Root rot of hydroponically grown lettuce caused by Phytophthora cryptogea

Abstract

In April 1989, lettuce (Lactuca sativa L.) plants showing severe root rot symptoms were received for diagnosis from a commercial hydroponic facility in Southern California. A species of Phytophthora was consistently isolated from necrotic roots. Pathogenicity trials were conducted under hydroponic conditions in a greenhouse at root temperatures of 18 and 28 C. Root necrosis, along with stem decay and plant death, occurred within 5-7 days after inoculation at both temperatures. Reisolation of the fungus from roots and stems of inoculated, but not from roots of non-inoculated plants, confirmed pathogenicity. The fungus was identified as Phytophthora cryptogea Pethybr. and Lafferty on the basis of cultural and morphological characteristics. However, crosses with A1 and A2 mating types of Phytophthora cryptogea and other species of Phytophthora were not successful. This is the first report of Phytophthora as a root pathogen of cultivated lettuce.