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Knowledge to Grow by Karen Weiland - Late Blight

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 1840s. If it is left unmanaged, it can completely destroy potato or tomato crops.

Wet, humid weather encourages the spread of this disease. Fungal spores will travel by wind to infect other potato or tomato crops. It also spreads from contact with infected plants 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.

Potato or tomato 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 ½ inch deep.

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. 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, completely chopping them up, and burying them. Do not put them into your compost pile.

Be certain that potato vines have been dead for 2-3 weeks before harvesting. The application of a fungicide may be continued until the vines are dead. When the foliage dies, so do the spores of the fungus that remains on the leaves. This practice will prevent the infection of the potatoes during harvest and the development of late blight while in storage.

As always, happy gardening!

Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange County.