This article appeared in the Summer 2015 issue of Pulse Crop News.

The ascochyta blight complex is an important field pea disease that occurs in virtually every pea field each year. The blight’s lesions reduce optimal leaf and stem area required for maximum photosynthetic activity, leading to reduced grade and productivity. When an epidemic establishes itself early, considerable seed yield loss can occur. Infected crop residue is the primary source of infection and airborne disease spores can travel a considerable distance. Given that several areas of the province received higher than average moisture conditions in recent years, the blight will have established itself at a heightened level in certain regions of the province. Fields seeded to pea in recent years require extra scouting diligence.

The Mycosphaerella pinodes species of the complex causes the most economic damage. Ascospores are produced via Mycosphaerella spore development on previous crop residue which spread to nearby plants via rain splash or are wind-blown for several kilometres. Frequent showers favour the blight and humid conditions in the lower canopy can lead to symptoms showing up on lower leaves, branches and the stem.

Initial symptoms appear as small, purplish-brown spots or flecks, which may enlarge and coalesce, potentially covering an entire leaf. Severe symptoms can lead to foot rot manifesting itself as stem girdling near the soil, leading to lodging and harvest difficulty. Prolonged moist conditions or lodging can cause symptoms to develop on pods. These symptoms may become extensive and lead to early pod dry-down causing seed quality loss in the form of small, shrunken or discoloured seeds.

There are no field pea varieties available with ‘good’ resistance to this disease. Most varieties have ‘fair’ resistance and a couple have ‘poor’. Fungicides can be effective if applied at the early flowering stage, however, thorough canopy penetration and good leaf coverage are essential. Begin scouting at the early vegetative stage and continue into the early flower stage to realize if symptoms are moving upward in the plant canopy. If symptoms don’t move higher than the lower one third of the canopy by the flowering stage, large yield losses are not expected.

A fungicide application may be necessary if the following conditions are present:

• the crop canopy thickness is moderate to heavy,
• 50 per cent of the bottom 1/3 of the canopy has symptoms and the symptoms appear to be moving into the middle 1/3 of the canopy;
• the weather has been humid and rain is in the forecast; and
• high yield is expected, justifying fungicide cost.

The field pea ascochyta scoring system developed by Ken Lopetinsky and Sheri Strydhorst is a helpful tool to determine whether a fungicide application is necessary. It appears below and on APG’s website.