Late Blight of Tomatoes & Potatoes

Image of late blight leaf lesions.
Late blight leaf lesions.

Late Blight of Tomatoes & Potatoes

Rainy conditions in late summer favor the development of Late Blight, caused by the fungus Phytophthora infestans, a disease affecting both tomatoes and potatoes.  It's the most devastating disease of tomatoes and potatoes worldwide, killing plants very quickly and spreading rapidly, and was responsible for the Irish potato famine of the 1840's.

Late blight is favored by periods of cool, wet weather.  Night temperatures in the 50's and days in the 70's, along with rain, fog or heavy dew are ideal conditions for disease development.   This perfectly describes the weather conditions eastern Nebraska experienced during the week of August 31- September 4, 2009.  Symptoms of infection are usually seen as pale green, water-soaked spots, circular or irregular in shape, on the leaves that rapidly expand becoming brown to black, killing entire plants or vines.  Black stem lesions also may be seen.  When the weather dries out, infected leaf tissues quickly dry up and the white mold growth on leaf surfaces disappears.

Image of late blight fruit lesions.
Late blight fruit lesions.

Infected tomato fruits develop large, firm, leathery-appearing lesions.  The lesions are brown in the center and may be yellow or goldish at the edges, penetrating deeply into the fruits.  Secondary fungi often enter these lesions resulting in a soft, wet rot.  Infected potatoes develop a shallow, coppery-brown, dry rot that spreads irregularly from the surface through the outer 1/8-1/2 inch or more of tissue.   Secondary fungi often enter infected potatoes, resulting in a slimy breakdown of the entire tuber.  Tubers may be infected in the soil by fungal spores washing down from infected leaves, or at harvest as they come in contact with infected vines. 

Your garden had late blight, so what can you do now? 

If you have not yet harvested your potatoes, allow the vines to lay completely dead in the field for 2-3 weeks before harvesting.  When the foliage dies, spores of the fungus that remain on the foliage will also die.  This will prevent infection of the tubers during harvest.  Harvest during dry weather.  The late blight fungus can only overwinter on living plant tissue, one common method being on infected potato tubers left in the ground.  So make sure you harvest all potatoes and discard diseased tubers.  Do not put any diseased plant material or cull potatoes in the compost pile and do not save seed potatoes for next year from infected crops.

Image of late blight stem lesions.
Late blight stem lesions.

Infected potatoes and tomatoes should be discarded and not used for canning.  The disease can cause changes in tomato acidity, which is critical in safely preserving tomatoes.  Tomatoes that have no lesions, even those from infected plants, can be safely eaten or used for canning. Potatoes showing signs of late blight infection should not be used for home canning.  Discard the whole potato rather than cutting off the disease portions since the fungus may spread to the interior. 

Next year eliminate any volunteer potato or tomato plants that emerge in the garden.  Purchase certified seed potatoes, which although not certified late blight free at least has a lower chance of harboring the disease.  Plant resistant varieties of potatoes, such as 'Allegany', 'Defender', 'Jacqueline Lee', 'Kennebec', 'Onaway', 'Ozette' and 'Sebago'.  Tomato cultivars 'Stupice' and 'Juliette' have shown some resistance to foliar late blight.