STUDIES ON THE INTERACTION BETWEEN SOIL MICROFLORA AND THE PATHOGENIC ORGANISMS CAUSING SORGHUM DISEASES

Abstract

Acremoniwn wilt disease caused by Acremonium strictum has been an important disease of grain sorghwn in Egypt and many of the sorghwn-growing countries. Eighteen isolates of A. strictum were collected from 7 govemorates in middle and upper Egypt and they were evaluated under greenhouse conditions for virulence on two sorghwn cultivars EGH-2 (resistant) and Giza 114 (susceptible). Isolate No. 9 was the most aggressive one. Reaction of 20 commercial and promising grain sorghwn cultivars were evaluated for resistance to A. strictum (isolate No. 9) at Giza and Sids Stations using soil infestation technique. Dorado was the highest resistant cultivar and Giza 113 was the highly susceptible.one. General survey for associated fungi with non sterilized grains of the two cultivars (Dorado and Giza 113) using standard blotter method indicated that Giza 113 grains had more load of associated pathogenic fungi than that of Dorado grains. Among fungi isolated, Alternaria alternata, Fusarium moniliforme, F. roseum, F. semitectum, Acremonium strictum, Bipolaris bicolor, Cepha/osporium sp., Aspergil/us flavus, A. niger, Curvu/aria sp. and C. lunata, were the most dominant associated fungi with grains of the two cultivars with different frequencies. Three seed health testing methods (agar plate, blotter and freezing method) were evaluated for detecting the most prevailing pathogenic fungi associated with sterilized Giza 113 grains. Freezing method was the most sensitive technique for detection of F. moniliforme, F. roseum and A. strictum which are considered the most important pathogens as they cause wilt diseases. : Out of 367 isolates (291 bacteria, 51 actinomycetes and 25 fungi) isolated from soil and the rhizosphere ofDorado 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 sp.) and 9 isolates of actinomycetes ( identified as Streptomyces sp.) showed the highest antagonistic effect. Isolate No. (9) of Bacillus sp. (identified as B. subtilis) and Streptomyces sp. No. (3) were the most antagonistic isolates. Culture filtrate as well as broth culture of both bioagents singly or in combination (I: I) inhibited the mycelial growth of A. strictum. Suspensions of the two bioagents and mixture of both of them (I: I) effectively enhanced germination and reduced associated pathogenic fungi of Giza 113 sorghum grains, as well as they effectively reduced Acremonium wilt disease incidence and increased grain yield/plant. Vermiculite proved to be the most effective carrier among the tested six carriers followed in descending order by talc powder and peat-moss. Seed coated with vermiculite based formulation of a combination of B. subtilis and Streptomyces sp. (I: I) gave a partial similar efficiency to that of the fungicide Occidor Plus (carbendazim 50% S.C.) in controlling the disease incidence and increasing grain yield/plant, this indicated that biological control can be successfully exploited as an effective method for soil-borne plant pathogens control to avoid the hazardous effects