First Report of Pythium aphanidermatum Infecting Guayule. J. D. Mihail, S. M. Alcorn, and P. J. Thrapp, Department of Plant Pathology, University of Arizona, Tucson 85721. Plant Disease 69:177, 1985. Accepted for publication 15 October 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-177c.
Pythium aphanidermatum (Edson) Fitzp. was recovered from rotted roots of field-grown guayule (Parthenium argentatum Gray) in Maricopa, Pima, Pinal, and Yuma counties of Arizona from 1980 to 1984. The disease occurred most frequently (26 of 28 instances) between June and October on plants 2–24 mo old. Initial root symptoms were tan to brown lesions on lateral roots. As the disease progressed, cortical tissue of lateral and eventually primary roots rotted and root piths blackened. Aerial symptoms included wilt, stunting, branch dieback, and eventually death. In five of the 28 instances, both P. aphanidermatum and Macrophomina phaseolina (Tassi) Goid. were isolated from the same naturally infected plant. Koch's postulates were completed with P. aphanidermatum in two growth-chamber and five greenhouse tests with seedlings 4–8 wk old. This disease is a potential constraint to guayule production in hot, arid climates, where copious water is used to establish stands and/or drainage is poor.