Relationship Between Postharvest Management of Grain Sorghum and Phymatotrichum Root Rot in the Subsequent Cotton Crop. C. M. Rush, Texas A&M University, Texas Agricultural Experiment Station, P.O. Drawer 10, Bushland 79012. T. J. Gerik, Texas A&M University, Blackland Research Center, 808 E. Blackland Rd., Temple 76502. Plant Dis. 73:304-305. Accepted for publication 1 November 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0304.

Research was conducted at four central Texas sites over a 3-yr period to determine the effects of postharvest sorghum treatments on incidence of Phymatotrichum root rot in the subsequent cotton crop. After sorghum harvest, stalks were disked (conventional treatment), sprayed with glyphosate at 420 g acid equivalent per hectare (glyphosate treatment), or allowed to regrow until killed by frost (ratoon treatment). In all cases, the incidence of cotton root rot the following year was higher with the ratoon treatment than with the glyphosate treatment. In three of four studies, the ratoon treatment also resulted in significantly more root rot than the conventional treatment. Results of an in vitro glyphosate toxicity study revealed that glyphosate restricts linear growth of Phymatotrichum omnivorum at concentrations ?10µg/ml. It is improbable that toxic levels of the herbicide accumulated in roots of treated sorghum in the field. Disease increase in ratoon plots was possibly due to an increase in inoculum density resulting from the prolonged availability of sorghum roots as a substitute for saprophytic growth.