Late Blight

by Karen Weiland, Advanced Master Gardener



Late blight is a major foliar disease caused by the fungus Phytophthora infestans.  It was responsible for the devastating potato famine that occurred in Ireland in the mid 1800’s.  This disease is not seen very often but if it is left unmanaged, it can completely destroy potato or tomato crops.  Thankfully, for us northern gardeners, the organisms that make up this disease have not overwintered due to our rather cold weather during the winter months.

Wet, cool weather encourages the spread of this disease.  Fungal spores can travel by wind to infect other potato or tomato crops.  It also spreads from contact with infected plants such as tomato transplants or seed potatoes.  To avoid bringing late blight into your garden from infected seed potatoes always buy certified seed potatoes from a reputable grower and never plant potatoes that have blemishes.  Look for varieties that are labeled as resistant to late blight.

Potatoes or tomatoes plants that are infected with late blight will have black spots or lesions on the leaves.  A cottony, white mold growth is usually visible on lower leaf surfaces at the edges of the lesions.  Greasy looking brown lesions may appear on the stems.  Infected potatoes will have brownish lesions on their surface and a reddish brown rot beneath the lesion that can be up to one half inch deep.

It is important to know what late blight looks like.  There are two other diseases that could be mistaken for late blight.  They are septoria leaf spot and early blight. Symptoms of septoria leaf spot usually appear on the oldest leaves after fruit starts to ripen while early blight symptoms usually first appear on the older leaves near the base of the plant with “target” looking, concentric rings.   These diseases will not kill your plants like late blight will.  Sometimes drought-stressed plants can show symptoms that are similar too.

To protect a potato or tomato plant, a fungicide will need to be sprayed on them before the fungal spores reach them.  Choose a fungicide that is directly labeled for potato and tomato late blight.  Organic growers will want to look for a copper soap fungicide and then apply it every 7 – 10 days.  Always be sure to read, understand and follow all label directions.

You should check your plants several times a week for signs of late blight and  more often during wet spells.  Destroy any infected plants by pulling them from the garden, seal them in a trash bag or covering them with black plastic where the sun’s heat will kill the spores.  Never place diseased material into your compost pile.

As always, Happy Gardening!


More information about gardening and related subjects is available online at Purdue University.   The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange Co.,636-2111 in Noble Co., 925-2562 in DeKalb Co. and 668-1000 in Steuben Co.