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