PEST MANAGEMENT: DISEASES Role of Soil Type in Predisposing Banks and Other Pi-ta Varieties to Rice Blast

The high-yield varieties ‘Banks’ and ‘Cybonnet’, released to seed growers during 2004, contain the Pi-ta R gene used in blast-resistant varieties for over 16 years. While Cybonnet fields were blast free, severe blast disease developed in drought-stressed Banks growing in a sandy production field in Clay County during 2004 and then again in sandy fields throughout Arkansas during 2005 and 2006. Race ‘IE-1k’, a rare but well researched minor race of the blast fungus Magnaporthe grisea, was isolated from diseased Banks plants. Soil samples were collected from Clay County production fields and University of Arkansas research stations near Colt and Stuttgart, Ark. Pi-ta and non-Pi-ta varieties growing in these soil samples under either drought-stressed-upland or continuous-flood treatments were inoculated with race IE-1k. Leaf blast severity was highest in drought-stressed-upland treated plants and was reduced by the continuousflood treatment. Soil samples were ranked according to blast severity over all varieties with the highest being an unidentified UA-PTES sandy loam sample followed by an UA-RREC Dewitt silt loam sample, a Corning 2005 Bosket FSL sample, an unidentified UA-PTES silt loam and, finally, a Corning 2004 Bosket FSL sample. This soiltype severity ranking was essentially the same, with minor variations, for each variety. Although leaf blast severity was higher with plants in specific soil samples, test results do not show increased blast susceptibility in drought-stressed Banks to be associated with a specific soil type. The test data show Banks to be more susceptible to race IE-1k than are other Pi-ta varieties. Although susceptibility is intensified with drought stress and soil type, Banks lacks known blast-resistance genes Pi-kh and Pi-ks. These genes and/or other unidentified genes apparently confer muc of the observed blast field resistance present in other Pi-ta varieties. Blast field resistance, cumulative in susceptible