VARIATION IN PATHOGENICITY AMONG SINGLE-OOSPORE ISOLATES OF SCLEROSPORA-GRAMINICOLA, THE CAUSAL ORGANISM OF DOWNY MILDEW IN PEARL-MILLET

Variation in pathogenicity (virulence and aggressiveness) of various s ingle-oospore isolates of Sclerospora graminicola, the causal organism of downy mildew in pearl millet, was studied. Oospore isolates were m aintained as sporangial progenies on the susceptible pearl millet geno type 7042-S through asexual generations. Twenty single-oospore isolate s obtained from samples of oospore inocula from three locations (the I CRISAT Centre, Coimbatore, and Hisar) in India, were tested for pathog enicity on 7042-S. All the isolates were virulent on 7042-S and they v aried considerably, between and within locations, in infection efficie ncy (11-44% infection). The S. graminicola populations from the ICRISA T Centre and Hisar were significantly more aggressive than those from Coimbatore. Considerable variation was found among the 10 single-oospo re isolates from the ICRISAT Centre in infection efficiency and for la tent period on a set of resistant and susceptible genotypes. Seven rep resentative isolates, two each from the ICRISAT Centre (ICOS) and Coim batore (CBOS), and three from Hisar (HSOS), exhibited variation in inf ection efficiency, sporangial production capacity, and oospore product ion rating on the susceptible genotypes 7042-S and NHB 3. A pathogenic fitness index (PFI) was calculated for these isolates as the product of the reciprocal of the latent period x percentage infected seedlings x log10 sporangia per cm2 leaf area x oospore production rating. Aggr essiveness of these isolates, measured as PFI, varied greatly. CBOS-I was the least aggressive with a PFI of 8.5 on NHB 3 and 69.6 on 7042-S ; the PFI of the other isolates ranged from 81.1 to 370.5. Downy milde w resistance in three of the five known resistant lines (700651, P7-4 and 7042-R) was consistently effective against all 10 ICOS isolates, i ndicating that the resistance of these three lines is likely to be mor e stable, at least at the ICRISAT location, than that of the other two resistant lines.