Recognizing late blight in potatoes and tomatoes

Our spring weather was perfect to encourage the fungal disease late blight that infects tomatoes, potatoes and sometimes eggplants and peppers. This disease caused the great potato famine in Ireland in the 1840s to 1850s by killing almost the entire potato crop. It thrives in high humidity with temperatures around 45 to 60 degrees at night and 70 to 85 degrees during the day, very much like our spring.

I recently heard on National Public Radio that late blight has seriously damaged crops in the northeastern United States this year due to all their wet weather. Entire fields can be lost in a few days.

Even though it is usually dry here, tomatoes and potatoes in Nevada can also get late blight. In fact, if the disease were to take hold here, northeastern Nevada's commercial potato crop could be seriously damaged at a great financial loss to the growers.

What does late blight look like? Pale green, bluish-gray or brown areas that look water-soaked develop on leaves. A grayish-white fungus may grow on the undersides of leaves when the weather is wet, but in dry weather the infected leaves dry up very quickly and the mold disappears. The leaves dry up, shrivel and eventually die.

The water-soaked spots may also occur on stems and fruit. On fruit, the spots turn brown, wrinkle and become leathery or cork-like. Infected fruit is inedible. Dying plants may smell bad. The plant can be infected at any stage of growth.

Late blight usually enters the garden on infected transplants, from air or water-borne spores from nearby infected plants, on infected equipment or on infected potato seed pieces.

The disease organism produces millions of spores. Spores overwinter in infected tubers left in the soil or on infected plant debris left in the garden or the compost pile.

Since this disease can wipe out your entire tomato and potato crop quite rapidly, it is important to buy only certified seed potatoes or healthy tomato transplants for planting, preferably ones that are late blight resistant varieties.

Overhead irrigation stimulates disease development. Fungicides work as protectants with repeated applications, but only if late blight is listed on the label. Always follow the label directions.

If you suspect your plants are infested, it may be best to remove and destroy them. Don't compost them. Because of the various diseases potatoes and tomatoes are susceptible to, I never compost either of these plants.

The old saying "an ounce of prevention is worth a pound of cure" definitely holds true for late blight.

JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at skellyj@unce.unr.edu or 887-2252.