Index >> Rhizosphere and Phyllosphere >> Fungistasis

Sclerotia may be a mass of loosely interwoven hyphae (e.g., Rhizoctonia solani) or they may have an organized structure- (Macrophomina phaseoli and Verticillium dahliae). Spores or sclerotia of many pathogenic fungi such as Rhizoctonia, Fusarium, Sclerotium, Aphanomyces, Pythium, Colletotrichum, Verticillium, Phytophthora and Plasmodiophora have been shown to germinate by the stimulus provided by the root exudates of susceptible cultivars of the host plants.

The stimulus for germination has been attributed to compounds exuded by plant roots which help to overcome, in some manner, the static nature of dormant reproductive structures in soil. The fungistasis of the sclerotia of Scelerotium cepivorum is released by certain volatile stimulators as­sociated with the roots of Allium. The volatile compounds have been iden­tified as alkyl sulphides which are produced when alkyl cysteine sulphoxides coming from roots of Allium are broken down by soil bacteria.

Root exudates may provide a food base for the growth of antagonists which could suppress the growth of pathogenic microorganisms in soil. Many instances have been reported where the rhizosphere of resistant varieties harboured more numbers of Streptomyces and Trichoderma than that of the susceptible varieties. Such observations have been recorded with reference of Fusarium wilts of plants.

On examination of rhizosphere soil of pigeon-pea (Cajanus cajan) planted in seven different soils, it was observed that 13-33% of the rhizosphere isolates of Streptomyces from resis­tant varieties inhibited the growth of Fusarium udum the causal organism of pigeon-pea wilt whereas only 6% of the isolates from the rhizosphere of susceptible variety were found to be inhibitory to the wilt causing pathogen.

High incidence of Trichoderma viride in the rhizosphere of varieties of tomato resistant to Verticillium wilt has been noted with its ability to minimize the severity of wilt on susceptible plants in the presence of the pathogen. Substances in plant root exudates may influence the formation of in­fection structures such as appressoria as in the case of Pellicularia filamen­tosa and Rhizoctonia fragariae, although contrary evidences have also been obtained with regard to many other fungi.