Submitted to: Biennial Grain Sorghum Research and Utilization Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 2/16/2003
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Sorghum downy mildew (SDM), caused by the fungus Peronosclerospora sorghi, appeared in several sorghum fields in Wharton and Jackson counties of Texas during the spring of 2001 and again in 2002. Our objectives were to determine whether isolates of P. sorghi from this area were tolerant to metalaxyl/mefenoxam. We planted a field trial in 2002 in a commercial sorghum field near El Campo, which had been previously cropped with sorghum and had an abundance of plants showing symptoms of systemic downy mildew in 2001. In a dose response experiment with two hybrids treated with four rates of mefenoxam (none; 0.125 oz.; 0.25 oz. and 0.5 oz. a.i./cwt), there was no significant control of SDM at any of these rates. With six other hybrids in the same field, there were no significant differences in SDM incidence between the control and seed treated with 0.5 oz a.i./cwt mefenoxam. Greenhouse tests using oospore and sporangial inoculum indicate that tolerance to mefenoxam exists in five other fields in Wharton county. Several factors interacting over many years have contributed to the recent outbreak of SDM. These primary factors are believed to be (1) low seed treatment rates of metalaxyl, (2) continued planting of pathotype 3 susceptible sorghum hybrids, (3) increased monoculture of grain sorghum, and (4) seed treatments with Concep III herbicide seed safeners. Fungicide resistance will make control of this disease more difficult in the future.