What is potato or tomato blight? What causes it?
A When gardeners on the West Coast refer to “blight”, they usually mean the disease called “late blight”, caused by Phytophthora infestans. This disease is famous as the cause of the Irish potato famine in the 1800’s. Phytophthora infestans is not a fungus or a bacterium or a virus. It belongs to a group of organisms called “protists”, although they are still commonly referred to as “fungi”. They are also called “water moulds” because they thrive and produce spores under humid, moist environment and cause infection only when free water is present on the plants. Late blight occurs in the Interior too, but “early blight” is more common than late blight in this drier region. Early blight is caused by a fungus called Alternaria solani. Both diseases cause leaf and stem lesions and fruit rot, so are sometimes confused. Although they are called “early” and “late” blight, both diseases can occur at any time from spring to fall if weather conditions are favourable. Early blight generally devel
