Potato Late Blight: How to Spot It in Time and Protect Your Harvest

Potato late blight (Phytophthora infestans) remains one of the most dangerous potato diseases in Lithuania – it can destroy most of a crop within days if a wet summer coincides with a delayed response. Knowing how to spot the first signs in time and when to act is one of the most important skills a grower can have.

Which conditions favor the spread of late blight

Late blight thrives on moisture. The greatest risk arises when it rains often, nights are cool, morning dew lingers, and the foliage doesn't get a chance to dry out. According to Lithuania's Integrated Pest Monitoring and Information System (IKMIS), conditions are especially favorable at around 15–21°C with high relative humidity and prolonged leaf wetness. Hot, dry weather slows the disease's spread, but the situation can change quickly after rain or heavy dew.

How to recognize the first signs

The disease doesn't only affect potato leaves – it can spread to petioles, stems, shoots, and even the tubers themselves. Infected tubers develop irregular, variously sized, slightly sunken brownish or purplish-brown watery spots on the surface. Cutting into an infected tuber reveals a yellowish-brown, granular rot. Important: not every brown spot on a leaf is late blight – potatoes are also commonly affected by early blight (Alternaria), which can look visually similar.

Why this isn't just a cosmetic problem

Spores of the pathogen can be washed by rain into the soil, infecting the tubers underground. That means severe late-season foliage infection isn't just a leaf problem – it directly threatens the harvest itself. If foliar infection is noticed too late, it's sometimes wiser to harvest the crop immediately rather than continue spraying to try to save an already heavily infected field.

What to do when you notice the first signs

After noticing a few spots on leaves, there can be significantly more within just a few days – the disease spreads very fast. That's why, during wet periods, it's worth walking and inspecting the field regularly, not only when going to dig the first tubers. Spraying or taking other protective measures makes the most sense while the plant still has something worth protecting – meaning prevention and early response always pay off more than trying to rescue an already heavily damaged crop.

After noticing a suspicious sign, it's essential to inspect the entire potato field – the disease may already be in its incubation stage elsewhere in the crop, even where symptoms aren't visible yet. Following IKMIS warnings for your region is one of the simplest ways to know in time when the risk has increased significantly.