Abstract
The infectivity of Ophiobelus graminis in wheat-field soils was found to he greatest after 2 or 3 years under wheat monoculture and least after 5 or 6 successive wheat crops. Added nitrogen enhanced the infectivity of all the soils tested. Short periods of contact with decline soils, or their suspensions, protected wheat roots from Infection by Ophiobolus when they were subsequently grown in non-decline soil. Attempts were made to dotermine the factor(s) responsible for this decreased infection. Investigation of the fungal flora of wheat roots grown in decline and non-decline soils showed little difference in the types of organism present, although more fungi were isolated from roots grown An non-decline soil. variations were not correlated with the presence or absence of Ophiobolus. More bacteria and actinomycetes were isolated from decline soil than from non-decline soil. A large proportion of the populations of both soils suppressed Ophiobolus in plate culture. Extracts of both soils wore also antagonistic. Inhibitory activity was removed from non-decline soil suspensions by filtering through a 2. 0um Mllipore filter. such filtration removed only a small part of the inhibitory activity of decline soil. Hyphae emerging from Ophiobolus inocula exhibited a positive growth response to the presence of wheat roots. This response was negatively correlated to the distance between inoculum and root. The presence of decline soil significantly reduced this growth response. Neither soil had any effect on growth rate or numbers of hyphae emerging from inocula. It Is suggested that the general antagonism in non-decline soil is mainly fungal in origin, whereas that in decline soil is mainly bacterial. It is further suggested that this general antagonism is not significant in decline. The reduction of hyphal response to wheat roots appears to be a major factor in reducing infection in the decline soils studied.