Effects of Deep Tillage and Roguing of Diseased Plants on Oospore Populations ofPeronosclerospora sorghiin Soil and on Incidence of Downy Mildew in Grain Sorghum

Abstract

Deep tillage and roguing of diseased plants were evaluated for effects on oospore populations of Peronosclerospora sorghi in soil and for control of sorghum downy mildew in grain sorghum. Roguing was conducted to simulate effects of 1- to 3-yr rotations with resistant crops by preventing completion of the disease cycle and addition of new inoculum to soil. Deep tillage (moldboard plowing to 30- 35 cm) was most effective when plowing was conducted in a downwind direction and not preceded by deep disking. Most oospores occurred at 0- 20 cm in soil with conventional tillage, whereas they were redistributed to 20- 40 cm with optimal deep tillage. Sorghum downy mildew incidence following optimal deep tillage was 14% of that obtained following conventional tillage. When deep tillage was conducted in an upwind direction or preceded by deep disking, oospore densities in upper soil and the incidence of sorghum downy mildew were less effectively reduced. Annual roguing of diseased plants from plots of susceptible cultivars reduced oospore densities to 22- 27%, and systemic downy mildew incidence to 11- 20% of levels observed in nonrogued plots after 3 yr. Results indicate that deep tillage, when properly conducted, and crop rotations or other treatments that prevent addition of new oospores to soil for 3 yr may provide economically beneficial control of sorghum downy mildew. However, some infection still occurred after almost 4 yr with little or no replenishment of oospore populations in soil