Late Blight disease is caused by the fungus-like organism Phytophthora infestans and can affect both tomatoes and potatoes. It should not be confused with Phytophthora blight (caused by P. capsici), which affects squash as well as tomatoes and peppers. Late blight has also been reported to affect petunias and nightshades.
Symptoms of late blight include dark, water-soaked lesions on leaves, often with fuzzy white sporulation on the underside. Lesions can also occur on stems. Sporangia (asexually produced spores) are dispersed by splashing water or by wind. Under optimum conditions, three to four days may elapse between infection and the appearance of symptoms.