VIEW ARTICLE

Ecology and Epidemiology

Influence of Vesicular-Arbuscular Mycorrhizae and Soil Phosphorus on Take-All Disease of Wheat. J. H. Graham, Department of Plant Pathology, University of California, Riverside 92521; J. A. Menge, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 72:95-98. Accepted for publication 5 May 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-95.

Wheat grown for 4 wk in P-deficient sandy soil (0.5 μg P/g soil) had significantly greater levels of root P as a result of either addition of 50 μg P/g soil as superphosphate or inoculation with the mycorrhizal fungus, Glomus fasciculatus. For plants grown in P-deficient soil, the formation of high levels (>70% root colonization) of vesicular-arbuscular mycorrhizae (VAM) increased root P content nearly to the level of P-amended plants, whereas 50 μg P/g soil severely inhibited VAM formation (<10%). Root exudation of amino acids and reducing sugars, which was lower in heavily mycorrhizal and P-treated plants than the untreated controls, was inversely correlated with root P content. When 4-wk-old plants of each P-VAM treatment combination were inoculated with Gaeumannomyces graminis var. tritici at two inoculum levels, P-deficient nonmycorrhizal plants developed more take-all at the high inoculum level (0.5% w/w, grams of oat kernel inoculum per gram of soil) than at the low inoculum level (0.1%). By comparison, P-treated plants developed fewer take-all symptoms, irrespective of inoculum density or presence of low levels of mycorrhizal infection. High levels of VAM infection in plants grown in P-deficient soil significantly decreased take-all at the low inoculum level only. The decrease in disease severity in P and VAM treatments was not a result of significant increases in root growth, but apparently was related to improved root P status and decreased root exudation prior to pathogen inoculation. The influence of soil phosphorus and mycorrhizae on take-all disease appears to be the same; that is, these factors increase the P status of the host, which leads to a decrease in net leakage of root exudates and thereby reduces pathogen activity.