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Differential Virulence of Phytophthora parasitica Recovered from Citrus and Other Plants to Rough Lemon and Tomato. M. E. Matheron, Extension Plant Pathologist, University of Arizona, Yuma Agricultural Center, Yuma 85364. J. C. Matejka, Research Assistant, University of Arizona, Yuma Agricultural Center, Yuma 85364. Plant Dis. 74:138-140. Accepted for publication 20 August 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0138.

The relative virulence of Phytophthora parasitica recovered from citrus and other plants to rough lemon (Citrus jambhiri, a citrus rootstock) and tomato (Lycopersicon esculentum) was examined. Isolates of P. parasitica from citrus were highly virulent to rough lemon seedlings, causing crown rot and significant reduction of root weight. Isolates of the pathogen from noncitrus hosts demonstrated low virulence to rough lemon, with no crown rot and only minor reduction of root weight. All tested isolates of P. parasitica were highly virulent to tomato seedlings, causing stem lesions and (usually) plant death. Apparently, isolates of P. parasitica from several noncitrus hosts do not pose a serious threat to citrus orchards. On the other hand, the high susceptibility of tomato plants to P. parasitica from several hosts, including citrus, suggests caution when planting tomatoes in areas adjacent to citrus orchards, other commercial crops, or landscape plantings that may be infested with this pathogen.