Infection of pearl millet by downy mildew [Sclerospora graminicola (Sacc.) Schrot.] has been shown to be influenced by soil pH, soil bulk density, soil moisture content and addition of farmyard manure and nitrogen fixing bacteria. 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 effects were not as marked as for pH. The addition of farmyard manure to soil or the addition of Rhizobium, Azospirillum or Azotobacter inocula as combined 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 population was lower in the resistant varieties but that it was increased in both susceptible and resistant varieties by infection with downy mildew. Bacterial and actinomycete populations were also lower in resistant varieties but in this case downy mildew infection decreased the rhizosphere populations of both groups.

Dehydrogenase activity of rhizosphere soil was higher in susceptible varieties but was decreased in both variety types by infection with S. graminicola.

The results are discussed with reference to possible mechanisms to explain the observed effects.