Why Are My Potato Plants Wilting?

A wilting potato plant — leaves drooping, stems losing their upright posture — can range in severity from a harmless afternoon response to heat, to the early sign of a serious stem disease that needs immediate attention. The most important thing to do when you see wilting is to check both the time of day and the stem base, because those two observations will tell you most of what you need to know. Do not assume it is just drought before you have looked more carefully.

Drought and heat stress

The simplest and most common cause of wilting is insufficient water during hot weather. Potato plants lose water through their leaves faster than roots can supply it when temperatures are high and soil moisture is low. This wilt typically affects the whole plant fairly uniformly, is worst in the afternoon heat, and the plants often partially recover by morning if soil has some moisture. Feel the soil 15 cm down: if it is dry, the plants need water. Deep, infrequent watering — at least 20 litres per square metre per week during dry spells — is far more effective than frequent shallow watering, which encourages shallow rooting.

Late blight collapse

Late blight caused by Phytophthora infestans can cause a sudden, dramatic collapse of foliage that looks like severe wilting but does not recover even after watering. Check the leaves for the water-soaked brown patches and white fuzzy growth on leaf undersides that characterise blight. In humid conditions blight can progress from the first spotted leaf to complete haulm collapse in less than a week. If blight is the cause, remove affected foliage immediately, do not water from above, and consider harvesting the tubers quickly — blight spores wash down into the soil and infect the tubers.

Blackleg — stem base rotting

If the wilting plant does not recover with watering and the affected plant looks distinctly worse than its neighbours, check the stem base at soil level. Blackleg (Pectobacterium atrosepticum) causes a black, slimy, foul-smelling rot at the base of the stem that cuts off water supply to the plant above, causing wilting that does not respond to watering at all. Affected stems often pull out of the ground easily because the root system has also collapsed. Remove and destroy infected plants immediately. Blackleg spreads through infected seed potatoes and favours wet conditions, so it tends to be worse after a wet spring.

Waterlogging

Counterintuitively, too much water can also cause wilting. Roots in waterlogged, oxygen-deficient soil cannot function effectively and the plant wilts even though the soil is saturated. If the soil has been wet for extended periods and the plant looks pale and limp rather than dark green and turgid, waterlogging is likely. Improve drainage by ridging the rows more steeply, improving the soil structure with organic matter, or in a seriously wet season, harvesting early to save what tubers have formed before they rot.