Late blight has become of increasing economic importance on tomatoes in California during the last few years. Previously, the disease, caused by the fungus Phytophthora in/estans, occurred mainly in coastal areas of California where environmental conditions favored its development. The disease may attack plants at any time during the growing season and, when it is severe, all plants in a field may be killed in a week or two. Late blight also results in great losses of tomatoes in transit, storage, and market.