Disease concerns in field peas

Article by Ruth Beck with contributions from Michael Wunsch.

Although field pea production in South Dakota has spanned two decades, acreage has been small enough and rotations sufficiently diverse so that disease issues have not typically been severe.

However a few diseases have appeared from time to time. The most common disease that is seen almost every year at some level in SD is bacterial blight. Bacterial blight is carried on the seed or resides in the soil. The pathogen will colonize on the surface of the emerging plant. If the plant is damaged by wind, hail or late frost, the disease will move into the plant through the wound. Bacterial blight symptoms appear on the above ground plant tissue. Initial symptoms are small shiny watersoaked spots on leaves, pods, and stems. These lesions eventually coalesce, turning brown and necrotic. At times holes will develop in the center of the lesions. See attached picture of pea plant exhibiting minor symptoms of bacterial blight on the lowest leaves.

Best controls include using clean seed, although at this time no one tests for bacterial blight on seed, and crop rotation. Bacterial blight tends to spread under wet conditions, particularly after a frost, wind storms, hail or other event that damages leaf and stem tissue. Many years as the season becomes warm and dry, the bacterial blight remains on the lower leaves and does not appear to spread to the new plant tissue. Damage and yield loss is limited in these situations. Since the causative agent is a bacteria, fungicides are of no value and may kill beneficial fungi. At times, producers use a copper sulphate treatment on the plant. There is little or no verification in the literature that indicates copper sulphate is an effective treatment for bacterial blight on field peas. Research indicates that copper sulphate does not help infected cells, it only impacts surface infections.

The other disease that can affect field peas in South Dakota is powdery mildew. Powdery mildew starts on the older leaves and spreads on the plant often affecting the pods. Leaf spots which are powdery white or gray on the upper surface of leaflets or stipules is an indicator of the presence of powdery mildew. Heavily infected plants do not mature normally. Typically powdery mildew arrives later in the life cycle of field peas, especially in South Dakota. Infections are rarely serious unless they occur early in the pea’s life cycle. Peas which are seeded later can be susceptible and often have more damage.

Most new varieties of field peas are quite resistant to present strains of powdery mildew. Fungicide treatments are rarely economical unless the crop is being grown for seed. For fungicide treatments to be effective they need to be applied in early flowering which is often before the disease can be detected. Decisions should be based on (1) the weather, as powdery mildew is favored by periods of warm and dry daytime conditions, and cool nights with heavy dews and (2) crop value and intended end use.

More information on diseases which affect field peas and on fungicide efficacy can be reviewed at the NDSU Plant Pathology or S.D. Pulse Growers websites.