Peas

Are caused by several fungi including Pythium, Rhizoctonia, Botrytis, and Fusarium species. These soil-borne fungi can attack any part of the root system up to a short distance above the soil surface and can attack any time between germination and maturity. Young seedlings may not emerge, and if they do, they will usually appear yellow and stunted, often dying at an early stage. These fungi are common in the soil, and infection is more likely if the soil around the seed is excessively wet. Warm and moist conditions generally favour these diseases, but cold and wet soil is also detrimental because the cool temperatures slow plant development and add additional stress to the plant.

Numerous products are registered for seed treatment in peas. Seed treatments offer protection to the developing seedling, especially under cool, wet conditions when emergence may be delayed. Seed treatments can reduce the viability of nitrogen-fixing Rhizobium inoculants. Fungicide seed treatments should be applied, allowed to dry, and then the inoculant should be applied just prior to seeding. Granular inoculant is not typically affected by fungicides.

Root rot can also be caused by Aphanomyces species. Aphanomyces euteiches has been identified across most of Saskatchewan. Although some seed treatments are rated for suppression there are no seed treatments that control A. euteiches and it can survive in the soil for many years. It is the most difficult pathogen among the root rot pathogens and is most aggressive under wet conditions.

Although widespread across Western Canada, powdery mildew is now rarely of significance because most grown varieties of peas are resistant to this disease. Without this built-in resistance, yield losses can be high (more than 60%).

Infection of susceptible pea varieties usually begins about mid-to-late July. By this time pea crops seeded in early spring have often progressed past the stage of economic impact, since pods and seeds are already formed. Delayed seeding of susceptible varieties increases the risk of an economic impact.

Powdery mildew thrives under warm and dry daytime conditions with nights that are cool enough to cause dew formation. Rain showers disrupt the spread of powdery mildew. Symptoms include the development of white, powdery spots on lower leaves, and stems that can quickly spread to the entire plant. Severely affected crops are covered in a white mat of powdery spores and may appear to have a bluish or silvery sheen. The undersides of infected leaves turn yellow below the powdery infection. The disease can reduce yield, delay maturity, and reduce uptake of desiccants. If fungicides are required, application should take place at the onset of symptoms.

Variety resistance is the best management strategy. Varieties can no longer be registered in Canada for production in Western Canada unless they have powdery mildew resistance.

Sclerotinia stem rot attacks many broadleaf crops and is usually more severe on sunflowers, dry beans, and canola, compared to peas. It overwinters as sclerotia, small, black resting bodies in the soil, apart from the host. Sclerotia may remain viable for three to five years.

Infection can take place in two ways. When in close contact with the pea root, sclerotia may germinate and cause infection at the base of the plant. Under a dense plant canopy, sclerotia will germinate and develop tiny mushroom like structures that produce spores. These spores colonize dead plant material such as fallen flower petals or hail damaged leaves. Spores can be scattered by the wind, so planting peas next to previously infected fields can assist in spreading the disease.

Once infection has occurred, it can spread very quickly by plant-to-plant contact, especially when there is moisture under a heavy crop canopy. If infection occurs late in the growing season, there may be little effect on yield, however, the buildup of sclerotia in a field may have a negative impact on subsequent broadleaf crops. There are fungicides registered to protect the pea plants from sclerotinia infection. Once symptoms are noted in a pea crop, it is too late to apply a fungicide. Fungicide application must be based on a forecast of risk.

Downy mildew fungal disease is common but does not usually cause serious losses unless cool, wet, and humid conditions exist. Alberta has been having increased problems with this disease, and it is increasing in importance in Saskatchewan. Symptoms show fluffy, greyish growth on the undersides of leaves that may cover the entire leaf underside surface. The top of the leaf becomes yellow and brown opposite to the infected area. Systemic infection, resulting from spores in the soil or on infected seed, causes stunting and distortion. The secondary disease cycle of plant to plant spread originates from these stunted plants and is favoured by moist weather. A two-to-three-year rotation will reduce the soil-borne inoculum but will not completely eliminate it as spores can be long lived in the soil, sometimes over 10 years. Presently, there are a few products registered for suppression, but no fungicides registered to control downy mildew.

Other fungal diseases of minor importance include bacterial blight, Anthracnose, Septoria blight, and rust. Virus-like diseases can also occur in peas. Except for pea seed borne mosaic virus (PSbMV), they are spread by aphids.

The pea leaf weevil has been expanding its infestation eastward across southern Saskatchewan. The grey adult weevil feeds on pea seedlings leaving scalloped or notched leaf margins in seedlings. The adult is difficult to observe, therefore economic thresholds are determined by the severity of notches in plants at various points in the field. Usually, pea plants will survive this defoliation, however, adult females will lay large numbers of eggs at the base of pea plants. The larvae hatch and burrow into the soil causing more serious damage when they feed on nitrogen fixing nodules on the roots of the plant. Research at AAFC-Lethbridge, Alberta, suggests the best yield response from insecticides results from the use of a seed treatment. If a foliar application is required, the current threshold for insecticide application is one notch per three clam leaf pairs (the most recently emerged leaves), or one-third of plants exhibiting feeding damage prior to the sixth node stage of crop growth.

The pea leaf weevil produces one generation per year. The adult (light grayish brown colour with three stripes on the back and on the wings) overwinters in alfalfa, other perennial legumes, or tree shelters. In May through June, eggs are laid on-or-near developing pea plants. The adult moves (up to a few kilometers) mainly by flying when temperatures are above 17°C but can also walk short distances. The larvae hatch in one to three weeks, and feed on the nodules on the roots. Once the larval stage is complete, the insect pupates and emerges as an adult once again in late July through September. It will search for any legume plants, including peas and other pulse crops, and continue to feed until it finds an overwintering spot.

Cutworms can cause damage to newly emerged pea seedlings, and at times may require insecticide application. Below-ground feeding cutworms (pale western and red-backed cut worms) cut plants off at-or-near the soil surface. Foliar feeding cutworms (dingy and army cutworms) feed above-ground, consuming the plant foliage. Pea crops can often recover from cutworm damage if cool, moist growing conditions occur. However, plants are set back and may not be competitive.

Fields should be monitored from late May to late-June. The economic threshold for cutworms is two to three per square metre (0.2 to 0.3/ft²) in the top 7.5 cm (three inches) of soil. Insecticides that are currently registered for the control of cutworms are listed in the Guide to Crop Protection. Best control, if using an insecticide with contact mode of action, is achieved by applying in the evening, due to cutworm nocturnal feeding habits.

Peas are not a preferred food source for grasshoppers, but they will feed on pea plants if no other food source is available. Damage is most likely to be seen along ditches and road allowances. Usually, grasshopper infestations of 10/m² and less do not cause economic losses in peas. Insecticides are registered for the control of grasshoppers in peas.

This insect tends to be more abundant in moist soils and in lower, damper areas of a field. They are the immature stage or larvae of click beetles. Although wireworms prefer grassy plants (cereal crops), wireworm damage has been noted in other crops including canola, chickpeas, and potatoes. Wireworms tend to shred the plant tissue below the soil surface. Initially, symptoms may show up as wilting in the central leaves of the main stem but can eventually cause death of the plant. Damage may not be noticed early enough in the season to reseed. There is no established economic threshold for wireworm in peas and there is no in-crop insecticide available to control wireworm. Controlling wireworm requires the use of insecticidal seed treatment.