Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Wet seasons such as 2013 increase the potential for root rots, wilts, and stem decay in peas. Root Rots are most prevalent in fields with significant compaction or with waterlogged soils. Root rots may be caused by one or more soil borne pathogens including Aphanomyces, Pythium, and Fusarium, Thielaviopsis, Ascochyta/Phoma, Rhizoctonia, and Sclerotinia sclerotiorum.
It may be difficult to determine which organism is causing root rots in advanced stages and plants may have several pathogens attacking them at the same time. In addition, nematodes may also be part of the root disease complex on peas.
The following are descriptions of the common pea diseases affecting roots or causing wilts or lower stem decay from the “Crop Profile for Peas in Delaware” http://www.ipmcenters.org/cropprofiles/docs/DEgreenpeas.pdf and “Root Rots of Peas” from the University of Illinois http://web.aces.uiuc.edu/vista/pdf_pubs/911.pdf.
Aphanomyces Root Rot
“Aphanomyces root rot is caused by the fungal pathogen Aphanomyces euteiches f. sp. pisi, although other soil-borne organisms contribute to the disease complex. Oospores can remain dormant in the soil for years. When conditions are favorable, the spores germinate and pass through several life stages before developing into hyphae that can grow through host plant tissue. Infection can occur at all temperatures favorable for pea development. Once pea roots are infected, the mycelium of the fungus begins to decay the root tissue. As roots decay, the oospores return the soil to serve as inoculum in years to come. Characteristic symptoms include water-soaking, softening, and slight discoloration of the taproot and lower stems of infected plants. The outer root tissue of infected plants can be easily sloughed off. Symptoms develop faster at warmer temperatures.”
“Aphanomyces root rot, or common root rot, is one of the most destructive diseases of peas. It occurs in most pea producing regions of the U.S., including the Mid-Atlantic. In the Northeast, average annual yield loss to this disease is about 10%, though losses in individual fields may be up to 100%. Wet soil conditions and poor drainage are associated with higher rates of infection. The disease is most damaging in years when a cool, wet spring is followed by an early, warm summer with low rainfall.”
Fusarium Wilt
“Fusarium wilt of peas is caused by the soil-inhabiting fungal pathogen Fusarium oxysporum f. sp. pisi. Near-wilt, a related disease, is caused by a different race of the same pathogen. Both diseases can be introduced from soil borne pathogens, but the symptoms and control strategies for the diseases differ somewhat. The fungus can survive in the soil as long as 10 years as chlamydospores and by association with the roots of non-host crops. The fungus penetrates the roots of peas and may colonize the vascular system of non-resistant varieties. The pathogen spreads in contaminated soil, seed, and plant debris, and can be transported from field to field by wind and water. Soil temperature, pea cultivar, and soil type can affect the rate of disease spread.
“Fusarium wilt is characterized by yellowing of the lower leaves and a general stunting of the plants. Leaf margins curl downward and, in some cases, the stem becomes swollen and brittle at the soil line. A discoloration of internal root tissue also occurs. At soil temperatures above 68°F, the disease progresses rapidly, plants may be killed, often in small patches (depending upon the race of the fungal pathogen). These dead plants serve as a reservoir of inoculum for spread of the disease. One of the outcomes of Fusarium wilt infection is uneven maturity among plants in the field, which leads to yield loss and reduced quality of produce. The symptoms of near wilt are similar, but the disease’s progress and plant death generally occurs more slowly than in Fusarium wilt. Some races of the fungus are widely distributed and can kill 1-3% of plants in infected fields. Because warm soil temperatures are conducive to the spread of inoculum, damage can vary significantly from year to year and the most severe losses occur in late peas.”
Fusarium Root Rot
Fusarium root rot is caused by the soil borne organism Fusarium solani f. sp pisi. “Fusarium root rot affects mainly the taproot with infection starting close to where the seed is attached. Reddish brown streaks form in the primary and secondary roots and later merge. The external portion of the stem shows brick red, dark reddish brown, or chocolate-colored lesions. The advancing lesion may be wedge-shaped with the point upward. The central part of the taproot is a deep red. Plant growth is stunted, the foliage turns grayish, then yellow, the lower leaves wither, and the plant eventually dies. The lower stem is often girdled, causing the plant to fall over. Pythium ultimum is often found in Fusarium-infected roots and vice versa.”
Ascochyta Blight
“Three species of related fungal pathogens cause important diseases of peas. Ascochyta pisi causes leaf and pod spot; Mycosphaerella pinodes, the perfect stage of A. pinodes, causes blight; and Phoma medicaginis var. pinodella, causes foot rot. All of these diseases are characterized by lesions on leaves, stems, blossoms and pods, and by discoloration of the hypocotyl, cotyledons, and roots. All of these pathogens are soil-borne and persist to a greater or lesser degree in or on soil and plant debris; A. pisi, however, is primarily carried on or in the seeds. Infested seeds may be infected and develop into weak, stunted plants that are unproductive or die. Leaf lesions vary in appearance, depending on the fungal species involved and on the geographic region. Stem lesions of Mycosphaerella pinodes can cause girdling. When flowers are infected by one of these species, sepals may become girdled, killing the developing pod or resulting in distorted pods. Leaves of infected plants become desiccated on all but the highest nodes. Root infection is often limited to the primary roots, but in some cases lateral roots are also destroyed. A. pisi is the most common in Delaware”
“It is difficult to estimate yield loss attributed directly to Ascochyta diseases. Phoma medicaginis var. pinodella, which causes foot rot, is the most serious of these pathogens.”
For control use fungicide treated seed. Follow a crop rotation scheme that provides at least 2 years without peas. Deeply incorporate crop debris immediately after harvest before the fungus can be dispersed by wind or rain.
Pythium Root Rot
“Pythium commonly causes seed rot as well as pre- and post-emergence damping-off of pea. Root rot of older plants also occurs, and often results in root-pruning that significantly reduces root length. Damage is most common in wet soils and is characterized by soft rot. Roots infected with Pythium are typically light brown in color and soft and watery to the touch. Infected plants are frequently stunted and pale green to yellow in color. Although it primarily causes a seed rot, damping-off, and seedling root rot, Pythium ultimum can cause a watery, soft decay of older plants in wet soils at an optimum temperature of 64° to 75°F.”
Rhizoctonia Root Rot
“Rhizoctonia root rot can attack plants at any stage of growth. Seeds may turn dark brown and decay. Water-soaked, then reddish brown to brown lesions form in the seedling epicotyl and hypocotyl. The growing point may die as it emerges from the soil. Seedlings damp-off or recover to produce a normal plant. On older plants, scurfy, reddish brown, sunken lesions form on the underground stem and roots. The stem may be girdled causing severe plant stunting and yellowing. The brown, thread-like filaments (mycelium) of the causal fungus may be seen with a hand lens on the surface of the lesion or canker.”