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Early symptoms of late blight infection include irregular olive or gray water-soaked lesions on the leaves and stems of tomatoes and potatoes. Both crops are vulnerable to this fungal disease at every stage of their growth cycle. A mildew-like white mold may also develop on or near these lesions. The disease spreads rapidly and can kill crops within a week of infection. (Late blight fungus, Phytophthora infestans, was the cause of the potato famine in Ireland in the 1850s.)
Unlike early blight, late blight does not cause the plant’s leaves to drop off; leaves usually remain attached even after they’ve turned brown and dried up. On tomato plants, infected fruits develop large sunken areas. These may not appear until after harvest. On potato plants, infection of leaves doesn't necessarily mean that tubers will be diseased. Tubers become infected by late blight when fungal spores wash down through the soil or tubers come in contact with foliage at harvest.
To prevent this from happening, remove and destroy all potato foliage and wait a week before harvesting to allow the tuber skins to cure. Monitor potatoes in storage for dry brownish rotted areas; disease symptoms may not appear until after harvest.
Wet weather with cool nights and warm days favors the spread of late blight. In warm parts of the United States, the pathogen can overwinter in soil and plant debris. In cold-winter areas, it survives only in potato tubers left in the soil, though the fungal spores move easily and quickly on the wind, spreading northward each growing season.
Last updated: 3/14/19
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