Interaction of Glomus fasciculatum and Pythium ultimum on greenhouse-grown poinsettia

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Abstract
Rooted Euphorbia pulcherrima (poinsettia) plants, mycorrhizal with G. fasciculatum or nonmycorrhizal, were grown in a potting medium with a P level of 5 .mu.g/g, containing P. ultimum at 0, 38, 66 or 132 colony-forming units (CFU)/g dried soil. Fertilization was at 75 ppm N-K weekly. Mycorhizal root colonization was greater in plants grown in P. ultimum soil at each density than in plants grown in soil without P. ultimum, as determined by the root segment method and at 132 and 38 CFU/g as determined by the root length method. Plant height and foliar P content of mycorrhizal plants were greater than of nonmycorrhizal plants when grown in soil infested with 38 CFU/g P. ultimum. Foliar Mn content was greater in nonmycorrhizal than mycorrhizal plants when grown in P. ultimum soil at each density. Mycorrhizal plants had lower final P. ultimum populations in rhizosphere soil than nonmycorrhizal plants when grown in soil infested with 38 CFU/g P. ultimum. Densities of P. ultimum were greater in rhizosphere compared with nonrhizosphere soil in the absence of mycorrhizae.

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