Disease

Late blight

Phytophthora infestans

The water mold behind the Irish potato famine. In cool, wet weather infected tomato and potato plants decline rapidly and die, and spores spread on the wind for miles. Outbreaks are tracked and reported.

Signs

• Dark, water-soaked spots that grow into large brown blotches with a gray-green edge.
• White fuzzy growth at the edge of the patch on the underside in humid weather.
• Brown to black lesions on stems; shiny, dark or olive blotches on tomato fruit; a dry, corky rot in potato tubers.

What favors it

Cool (60-70 °F, 15-21 °C), damp weather. Spores blow in from infected crops miles away.

Prevention

• Plant certified seed potatoes and destroy volunteer potatoes and cull piles.
• Grow resistant tomato varieties where it occurs.
• Watch outbreak reports (usablight.org in the US) and start protectants when it is near.

Crops it shows up on

Tomato, pepper & eggplant; Potato.

Often confused with

Sources

• Late blight of tomato and potatoUniversity of Minnesota Extension
• Late Blight of Tomato and PotatoUniversity of Maryland Extension

Why →