Evidence for a new virulent pathotype of Sclerospora graminicola on pearl millet.

Field surveys of pearl millet (Pennisetum glaucum) crops in Maharashtra, India during 1993-96 indicated a high incidence of downy mildew (Sclerospora graminicola) on several new F1 hybrid cultivars. Disease incidence varied considerably within and across fields of several cultivars. Some of the more popular private sector hybrids (MLBH 104, MLBH 267, BK 560, Eknath 201, JKBH 26, Pioneer 7602) recorded downy mildew incidence up to 80-100% in some years in certain fields. In contrast to F1 hybrids, open-pollinated cultivars Mallikarjuna and ICTP 8203 recorded trace or no downy mildew incidence. In a greenhouse experiment, S. graminicola isolates Sg 008 (from NHB 3), Sg 010 (MHB 110), Sg 021 (MLBH 104), Sg 024 (BK 560) and Sg 026 (Nath 4209) showed differential virulence on a set of host differential lines (NHB 3, 5141A, MBH 110, BK 560 and 852B). DNA fingerprinting of isolates, using a microsatellite (GATA)4 revealed high levels of polymorphism among the isolates. Both virulence and DNA fingerprinting analyses showed that isolate Sg 021 from a popular hybrid MLBH 104 was quite distinct from those from MBH 110, BK 560 and others. The cluster analyses of virulence data and DNA fingerprinting data classified the isolates into 4 groups, although there was no complete agreement between the 2 groupings. The results indicate the evolution of a new virulent pathotype (Sg 021) specific to a widely grown hybrid MLBH 104, which has caused substantial damage to the crop in Maharashtra during the past 3 years. A strategy to monitor and manage downy mildew in pearl millet is discussed.