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Virulence Characterization of Single-Zoospore Isolates of Sclerospora graminicola from Pearl Millet

Sclerospora graminicola, the causal agent of downy mildew in pearl millet, is well-known for variation in its virulence pattern. Nine single-zoospore isolates (Sg 026-Z-1 to Sg 026-Z-9) derived from an oosporic isolate Sg 026 from a pearl millet F₁ hybrid cultivar Nath 4209 grown in a farmer's field in a village, Veelad, in Maharashtra state, India, and three controls (Sg 026, Field-1, and Field-2) were evaluated for their virulence in two experimental runs in a greenhouse. The isolates were maintained on pot-grown seedlings of a highly susceptible pearl millet line, 7042S, in a greenhouse through asexual (sporangial) generations. Pot-grown seedlings of six pearl millet potential differential lines/cultivars (7042S, NHB 3, MBH 110, ICMH 451, 843B, and 852B) were spray-inoculated with a sporangial suspension (5 × 10⁵ sporangia ml^-1) and maintained in a greenhouse at 25 ± 2°C. Data were recorded for latent period (days) and disease incidence (%), from which a virulence index (incidence × latent period^-1) was calculated to quantify disease-causing potential of isolates. Results indicated significant variation in latent period, incidence, and virulence index among isolates. The isolates were classified into four distinct pathotype groups based on their virulence indices on six pearl millet lines. Because of the significant variation for virulence in the S. graminicola population infecting Nath 4209, it is recommended that the hybrid be regularly monitored for downy mildew infection in farmers' fields, and be replaced by a resistant cultivar that is genetically unrelated to the parental lines of Nath 4209. This will help delay or avoid development of downy mildew epidemics and the resulting heavy loss to pearl millet farmers in the region.