Field pea has been an important crop for Saskatchewan producers for over three decades.  Pea production has many benefits when grown in a crop rotation, such as producing part of its own required nitrogen, providing a diversified crop rotation, and interrupting pest cycles in other crops. There are a few disease restraints to field pea production in Saskatchewan. 

Field pea is susceptible to diseases such as damping off, seedling blights and root rots (Pythium, Rhizoctonia, Botrytis, Fusarium spp.), stem and pod rot (Sclerotinia sclerotiorum), powdery mildew (Eryisphe pisi), bacterial blight (Pseudomonas syringae pv. pisi), and ascochyta blights (Ascochyta spp.).  Of these, the most prevalent disease is mycosphaerella blight, which is one of the ascochyta diseases (Figure 1).

There are three different fungi responsible for the ascochyta blight complex, and these occur in all pea-growing regions of the world.  The most common fungus in western Canada is Ascochyta pinodes, which can affect roots, leaves, stems, flowers and pods.  The sexual stage of this fungus is Mycosphaerella pinodes, which is why the disease is also referred to as mycosphaerella blight. 

The two other fungi involved in the ascochyta blight complex of field pea are Ascochyta pinodella (synonym: Phoma medicaginis var. pinodella), causing foot rot, and Ascochyta pisi, causing leaf and pod spot.  Ascochyta pisi has not been known to cause significant yield loss, but has become more common in the field and in harvested seed in the past five years.  Researchers continue to study the various ascochyta fungi and work towards finding varieties with improved resistance to disease.

Ascochyta blight is found at some level in pea crops in most years in Saskatchewan, but not usually at levels that warrant a fungicide application, except in seed crops.  Yield losses of five to 15 per cent are common in regions where the disease is established and if wet conditions continue during pod development.  Studies have reported yield losses as high as 50 per cent in susceptible varieties, but such losses are rare in commercial fields.  It is estimated that for every 10 per cent of infected stem area, about five per cent yield loss occurs.

Figure 3. Ascochyta foot rot on field pea. Photo by Saskatchewan Agriculture

Early symptoms are most commonly observed under the plant canopy, on lower leaves, stems, and tendrils, where conditions are more humid (Figure 2).  Symptoms first appear as small, purplish-brown, irregular flecks.  Under continued humid conditions, the flecks enlarge and coalesce, resulting in the lower leaves becoming completely blighted. 

Severe infections may lead to girdling of the stem near the soil line, which is known as foot rot (Figure 3).  Foot rot lesions are purplish-black in colour and may extend above and below the soil line.  Foot and stem lesions girdle and weaken the stem, leading to crop lodging and yield loss.

Disease lesions develop on pods under prolonged moist conditions or if the crop has lodged.  Pod lesions are initially small and dark, but may become extensive and lead to early pod senescence (Figure 4).  Severe pod infection may result in small, shrunken or discoloured seed; or alternatively, seed may show no symptoms (Figure 5). 

Lesions caused by the less common Ascochyta pisi fungus are slightly different from the more prevalent M. pinodes.  Ascochyta pisi lesions are typically tan or brown in colour and have a distinct dark brown margin (Figure 6).  Small, dark fruiting bodies called pycnidia are usually visible within the lesion.  Pycnidia will also develop in lesions caused by M. pinodes, but will be less obvious as they blend into the already dark lesion.

Early infections from seed- or soil-borne inoculum can cause lesions on seedlings at the soil line (Figure 7).  Depending on growing conditions and severity of infection, the infected seedling may develop foot rot and be less vigorous, or recover and produce a viable plant.  However, the importance of infected seed in establishing disease in pea crops has not been fully determined. 

Mycosphaerella pinodes is the only species that develops a sexual spore stage on infested residue.  This stage results in the production of wind-blown ascospores.  Ascospores can be dispersed for several kilometres, limiting the effectiveness of crop rotation as a disease management tool.  Ascospore release begins in the spring and can continue into the summer if moist conditions prevail. 

All of the Ascochyta species have an asexual spore stage which is responsible for short-distance spore dispersal during the growing season.  Pycnidia release conidia (spores) which are rain-splashed to surrounding plants.  Symptoms start to appear as early as two to four days after dispersal and initial infection.

Ascochyta blight is favoured by wet weather, particularly frequent showers.  The optimal temperature for infection and lesion development is around 20°C.  If the canopy remains dense and wet into the flowering stage, lesions will continue to develop on lower leaves and stems.  In the absence of rain, both spore dispersal and lesion growth will be slowed or completely arrested. 

Crop rotation is also a valuable tool to control pea diseases such as seedling blights, root rot and sclerotinia stem and pod rot.  Even though the Ascochyta species infecting field pea will not infect chickpea or lentil crops, it is still advised not to plant pulses more than once in four years in the same field to reduce other diseases.

2. Stubble Management:  Zero or minimum tillage does not appear to affect the development of ascochyta blight in field pea.  Weather conditions and crop rotation are the most important factors determining disease risk.  Stubble management practices such as straw-chopping during combining, or harrowing to spread out residue on the soil surface, can help speed the decomposition of pea residue.

