SUMMARY: The microbial populations of rhizosphere soil from pea varieties On-ward, susceptible to wilt by f. 1, and Wilt-Resistant Alaska were assessed at six successive plant-growth stages, by the dilution plate technique. The most commonly found rhizosphere organisms were: spp., bacteria, spp., and . Root exudate from variety Onward stimulated growth and sporulation of some of the more prevalent 24 of 60 different species among 137 morphologically different isolates of fungi and bacteria. Testing the rhizosphere isolates for their in witro effects on the pathogenic Fusarium showed that 15 yo had no effect, 29 yo were slightly inhibitory, 42 yo con-siderably more and 14 yo strongly inhibitory, Prominent among the strongest inhibitors were a few bacteria, and strains of , the last showing that intraspecific inhibition occurs among the Fusaria.

When ten of the more prevalent rhizosphere fungi were grown in media containing rhizosphere soil extract, root exudate or both, their ability to inhibit the pathogenic Fusarium greatly increased; the most inhibition was obtained with culture filtrates of fungi grown in the presence of both rhizosphere soil extract and root exudate. Morphologically different isolates of and inhibited the pathogenic Fusarium to different extents, showing that these species contain physiologic strains that could act differentially towards f. in the rhizosphere.

The rhizosphere of the wilt-resistant variety Alaska contained no more inhibitory isolates than the rhizosphere of the wilt-susceptible Onward. Although there were more micro-organisms per unit of dry rhizosphere soil of the susceptible variety, the species isolated from the rhizospheres of the susceptible and resistant varieties did not differ qualitatively.

Rhizosphere micro-organisms that were more prevalent up to the time that Fusa-rium invaded the host roots were not prominent among the group most antagonistic towards the Fusarium. In addition, there was no correlation between stimulation of rhizosphere organisms by root exudate and their antagonism towards the Fusarium. This implies that competition between pathogenic Fusarium and the other rhizo-sphere micro-flora for nutrients in root exudates may be at least as important as overcoming antibiosis in maintaining successful growth of the pathogen near host-root surfaces. Although these results were obtained in vitro, they suggest that rhizo-sphere soil extract and microbial metabolites together deter the growth of Pusarium oxyspmm f. pisi near pea roots, and that the growth-promoting effects of root exudate from the wilt-susceptible pea variety Onward are partially offset by its ability to increase inhibition by some of the other rhizosphere inhabitants.


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