Texas Has a New Pathotype of Peronosclerospora sorghi, the Cause of Sorghum Downy Mildew

Plant Disease
T Isakeit, J Jaster

Abstract

Three pathotypes of Peronosclerospora sorghi were known to occur in Texas as of 1980, with pathotype 3 (P3) predominant on sorghum (Sorghum bicolor) grown in the Upper Coast area. Following the use of hybrids resistant to P3, combined with metalaxyl or mefenoxam seed treatment, sorghum downy mildew (SDM) became a minor disease in Texas until the occurrence of a widespread outbreak caused by a P3 strain resistant to metalaxyl and mefenoxam in Wharton County in 2001 (2). During July 2004, <1% of plants in a commercial field in Wharton County planted to two Pioneer Brand P3-resistant hybrids had white stripes on the leaves and leaf shredding typical of systemic SDM. To obtain inoculum for pathogenicity studies, several infected plants were removed from the field and transplanted to pots for growth in a greenhouse. Systemically infected leaves suitable for inoculum production subsequently developed from tillers. Conidia were collected from leaves using a tiered temperature system (1). One-week-old seedlings of 10 sorghum lines used as pathotype differentials for Texas were sprayed until runoff with a conidial suspension (8 × 104 per ml) and incubated for 24 h at 20°C and 100% relative humidity. Seedlings were grown for 6 days in th...Continue Reading

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