3. Variety Selection:  Refer to Saskatchewan Agriculture and Food's Varieties of Grain Crops for a current listing of disease and lodging ratings for field pea.  There are no varieties with "good" resistance to ascochyta blight.  However, some varieties are rated as having "fair" resistance to the disease and are less likely to exhibit lodging or yield loss if infected.

4. Agronomics:  Yields tend to be higher for field pea planted early in the spring.  However, if seed is planted too early or deep when soils are cold, the emergence rate will be reduced and plants will be more susceptible to stresses such as soil- and seed-borne diseases. 

Dense pea stands favour disease by reducing air movement within the canopy and holding humidity near the lower stems where stem and foot lesions develop.  Aim for the target plant population of 86 plants per m2 (eight per ft2).  Seeding rate will change according to seed size, so it is important to calculate seeding rate based on percentage germination and 1000 seed weight for the specific seed lot.  For information on this calculation, refer to the provincial publication Dry Pea in Saskatchewan.

Furthermore, agronomic practices that maintain an upright crop, such as choosing a variety with a "good" lodging rating and avoiding fields with excess nitrogen, help reduce the impact of disease by preventing lodging.

5. Seed Quality:  Plant the best seed available to reduce the risk of disease-related losses.  Seed should have a high level of germination and a low level of ascochyta blight and other diseases.  Growers are recommended to have their seed tested at an accredited laboratory for levels of germination and seed-borne diseases.  Labs will plate out pea seed on a growth medium to determine the percentage of seed that is infected with seed-borne fungi (Figure 8). 

Although the seed-to-seedling transmission rate of the ascochyta fungus is low in field pea, it is still advised to plant seed with less than 10 per cent ascochyta infection.  This level should not significantly affect plant establishment and yield as long as soil conditions promote quick germination.  Some seed lots may have as high as 20-40 per cent infection if produced under conditions favouring disease.  Planting highly infected seed reduces germination and vigour, resulting in poor stand establishment and reduced yield.  If better quality seed cannot be sourced, seed with infection levels greater than 10 per cent should be treated with a fungicide seed treatment.  In regions where infested field pea residue is common, planting disease-free seed becomes less critical. 

For more information on pulse seed quality, refer to the Saskatchewan Agriculture publication Guidelines for Seed-borne Diseases of Pulse Crops.  

6. Seed Treatments:  Seed treatments provide protection against both seed- and soil-borne diseases.  The products registered for seed-borne ascochyta on pea are Apron Maxx RTA® and Vitaflo 280®.  A number of other seed treatments are registered for use in pea against seed rot and seedling blight caused by Pythium, Botrytis and Fusarium species.  For a complete list of seed treatments registered in field pea, refer to Saskatchewan Agriculture's Guide to Crop Protection.

7. Scouting:  It is important to scout for early symptoms as well as determine the economics of a fungicide application.  Accurate identification of ascochyta blight is critical.  Begin scouting for disease during the vegetative stage.  Continue to scout into the early flowering stage to observe whether disease symptoms are moving upwards in the plant canopy and are present on tendrils and flowers.

If symptoms do not move beyond the lower third of the plant canopy at the flowering stage, risk of yield loss is low and fungicide should not be applied.  

Fungicide control may be warranted if:

1. At least 50 per cent of the bottom third of the crop canopy is showing symptoms and symptoms are progressing into the middle third of the canopy; AND
2. The weather has been humid and rain is in the forecast; AND
3. A high yield is expected that would justify the cost of the fungicide. 

Pea varieties rated as having "fair" resistance to ascochyta blight rarely benefit from a fungicide application.   Regardless of yield benefits, seed growers may want to make a fungicide application to protect the quality of the seed.

8. Foliar Fungicides:  The foliar fungicides registered for use in field pea for the control of ascochyta blight include Bravo 500®, Headline EC®, Lance®, and Quadris®.  The ideal timing for fungicide application in field pea is at early flower.  Foliar fungicides work by protecting the healthy green plant material but will not repair plants affected by foot rot.  It is necessary to use high water volumes to ensure thorough coverage of leaves and to penetrate the plant canopy as much as possible.  

It is essential to calculate the potential economic return from a fungicide application for your specific situation before applying a fungicide.  The cost of fungicide and application can range from $14 to $30 per acre.  Depending on pea prices and expected yield, a yield response as a result of a fungicide application would have to be at least 10 per cent in order to be economical.

Refer to the Saskatchewan Agriculture publication Guide to Crop Protection for a complete list of foliar fungicides registered in field pea. 

Researchers with Agriculture and Agri-Food Canada in Saskatoon investigated a fungicide decision support system for ascochyta blight management in field pea.  Their research found that after 23 site-years, fungicide application was rarely economical due to the late onset of disease, variable yield potential, and low crop prices. 

More recently, Alberta Agriculture and Food has worked to develop a disease prediction system for ascochyta blight in field pea.  The project began in 2005 and results have been promising.  Further field-proofing of the prediction system is now underway.  For more information on AAFRD's fungicide decision support system, visit their website at:  http://www.agric.gov.ab.ca/

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