Sanjeet Kumar
Microorganisms in the immediate vicinity of the root together with those on its surface play active, effective and decisive role in the success or failure of a pathogen in causing root infections. It is due to “Rhizosphere effects” directly linked with root exudates and rhizosphere microorganisms. The rhizosphere effect is strongest at the root surface, the rhizoplane and declines with increasing distance from it.
The rhizosphere effect is a concept to indicate the state of rhizosphere health. It is expressed as a ratio of number of microorganisms in the rhizosphere and the number of microorganisms in the soil beyond the influence of the roots, calculated on the basis of dry soil. The rhizosphere / soil ratio is usually considered as a positive effect in the case the ratio exceeds one and as negative if below one. On the best known example of rhizosphere effect is flax wilt caused by Fusarium oxysporum. The rhizosphere of a resistant variety was reported to contain HCN that favoured the growth of saprophytes such as Mucor, Penicillum etc whereas susceptible varieties contained HCN in trace and favoured the growth of potential pathogens like Fusarium, Verticillium, Alternaria and Helminthosporium.
