VIEW ARTICLE

Ecology and Epidemiology

Relationship of Soil Cations to the Distribution of Phymatotrichum omnivorum. J. P. Mueller, Department of Plant Pathology, University of Arizona, Tucson 85721; R. B. Hine(2), D. A. Pennington(3), and S. J. Ingle(4). (2)Department of Plant Pathology, University of Arizona, Tucson 85721; (3)Department of Soils, Water, and Engineering, University of Arizona, Tucson 85721; (4)U.S. Department of Agriculture, Fruit Protection and Production Laboratory, Weslaco, TX 78596. Phytopathology 73:1365-1368. Accepted for publication 24 March 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1365.

The observation that root rot of cotton, caused by Phymatotrichum omnivorum, occurs in sharply defined, localized areas in Arizona cotton fields led to the hypothesis that a specific soil chemical factor may limit distribution of the fungus. Comparisons were made of the sodium, potassium, calcium, and magnesium contents of infested and adjacent noninfested soils in 13 fields. At 11 sites, there were no significant differences in the sodium contents of infested and adjacent noninfested areas (P=0.01). Sodium levels were higher in infested areas than in adjacent noninfested areas at two sites. Calcium levels were higher in the noninfested area than in the infested area at one site. Magnesium levels were higher in the noninfested areas at two sites. Contrary to previous reports, the fungus survived in soils with sodium contents as high as 5.1 milliequivalents (meq) per 100 g of soil, as indicated by the occurrence of severe disease in such areas in several successive seasons. Applications of sodium chloride at rates ranging from 1,800 to 4,000 kg/ha (equivalent to increasing exchangeable sodium by 1.4- 3.1 meq per 100 g of soil) did not reduce disease severity or increase yield significantly in 10 field trials in several locations in Arizona. Also, there was no reduction in disease severity when cotton was planted into infested soil that had been treated with sodium chloride the previous year.