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Phytophthora Root Rot Development on Mycorrhizal and Phosphorus-Fertilized Nonmycorrhizal Sweet Orange Seedlings. J. H. Graham, Associate Professor, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. D. S. Egel, Biological Scientist, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. Plant Dis. 72:611-614. Accepted for publication 5 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0611.

Comparable sweet orange seedlings were grown in soilless medium and either were inoculated with the vesicular-arbuscular mycorrhizal fungus Glomus intraradices (VAM) or were nonmycorrhizal and fertilized with soluble P (NM). Seedlings, 8 or 10 mo old, were transplanted into a low P (3.5 g/g) citrus soil that was noninfested or infested with 1 or 10 chlamydospores of Phytophthora parasitica per cubic centimeter of soil. In one experiment, root dry weight and leaf P content of noninfested VAM seedlings were greater than those of NM plants, which were nearly deficient in P. P. parasitica reduced leaf P status of VAM and NM seedlings alike but reduced dry weight of only VAM plants. There were significantly fewer rotted root tips on VAM seedlings. P. parasitica reduced VAM colonization as a result of the loss of root tips. In a second experiment, at the higher inoculum density of P. parasitica, NM and VAM seedlings were similar in size and had sufficient levels of leaf P. P. parasitica infestation reduced leaf P content and root dry weight irrespective of mycorrhizal treatment. Root rot development occurred to about the same extent on NM and VAM root systems. G. intraradices did not increase the resistance or tolerance of sweet orange to Phytophthora root rot unless mycorrhizae conferred a P nutritional advantage over the NM plant.

Keyword(s): phosphorus nutrition.