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Root Rot of Hydroponically Grown Lettuce Caused by Phytophthora cryptogea. A. R. Linde, Department of Plant Pathology, University of Arizona, Tucson 85721. M. E. Stanghellini, and M. E. Matheron. Department of Plant Pathology, University of Arizona, Tucson 85721. Plant Dis. 74:1037. Accepted for publication 6 September 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-1037D.

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 plants, but not from roots of noninoculated plants, confirmed pathogenicity. The fungus was identified as P. cryptogea Pethybr. & Lafferty on the basis of cultural and morphological characteristics. However, crosses with AI and A2 mating types of P. cryptogea and other species of Phytophthora were not successful. This is the first report of Phytophthora as a root pathogen of cultivated lettuce, although a stem rot of lettuce caused by P. porri Foister has been reported (1).

Reference: (1) P. Sitepu and M. Bumbieris. Australas. Plant Pathol. 10(3):59, 1981.