INFLUENCE OF PLANT INTERACTIONS ON VESICULAR-ARBUSCULAR MYCORRHIZAL INFECTIONS .2. CROP ROTATIONS AND RESIDUAL EFFECTS OF NON-HOST PLANTS

The effects of crop rotation on mycorrhizal development were examined in pot experiments, using sterilized soil inoculated with Glomus fasciculatus ''E3'' or Gigaspora margarita or unsterile soil containing mainly a type of Glomus macrocarpus var. geosporus. In 4 crop pairs tested, the amount of VA mycorrhizal infection in a host plant was not depressed in soil previously cropped with a non-host plant, even where roots of the preceding non-host plants were retained intact in the soil. Indeed the early establishment of VA infection in barley [Hordeum vulgare cv. Julia], lettuce [Lactuca sativa cv. Fortune] and maize [Zea mays cv. Cargill Primeur 170] inoculated in sterilized soil was stimulated by the non-hosts oilseed rape [Brassica napus cv. Victor], cabbage [Brassica oleracea cv. Ormskirk] and sugar beet [Beta vulgaris var. rapa cv. Kleine], respectively, [also Kale, Brassica oleracea cv. Maris Kestrel]. VA hyphae were sometimes observed growing in moribund non-host roots. Effects of crop rotation in unsterile soil were similar, but less marked. B. margarita in sterilized soil infected lettuce considerably less than other crops.