Development of sorghum downy mildew on sequential plantings of maize in Uganda

Development of sorghum downy mildew (SDM), caused by Perenosclerospora sorghi (Weston and Uppal) C.G. Shaw, was monitored in three to five sequential plantings of six maize genotypes for three seasons in two locations in Uganda. Generally, incidence of SDM was not influenced by time of planting, suggesting that infection was initiated by oospores, the primary inoculum, and not conidia, the secondary inoculum. Genotype effects were highly significant (P<0.001); disease was high in Farida, Pool 16 and NZ 2, but low in Suwan 1. The six maize genotypes were grouped into three categories: resistant (Suwan 1), moderately resistant (102 and Longe 1), and susceptible (Farida, Pool 16 and NZ 2). Sporulation of P. sorghi varied significantly with the genotypes, being highest in the most susceptible genotype (Farida) and least on 102 and Longe 1. Suwan 1 was not tested for sporulation because of low infection. Thus, the resistant genotypes exhibited low incidence of SDM and low sporulation of P. sorghi.