely mild winters, they may remain active on infected volunteer plants. The ascochyta blight organisms do not live indefinitely in the soil as in the case of the root rot and wilt fungi, but remain there only as long as the infected pea straw is not completely decomposed.

When infected seed is planted, the disease first appears as a foot rot on the young seedlings at the point of seed attachment and often kills or weakens the young plants. Spores are produced during wet weather on such plants and spread the disease to nearby plants.

Infected pea straw in the soil from the crop of the previous season may give rise to both pycniospores and ascospores. As pycniospores need spattering rain for their dissemination, they spread the disease only a few feet from the source. Ascospores, however, are shot out from the spore-bearing structures in the old plant tissue and are carried quite far by air currents. They usually are more abundant than pycniospores and spread the disease uniformly over the field rather than in small patches. If conditions are favorable, both types of spores may be formed continually on the dead parts of the infected plant. Because the ascospores are more widely and uniformly distributed, however, infection by M. pinodes (the only species producing ascospores) is more damaging.

Because moisture is required for spore discharge and infection, rainfall, dews, and high humidity are the most important environmental factors in the development of ascochyta blight. The number of periods of wet weather during the pea season largely determines how bad the disease will be.

Because there is no practical resistance in the pea to the ascochyta blight complex and because nothing can be done at present to change the weather, the disease is best controlled by avoiding, eliminating, or reducing the causal organisms by the use of disease-free seed, crop rotation, and sanitation.

It is unwise to plant seed grown in humid sections of the East and Middle West. That seed is likely to be infected with the disease organisms. Furthermore, the straw from a seed crop is apt to be more heavily infected with the organisms than a crop cut in the green stage for processing. That increases the inoculum potential on the overwintered straw and affords a better chance for the disease to establish itself earlier and more severely. Seed grown in drier regions of the West is the best insurance against seedling infection. Treatment of seed with fungicides reduces the surface contamination but will not eliminate internal infection.

Rotation to control pea blights implies more than merely avoiding planting pea crops on the same land more than once every 4 or 5 years. Such a rotation undoubtedly would eliminate the organism from the soil. But, in addition, every effort should be made to locate new plantings as far as possible from those of the previous season. Even though most of the vines from the old fields are removed to the viner station, there may be enough infection on the stubble and debris to provide for the dispersal of ascospores the following spring.

Of equal importance is sanitation. Any infected plant parts whether Stubble in the field or vines on the viner stacks offer a constant source of inoculum for the next crop. After the ensilage has been removed from the stacks, the outside part of the stack, which usually is raked off, should be destroyed before seeding the next spring. If mobile viners are used in pea fields, the vines and pods, instead of just the stubble, are left in the fields. That will create a situation comparable to one in which seed peas and processing peas are grown in the same area, for the fresh vines will dry, and the organisms will continue to develop, thereby increasing the inoculum potential for the next season.

A good practice would be to plow down all pea stubble and vines immediately and plant the field to a crop, such as grain, in which the soil will not be cultivated during the next season. In some places it is customary to plant early varieties alongside late ones or to stagger planting dates. In either event, ascospores may develop on the stubble of the earlier planting and provide for abundant dispersal of spores to the younger plantings nearby. Such practices should be discouraged.

ROOT ROT is caused by a number of fungi, which singly or together attack the cortex, the tissue outside the water-and food-conducting cylinder of the roots and lower stem. Invasion of the cortex may be quite general, or it may be somewhat localized. The rot may be soft and watery, or it may be more like a dry, corrosive decay, depending on the causal organism. The various types of root rot are designated by the names of the fungi that cause them, such as aphanomyces root rot, fusarium root rot, and ascochyta root rot.

The damage done by root rot varies With the season and the causal organism. One year the crop may fail; the next year on the same piece of land the crop may be all right. The disease may act as a seedling blight and kill scattered plants at an early stage, or it may not attack the plant until quite late in its development. Two different pathogens attacking the same plant frequently cause more harm than either alone. Complete crop failures can occur. More often, however, yields are reduced in varying degrees as a result of an impaired or restricted root system. Under those conditions, the affected plants look as though they were suffering from malnutrition.

The disease pattern or symptoms and the conditions favorable for root rot development depend upon the specific pathogen. Practically all the types may occur wherever peas are grown, but certain ones are more prevalent in some areas than in others.

APHANOMYCES ROOT ROT, caused by Aphanomyces euteiches, occurs to some extent wherever peas are grown and is often referred to as common root rot. It is especially prevalent in the older pea-growing regions of New York, New Jersey, Wisconsin, and Minnesota. Some 10,000 acres of canning peas were lost in 1942 in Wisconsin as a result of the disease.

The disease is first detected by a soft and watery condition of the stem an inch or two above the soil line. By then the roots have become similarly affected. The diseased tissue becomes discolored because other fungi invade the softened tissue. Ordinarily most of those fungi cannot infect the healthy tissue.

In time the affected tissue on the stem above the soil line collapses and shrivels. The outer tissues of the roots then are so rotted that when the plant is pulled all that remains of the root system is the stringy central core of the taproot. That condition usually distinguishes the aphanomyces from other root rots in the field.

Severe infection of young plants usually kills them before they blossom. Plants infected later seldom die but, because of a restricted root system, they are stunted; their leaves turn yellow and die from the ground up.

The fungus that causes aphanomyces root rot belongs to a group of fungi commonly called water molds. It has two kinds of spores. One, the oospore, or resting spore, is the thick-walled spore generally found in diseased tissue. It may also occur in culture on artificial media. The other, the zoospore, or free-swimming spore, has two long hairs, which enable it to move about in water.