Bacterial blight is a disease commonly caused by the bacterium Pseudomonas syringae pv. pisi. However, several other bacteria can also cause similar symptoms on peas. The Pseudomonas syringae bacterium survives winter in infected debris on the soil surface or in infected seed. In the spring, bacteria are dispersed from seeds or debris to healthy plants by rain splash, overhead watering, and human activity in the garden when plants are wet. Bacteria may enter plants through natural openings or wounds, such as those created by frost injury.
Bacterial blight initially appears as shiny dark green (water-soaked) spots on the upper and lower leaf surfaces. These spots are irregular in shape and often contained by the veins. Older spots may appear brown, papery, and translucent, with the center of the lesion lighter in color. As the disease progresses, spots may develop on all above ground parts of the plant, including the pods. Occasionally, the bacteria will penetrate the pod and infect the seed. However, infected seed usually remains symptomless. Severely infected plants may drop blossoms or young pods, turn brown, or die.
To prevent bacterial blight, plant commercially grown disease-free seed. Do not save seed from infected plants, even if pods appeared healthy. If bacterial blight occurs, be sure to remove and discard all debris in the fall. Rotate crops yearly if possible. Water plants at the base and properly space plants to promote drying of leaves. Do not harvest or work in the garden while leaves are wet.
Ascochyta blight is a disease complex involving three different fungi, Ascochyta pisi, Phoma medicaginis var. pinodella (A. pinodella), and Mycosphaerella pinodes (A. pinodes). All of these fungi survive winter in plant debris or enter the garden in the spring on infected pea seeds. Spores produced in the spring are rain-splashed or windblown to healthy plants, where they cause new infections. Adequate moisture is required for spore release and infection.
The symptoms of Ascochyta blight vary with the fungus causing infection. However, plants generally show blackening of the stem from the soil line to a height of 15 cm. Plants exhibit yellow foliage, brown spots on leaves and stems, and bud drop. Pods and seeds can also be infected. Severe infections may kill seedlings. Phoma medicaginis usually infects the stem near the soil line causing lateral roots to die and lesions to develop on the stem at or below the soil line. Ascochyta pisi infects the leaves, stems, and pods causing slightly sunken, tan to brown lesions with distinct dark borders. Mycosphaerella pinodes can also severely infect aboveground portions of pea plants. Dark flecks on the leaves, pods, and stems eventually enlarge into a concentric ring pattern with alternating rings of brown and tan.
Ascochyta blight may be managed by removing and disposing of diseased plants and by planting commercially grown disease-free seed. Remove infected plants as soon as the disease appears and the remainder of the plants after harvest. Rotate peas with non-susceptible crops yearly. Currently there are no resistant cultivars and no registered fungicides for Ascochyta blight.
Seed Decay, Damping Off, and Root Rot
Seed decay, damping off, and root rot are caused by a number of fungi including Rhizoctonia solani, Pythium sp., Fusarium sp., and Aphanomyces euteiches. Cool, wet soils favor seed decay and damping off, causing seeds to become soft, rotted, and dark colored. Infected seedlings may fail to emerge or collapse after they emerge due to sunken stem lesions. Older seedlings may also fail, as they develop a root rot. Root rot is a common problem when peas are planted in heavy, wet soil. Plants infected by root rot fungi may appear yellow, stunted, wilted, or dead. Symptoms may be less apparent in cool, damp weather, but plants often die quickly once the weather turns hot and dry.
Infected roots may appear brown, black, or red. Often the outer layer of root tissue easily pulls away from the inner root (stele). Some infections may cause lesions to form, but most simply rot the root causing soft, mushy roots to develop. All of these fungi overwinter in the soil and in infected plant material. Fungi are dispersed from plant to plant be direct contact, water, and soil movement.
To minimize damping off, seed decay, and root rot, purchase commercially grown disease-free seeds. Seeds pre-treated with a fungicide will also help minimize these diseases. Choose cultivars with smooth seeds and darkly pigmented seed coats since they tend to be more resistant. Rotate crops and plant in well-drained soil. Space plants properly and avoid over- watering.
Hagerdorn, D.J. 1984. Compendium of Pea Diseases. APS Press, St. Paul, MN.
Howard, R.J., Garland, J.A., and Seaman, W.L. 1994. Diseases and Pests of Vegetable Crops in Canada. The Canadian Phytopathological Society and Entomological Society of Canada. pp 202-204.
Sherf, A.F. and MacNab, A.A. 1986. Vegetable Diseases and Their Control. 2nd ed. John Wiley & Sons, New York. pp 471-475, 481-487.
Chad Behrendt, Crystal Floyd