Why Do My Pea Leaves Have Grey Furry Patches Underneath?

A grey to purple-grey furry or powdery coating on the underside of pea leaves, accompanied by pale yellow or bleached patches on the upper leaf surface directly above, is pea downy mildew — caused by Peronospora viciae. This is a water mould rather than a true fungus, and unlike powdery mildew it favours cool, damp, humid conditions. Spring sowings in wet years are most commonly affected. The disease can spread rapidly under suitable conditions and reduce yield significantly if not managed.

How to confirm downy mildew

Turn a leaf over and look at the underside — the grey, furry, or powdery growth of sporangiophores (spore-bearing structures) on the underside is diagnostic. The upper leaf surface shows corresponding pale, angular, or irregular yellow patches bounded by leaf veins. These are clearly different from the white surface coating of powdery mildew, which occurs on the upper leaf surface. In severe infections, stems and pods may also be affected, and young plants can be stunted or killed. The disease is most active when temperatures are 10–15°C and humidity is high.

Why it spreads in damp conditions

Downy mildew requires free moisture — dew or rain — to release spores and for them to germinate and infect leaf surfaces. Dense plantings where leaves stay wet for long periods after rain or dew are particularly susceptible. Morning sun and good airflow that dries the foliage quickly after rain reduces infection opportunities. Avoid watering overhead in the evening, which keeps foliage wet overnight. Watering at the base of the plant in the morning is much lower risk.

Management in an affected crop

There are no fungicides approved for downy mildew on peas in home gardens. Cultural management is the only option: remove and bin (do not compost) heavily infected leaves or whole plants to reduce spore production. Ensure plants are well supported so air circulates through the canopy. If the infection is severe early in the season, replacing the affected plants with a fresh sowing in a different part of the plot (after a break of two to three weeks for conditions to improve) may be worthwhile. In wet years, downy mildew may be unavoidable — prioritise choosing resistant varieties.