THE PEACH REPLANT PROBLEM IN ONTARIO: IV. FUNGI ASSOCIATED WITH REPLANT FAILURE AND THEIR IMPORTANCE IN FUMIGATED AND NONFUMIGATED SOILS

Abstract

Soil fumigation with MC-2 (methyl bromide) inhibited many of the fungi found in the rhizosphere of peach replants. This chemical, as well as DD (a propane–propene mixture), induced greater growth of peach seedlings in most soils than either Dowfume W40 (ethylene dibromide) or CBP HI 806. Methyl bromide fumigation also permitted second year replants to develop free of root rot. On the other hand, the use of Dowfume W40, which was ineffective against the fungus flora, usually resulted in greater growth than that obtained in check soils. The soil fumigants used did not modify the depressing effect of toxins on the growth of replants in tree sites although this effect gradually diminished with the passage of time. Fourteen species of fungi out of several hundred isolated from the rhizosphere and root tissues of peach replants in fumigated and non-fumigated peach orchard soils were found to be facultative parasites on the roots of peach seedlings when grown under greenhouse experimental conditions. Of these, Fusarium equiseti, Fusarium moniliforme, Fusarium oxysporum, Fusarium solani, Alternaria tenuis, Myrothecium verrucariae, and members of Mycelia Sterilia were isolated most frequently while Papularia sphaerosperma, Gliomastix convoluta, and species of Coniothyrium, Curvularia, Humicola, Peyronellaea, and Truncatella appeared in the rhizosphere of certain peach trees and not in that of others. The experimental conditions most favorable to the infection of roots by these fungi proved to be low soil moisture combined with high soil temperature.