Ascochyta blight on peas — symptoms and treatment

Severity: high

What is Ascochyta blight

Ascochyta blight is a complex caused by Ascochyta pisi, Mycosphaerella pinodes and Phoma medicaginis var. pinodella. Three closely related fungi, often present together, produce overlapping symptoms — which is why identifying which one you have matters less than managing the group. Its high severity comes from its reach: it attacks every part of the pea plant, travels on the seed, and survives in crop residue, so one infected sowing can seed the problem for years.

Symptoms

Lesions appear as dark brown spots and blotches on leaves, stems and pods. On leaves they are often sunken and clearly bounded; on pods they are dark and sunken, and the fungus can pass through the pod wall to the seed inside. The most damaging phase is on the stem: lesions expand until they girdle it, cutting the plant's supply line, and girdled vines lodge — they collapse into the wet soil where everything worsens at once. At the stem base and just below the soil line a dark foot rot develops. Seed from an infected crop shows discoloration: brown or purplish staining, sometimes shrivelling.

• Early signs: small dark brown spots on the lower leaves and stems
• Advanced signs: stem girdling and lodging, dark foot rot at the stem base, sunken pod lesions, discoloured seed
• Confusable with: other pea leaf spots — foot rot together with stem girdling points to the Ascochyta complex

Causes and conditions

The fungi survive between seasons in two places that matter: infected crop residue on or near the soil surface, and the seed itself. Sowing infected seed puts the disease in the row from day one, and a seedling can be infected before it emerges. From residue, spores are splashed up onto the lower leaves by rain, then move progressively upward through the canopy. Wet seasons are the driver — prolonged rain, extended leaf wetness and dense canopies all accelerate spread, and a wet flowering period is the worst case. Peas grown too often on the same ground build residue and inoculum year on year.

Treatment

Clean seed and rotation — cultural

Timing: pre-planting. Use certified disease-free seed. Rotate for 3-4 years. Bury crop residues by deep plowing. These address both survival routes: clean seed keeps the fungi out of the row, rotation starves the residue-borne inoculum while it breaks down, and burying residue takes the spore source out of splash range.

Chlorothalonil — chemical

Timing: at flowering. Apply fungicide at flowering stage in wet seasons. Seed treatment with thiram also helps. The flowering sprays are protective, keeping the fungus off the pods and out of the developing seed when a wet spell does most damage. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Sow certified disease-free seed and never save seed from a crop that showed symptoms
• Rotate for 3-4 years before returning peas to the same ground
• Bury crop residues by deep plowing so splash cannot lift spores onto new plants
• Space and support plants for airflow, and stay out of the vines while the foliage is wet

Frequently asked questions

Can I eat peas from an affected crop? Yes — sound, unblemished pods are fine. Discard pods with dark sunken lesions and any discoloured seed, and observe the pre-harvest interval for any fungicide applied.

Can I save seed from this crop? No. This is the one thing not to do. The fungi travel inside the seed, so saved seed replants the disease directly into next season's row — the most common way Ascochyta blight persists in a garden.

Why does it come back every year? Because it survives in crop residue and in seed. If peas return to the same ground too soon, or saved seed is used, the source is still there regardless of how well you sprayed.