Systemic Fungicides and the Control of Oomycetes

Abstract

Diseases caused by oomycetous fungi of the order Peronosporales present major problems worldwide. Important foliar diseases include late blight on potatoes, blue mold on tobacco, grape downy mildew, plus a wide range of other foliar blights and downy mildews on cereals, fruits, vegetables, and ornamentals. Soilborne Phytophthora and Pythium spp. are equally wide­ spread and are the cause of major losses on crops as diverse as soybeans (148) and avocados (33, 51). In addition, Phytophthora and Pythium spp. are responsible for many preand postharvest problems on fruits and vegetables, including late blight of potato tubers (10), brown rot of citrus (35-37, 94), and black pod of cocoa (125, 126). Less than a decade ago our means of adequately controlling many of these diseases was extremely limited. Control of potato late blight and downy mildews involved repeated applications of protectant fungicides, starting well in advance of the general appearance of the disease. The control of many soilborne pathogens was even more limited. Fumigation with methyl bromide gas, or soil drenches with nematicides, offered only limited control for some high-value annual crops. Soilborne pathogens of field and perennial tree crops were not controlled by chemical methods, because of either cost or lack of efficacy of available chemicals. However, the discovery of systemic fungicides with good activity against Oomycetes in the early 1970s continues to revolutionize our concepts of