Influence ofGlomus fasciculatusand Soil Phosphorus on Phytophthora Root Rot of Citrus

Abstract

In soil fertilized with less than 15 .mu.g P/g soil, roots of sweet orange seedlings infected with P. 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 .mu.g 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 1 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. Tolerance to P. parasitica root rot in citrus infected with G. fasciculatus is evidently caused by the ability of mycorrhizal roots to absorb more P and possibly other minerals than nonmycorrhizal roots.