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Influence of Glomus fasciculatus and Soil Phosphorus on Phytophthora Root Rot of Citrus. R. M. Davis, Assistant professor, Texas A & I University Citrus Center, Weslaco 78596; J. A. Menge, assistant professor, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 70:447-452. Accepted for publication 7 November 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-447.

In soil fertilized with less than 15 μg P/g soil, roots of sweet orange seedlings infected with Phytophthora parasitica and the mycorrhizal fungus Glomus fasciculatus were healthier and weighed more than roots infected with P. parasitica alone. However, the beneficial effect of G. fasciculatus was eliminated by P. parasitica in mycorrhizal seedlings fertilized with more than 56 μg P/g soil. In citrus seedlings with split root systems inoculated with P. parasitica, the percentage of healthy nonmycorrhizal roots was significantly greater when these roots were split from mycorrhizal roots than when they were split from nonmycorrhizal roots. Numbers of propagules of P. parasitica were greater in the rhizosphere of roots in which one or both sides of the root system were mycorrhizal than in the rhizosphere of roots in which both sides of the root system were nonmycorrhizal. All evidence suggests that tolerance to P. parasitica root rot in citrus infected with G. fasciculatus is caused by the ability of mycorrhizal roots to absorb more phosphorus and possibly other minerals than nonmycorrhizal roots.

Additional keywords: vesicular-arbuscular mycorrhizae, endomycorrhizae, soilborne fungi.