Peas

Several fungi, including Pythium, Rhizoctonia, Botrytis, and Fusarium species, are responsible for these diseases. These soil-borne fungi can attack any part of the root system, extending up to a short distance above the soil surface, and can infect at any time between germination and maturity. 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, wet soil is also detrimental because the cool temperatures slow plant development and cause additional stress to the plants.

If infected, young seedlings may not emerge, and if they do, they will usually appear yellow and stunted, often dying at an early stage.

Several registered seed treatments offer protection to developing seedlings and are especially worthwhile under cool, wet conditions, when emergence may be delayed.

It is worth noting that 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 before seeding. Granular inoculant is typically not affected by seed-applied fungicides.

Aphanomyces species can also cause root rot. 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 to manage among the root rot pathogens and is most aggressive under wet conditions.

Although widespread across Western Canada, powdery mildew is rarely a significant issue in commercial operations, as most modern pea varieties are resistant to the disease.

However, if susceptible pea varieties are impacted, the disease can reduce yields, delay maturity, and reduce uptake of desiccants.

Infection usually begins by mid-to-late July. By this time, early-seeded pea crops have often progressed past the stage of economic impact as pods and seeds are already formed. Late-seeding of susceptible varieties increases the risk of financial loss if powdery mildew infection occurs.

Powdery mildew thrives under warm, dry daytime conditions with cool, dewy nights. Rain showers can expedite the spread of powdery mildew by spreading spores.

Symptoms of the disease include the development of white, powdery spots on the lower leaves and stems, which 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 beneath the powdery infection.

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. If fungicides are required, application should take place at the onset of symptoms.

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

Infection can occur in two ways: via root colonization, where sclerotia germinate into mycelia and infect plant roots, or via apothecia (small, mushroom-like structures) that release ascospores, which spread through the wind or from plant to plant. Because of this, seeding peas near previously infected fields increases the risk of disease spread.

Infection can spread rapidly across a crop, 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 accumulation of sclerotia in a field may infect subsequent broadleaf crops. Once symptoms are noted in a pea crop, it is too late to apply a fungicide. Instead, fungicide application must be based on a forecasted risk.

Downy mildew is a common fungal disease that typically only poses a concern when cool, wet, and humid growing conditions persist. Downy mildew pressure has increased in Alberta in recent years, and this trend appears to be moving into Saskatchewan.

Symptoms include fluffy, greyish growth on the undersides of leaves, and yellow and brown discoloration on the tops of leaves. Systemic infection, resulting from spores in the soil or on infected seed, causes stunted plants and distortion. The secondary disease cycle of plant-to-plant spread originates from these stunted plants and is favoured by moist weather.

While crop rotations of at least two to three years can reduce inoculum in the soil, spores can survive for 10 years or more, allowing the disease to reappear years later. Presently, few fungicide products are registered for disease suppression, but none are registered for complete control of downy mildew.

Other fungal diseases of minor importance in pea 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.

Pea leaf weevils cause the most serious damage during the larval stage, when they feed on nitrogen-fixing root nodules of pulse crops, most notably pea and faba bean. Adult weevils feed on pea seedlings, leaving scalloped or notched leaf margins.

Since adult weevils are difficult to spot, the economic threshold for pea leaf weevil is based on leaf damage. The 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 before the sixth node stage of crop growth.

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 cutworms) 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 be less 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. The best control, when using an insecticide with a contact mode of action, is achieved by applying it in the evening, as cutworms have 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 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 shred the plant tissue below the soil surface. Initially, symptoms may appear as wilting in the central leaves of the main stem, but can eventually lead to the plant’s death. Damage may not be noticed early enough in the season to allow for reseeding. There is no established economic threshold for wireworm in peas, and no in-crop insecticides are available to control wireworm. Controlling wireworm requires the use of insecticidal seed treatment.