Late Blight

• Large, irregularly shaped water-soaked lesions on leaves — pale green to brown
• White, fuzzy sporulation visible on the underside of lesions in humid conditions
• Rapid progression — an entire plant can collapse in three to five days under ideal conditions
• Dark brown, firm lesions on stems
• Fruit develops greasy, dark-brown, firm rot that penetrates deep

Phytophthora infestans is an oomycete (water mold), not a true fungus. It thrives in cool (60-70°F), wet, humid weather and can spread explosively. Spores travel miles on wind during wet conditions. The pathogen does not overwinter reliably in cold soil but can survive in potato tubers left in the ground or in compost. It was the cause of the Irish Potato Famine. Garden tomatoes can be wiped out in a week during a late-blight event.

1. 01Plant resistant varieties — many modern cultivars have Ph-2 or Ph-3 resistance genes
2. 02Remove and bag (do not compost) any plant showing symptoms immediately — late blight spreads to neighboring gardens
3. 03Apply copper-based fungicide preventively when late blight is reported in your region or forecasted weather is cool and wet
4. 04Avoid overhead irrigation — keep foliage as dry as possible
5. 05Do not leave potato tubers in the soil over winter in areas where late blight has occurred
6. 06Check your state extension service's late blight forecasting alerts in late summer

If this keeps happening in your garden, the single most effective change is often the seed packet. These varieties carry documented resistance.

Late blight is a reportable plant disease in some states and can affect neighboring farms. If you suspect late blight, contact your local extension office or state plant diagnostic lab immediately — they typically respond quickly and can confirm via lab test, which is worth doing before removing plants.

• Late Blight of Tomato and Potato— University of Minnesota Extension
• Late Blight— Penn State Extension