Role of edaphic factors in the development of downy mildew (Sclerospora graminicola) in pearl millet

Infection of pearl millet by downy mildew [Sclerospora graminicola (Sacc.) Schrot.] has been shown to be influenced by soil pH, soil bulk density, soi l moisture content and addition of farmyard manure and nitrogen fixing bact eria. A pH value of 8.5 allowed disease to develop the most, with increases in acidity to pH 7.5 producing 70 % reduction in disease. Higher soil bulk density and moisture content also led to reductions in disease but the eff ects were not as marked as for pH. The addition of farmyard manure to soil or the addition of Rhizobium, Azospirillum or Azotobacter inocula as combin ed seed and soil treatment also reduced disease with the best effects being from a cluster bean isolate of Rhizobium and from Azotobacter chroococcum. Assessment of rhizosphere microorganisms associated with the resistant and susceptible varieties of pearl millet showed that, overall, the fungal pop ulation was lower in the resistant varieties but that it was increased in b oth susceptible and resistant varieties by infection with downy mildew. Bac terial and actinomycete populations were also lower in resistant varieties but in this case downy mildew infection decreased the rhizosphere populatio ns of both groups. Dehydrogenase activity of rhizosphere soil was higher in susceptible variet ies but was decreased in both variety types by infection with S. graminicol a. The results are discussed with reference to possible mechanisms to explain the observed effects.