Fungal communities in winter wheat roots following crop rotations suppressive and nonsuppressive to take-all

Abstract

Mycofloral communities colonizing winter wheat roots were quantified in five crop sequences involving winter wheat preceded by either spring wheat, spring oats, spring barley, spring canola, or flax. Fungi were categorized into groups (major pathogenic, minor pathogenic, and nonpathogenic) based on their ability to cause root disease. Forty-six species of fungi were identified, of which the principal species were Alternaria alternata, Fusarium acuminatum, and Microdochium bolleyi. Isolates from the genus Fusarium formed the largest proportion of the mycofloral community. Fungal species that comprised root-colonizing communities were generally the same, and there were no significant differences in the total numbers of fungal species and isolates recovered in any of the crop sequences examined. However, the ratio of major pathogenic isolates to other isolates (minor pathogenic and nonpathogenic) was significantly different (in 1985) and a trend towards lower levels of root disease with decreasing ratios was seen. The most destructive of the root rot pathogens identified was Gaeumannomyces graminis var. tritici. Generally, the lowest ratio of major pathogenic to all other isolates and the lowest levels of root disease and root colonization (expressed as numbers of fungal isolates and species recovered) occurred in sequences involving flax and canola.