Population densities of bacteria, sporeformers, actinomycetes and fungi were
enumerated in soil and rhizosphere samples collected from fields cultivated either with
a susceptible(Giza-113) or a resistant (Dorado) grain sorghum cultivars after 15,30,45,
and 90 days of sward establishment. Results revealed that the rhizosphere of both
cultivars harboured higher counts of the examined microorganisms than those found in
soil apart .Counts of total bacteria, spore-formers and fungi increased as the plant
growth increased till the45-day period and gradually decreased thereafter. Bacteria
were the highest followed by actinomycetes, fungi and sporeformers being 34.2, 8.1,
0.6 and 0.3 x 10⁶ cfu/g soil,respectively. During the earlier plant growth periods, counts
of each type of microorganisms increased in the rhizosphere of both cultivars being
higher with resistant plants.Out of 367 isolates (291 bacteria, 51 actinomycetes and
25 fungi) isolated from soil and the rhizosphere of Dorado and Giza 113 plants, 183
isolates (151 bacteria and 32 actinomycetes) proved to be effective against A.
strictum. Twenty isolates of bacteria (identified as Bacillus spp.) and 9 isolates of
actinomycetes (identified as Streptomyces spp.) showed the highest antagonistic
effect. Isolate No. (9) of Bacillus spp. (identified as B. subti/is) and Streptomyces spp.
No. (3) were the most antagonistic isolates. Culture filtrates of both bioagents
individually singly or in combination (1: 1) inhibited the mycelial growth of A. strictum