Charcoal Rot of Guayule in Arizona. J. D. Mihail, Department of Plant Pathology, University of Arizona, Tucson 85721. S. M. Alcorn, T. V. Orum, and D. T. Ray. Department of Plant Pathology, and Department of Plant Sciences, University of Arizona, Tucson 85721. Plant Dis. 74:219-224. Accepted for publication 26 September 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0219.

Fifteen experimental lines of guayule were tested for resistance to the charcoal rot pathogen, Macrophomina phaseolina, in greenhouse and field tests. Although no immunity to the pathogen was shown in greenhouse or field tests, charcoal rot incidence in the field was quite low, indicating a high level of field tolerance. We could not confirm previously reported differences among lines. Greenhouse tests of the interaction between saline irrigation water and charcoal rot demonstrated enhanced mortality when tap water was amended with NaCl or KCl at –0.05 to –0.5 MPa. In a field experiment with nonsaline and saline irrigation water, however, no difference was observed in rates of mortality caused by M. phaseolina. The wide distribution of symptomless infection in this field plot suggests a potential for widespread disease should the appropriate environmental triggers occur.