Colonization of crop and pasture species with vesicular–arbuscular mycorrhizal fungi and a negative correlation with root infection by Bipolaris sorokiniana

The roots of 37 crop and pasture species were assessed for vesicular–arbuscular mycorrhizae in a vertisol containing spores of vesicular–arbuscular mycorrhizal fungi mainly Glomus mosseae, and of the pathogenic fungus, Bipolaris sorokiniana. The level of mycorrhizal colonization of different hosts is an important aspect of managing crop sequences to reduce "long fallow disorder." All species except rapeseed in the Cruciferae and lupin in the Leguminosae were hosts, although perennial rye grass in the Gramineae had only very slight colonization. The percent root length colonized as assessed by the grid-intersect method ranged up to 60.5% for wheat in the winter series and up to 98.4% for mungbean in the summer series. Greatest weights of mycorrhizal roots were produced by phalaris grass, chickpea, safflower, cocksfoot, lucerne, and barley in the winter series and by lucerne, maize, canary seed, Sudan grass, grain sorghum, and buffel grass in the summer series. Although Gramineae as a group tends to have fine roots with a low percentage of mycorrhizal colonization, the total weight of mycorrhizal roots can be large, and they should be at least equal to legumes in effectiveness for breaking long fallow disorder. Percentages of mycorrhizal colonization determined by the grid-intersect and three slide methods were generally well correlated with one another, but all were less strongly correlated with weight of mycorrhizal roots for winter crops and were entirely uncorrelated with weight of mycorrhizal roots for summer crops. Significant inverse-regression relationships were obtained between infection of root segments (but not of stem bases) by B. sorokiniana and root colonization with vesicular–arbuscular mycorrhizae, indicating that vesicular–arbuscular mycorrhizal fungi antagonise root infection by B. sorokiniana.