Pea crops are subject to a number of diseases that can reduce yield and quality. Infection can come from a variety of sources. Seed-borne, soil-borne and residue-borne diseases can be minimized through preventative management. Disease prevention recommendations include:
1. Use of effective crop rotations. Plant pea only once every four years in the same field. Continuous production of broadleaf crops can increase some seedling diseases and sclerotinia.
2. Use of the best seed available. A seed test will indicate the presence of seed-borne diseases.
3. Use of a registered fungicide seed treatment may be warranted, especially if seeding early into cool wet soils.
4. Use varieties with disease resistance, such as powdery mildew resistance.
5. Early seeding.
6. Monitoring of fields for diseases.
Mycosphaerella blight: this fungal disease is the most common and economically significant disease of pea in Saskatchewan. It is one of the ascochyta diseases often referred to as the Ascochyta disease complex. Three ascochyta species infect pea and are hard to distinguish from each other. In Saskatchewan, the most common species is Ascochyta pinodes. The sexual stage of this species is Mycosphaerella pinodes and is the reason this disease is also referred to as mycosphaerella blight. Losses attributed to this disease have been reported as high as 80 per cent.
The other fungi involved in this complex are Ascochyta pinodella, causing foot rot and Ascochyta pisi causing leaf and pod spot. Ascochyta pisi has been considered a minor problem in the past, but appears to becoming more common the last few years.
Most Ascochyta symptoms observed in the field will be those of mycosphaerella blight. Early symptoms first appear as small, purplish-brown, irregular shaped spots on lower leaves, stems and pods. These spots grow together and spread up the plant. Infection at the base of the plant may lead to girdling of the stem near the soil line, which is known as foot rot. Girdling weakens the stem and often results in premature lodging. The impact on yield depends on the timing of initial infection and on weather conditions. Early infection followed by wet conditions lead to higher potential losses.
Infested crop residue is the primary source of infection. Infected seed is not considered a major source of inoculum, but may play a role in introducing the disease to new areas. Mycosphaerella pinodes is the only ascochyta species that forms a sexual spore stage. Those spores result in airborne ascospores allowing transmission of the disease over longer distances. As well, the fungus is capable of living in the soil for long periods of time. This means extended crop rotations do not guarantee low levels of this disease, although it is still an important component of managing ascochyta and other diseases. No commercial pea varieties are resistant to mycosphaerella blight but some are more susceptible than others.
Scouting early and monitoring the disease is critical for control. If symptoms move upward in the plant canopy beyond the lower third of the plant, fungicide application may be warranted. Maximum effectiveness of the fungicide occurs if applied at early flowering. Thorough penetration of the canopy with good leaf coverage is essential. Fungicides work by preventing infection of healthy green plant material. It will not repair plant damage that has already occurred. The timing of infection, yield potential of the crop, weather conditions and value of the harvested seed will all influence the economics of fungicide application. To help growers with decisions on risk of losses due to ascochyta or benefits of fungicide, there is an aschocyta scoring system developed by Alberta Agriculture and Rural Development.
Seedling blight, root rot, and seed rot: 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 pea. 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 and the Aphanomyces euteiches is the one that has been identified across most of Saskatchewan. 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.
Powdery mildew (Erysiphe polygoni [pisi]): although widespread across Western Canada, powdery mildew is now rarely of significance because most commonly grown varieties of pea are resistant to this disease. Without this built-in resistance, yield losses can be high (in excess of 60 per cent).
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 actually 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 underside 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.
Sclerotinia stem rot (Sclerotinia sclerotiorum): this disease attacks many broadleaf crops and is usually more severe on sunflower, dry bean, and canola, compared to pea. 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. Firstly, when in close contact with the pea root, sclerotia may germinate and cause infection at the base of the plant. Secondly, 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 pea 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 later broadleaf crops. There are fungicides registered for the control of sclerotinia in pea. Once symptoms are noted in a pea crop, it is too late to apply a fungicide. Fungicide application has to be based on a forecast of risk.
Downy mildew (Peronospora viciae): this 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. In 2009, field disease surveys showed this disease occurred in 30 per cent of the pea fields included in the survey. Symptoms are fluffy, greyish growth on the undersides of leaves and can 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 not completely eliminate it as spores can be long lived in the soil, sometimes over 10 years. Presently, there are no fungicides registered to control downy mildew.
Other Diseases: other fungal disease of minor importance include bacterial blight, anthracnose, septoria blight and rust. Virus-like diseases can also occur in pea. With the exception of one disease, pea seed borne mosaic virus (PSbMV),they are spread by aphids.
For more information on disease of pea refer to Diseases of Field Crops in Canada available from the University Extension Press, University of Saskatchewan at (306) 966-5565.
Managing Fungicide Resistance
As with herbicides and weed resistance, managing fungicide use to prevent or slow the development of fungicide resistance is in the long-term interests of all pea growers. Use foliar fungicide only when disease risk and potential loss are considered to be economically damaging (i.e. greater than the cost of control).
Development of resistance of several fungal pathogens to the strobilurin group of fungicides in other crops has been reported in Europe and in Saskatchewan, and is of great concern. No more than two applications per year of any strobilurin fungicide should be made to the same field, as disease resistance could develop. The continuous use of strobilurin fungicides without fungicide rotation greatly increases the threat of disease resistance.
Any fungal pathogen population may contain some strains naturally insensitive to various fungicides. A gradual or total loss of disease control may occur over time if these fungicides are used repeatedly in the same fields.
The following strategy should be considered and implemented to delay fungicide resistance/insensitivity:
• Use a fungicide rotation - rotate the use of fungicides with others from different groups that control the same pathogens.
• Tank mix fungicides that have a high risk of developing insensitivity with other fungicides from a different group.
• Do not apply more than the maximum number of applications listed on the label. Avoid consecutive sprays of the same fungicide, or other fungicides in the same group, in a season. Fungicides belonging to the strobilurin group should not be applied more than twice a season in the same field.
• Fungicide application should be based on an integrated pest management (IPM) program that includes scouting and accurate recording related to pesticide use and crop rotation.
• Monitor treated fungal populations for signs of fungicide insensitivity. If disease continues to progress after treatment with a product, do not increase the use rate. Discontinue use of the product and switch to another fungicide with a different target site of action.