The spores can remain viable for several hours depending upon environmental conditions, and in warmer and in warmer weather can germinate to directly infect the plant. high humidity/rainfall) thousands of sporangia develop, which then are aerially dispersed several miles. Under ideal environmental conditions (i.e. Sporangia release zoospores that infect leaves How the pathogen spreads Characteristic lesion on tomato plant Sporangia on sporangiophores. leaf) in a damp zip-lock bag for a day to see if white sporulation appears. Therefore, to diagnose late blight under dry conditions, place the infected sample (i.e. Both plants display similar symptoms which consist of dark lesions surrounded by white sporulation or ‘fuzz’ under humid/damp conditions but may appear just as dry dark lesions under dry conditions. Phytophthora infestans infections produce characteristic symptoms that can form on the leaves and/or stems of tomato and potato plants. infestans Lesions on tomato stems with white sporangia Infected tomato fruits Tomato leaves with characteristic brown lesions Symptoms However, once an outbreak occurs the pathogen can rapidly infect surrounding fields. These asexual spores cannot overwinter without plant tissue, so the main sources of inoculum are contaminated plant debris/tubers that were improperly disposed, volunteer plants growing from infected tubers, transplants, and compost piles. The pathogen reproduces asexually in the United States. Late blight is caused by the water mold Phytophthora infestans.
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Due to its ability to travel long distances and destroy entire fields, growers need to know how to diagnose and manage the disease. It causes fruit rot and plant death on tomatoes and potatoes. Late blight is a notorious disease that is often associated with the Irish Potato Famine, yet even today it remains a destructive disease.
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