Incidence of Mycorrhizal Fungi on Six Field Crops in Monoculture on a Newly Cleared Woodland Site

SUMMARYThe yearly incidence of root colonization and extramatrical spores of vesicular-arbuscular (-VA) mycorrhizal fungi were determined for six agronomic crops grown in monoculture for 7 yr on a newly cleared woodland site in northwest Florida. Thirteen species of VA-mycorrhizal fungi were identified from the test site from 1972 to 1978. Sorghum yielded the greatest number of species (12) from a single crop during this period. The highest number of spores was associated with soybean and the lowest number of spores was found in the native woodland. Spores of Gigaspora margarita, G. gregaria, and G. gigantea were most numerous from soil around soybeans while Glomus fasciculatus and G. clarus were most numerous around roots of bahia grass. Acaulospora spp. were most abundant from soil around cotton and peanut. The yearly incidence of Gigaspora margarita spores increased while G. gregaria and G. gigantea decreased in numbers during the 7-yr study. Glomus macrocarpus var. geosporus and G. fasciculatus were observed in 1971 and/or 1972 but were not recovered again until 1978. Only two species, Gigaspora margarita and G. gregaria, were recovered each yr that samples were taken. The highest level of root colonization occurred in 1972 and was followed by a decline in 1973–1974, but mycorrhizae showed a general increase in 1975–1976. It is suggested that the observed changes in the incidence of VA-mycorrhizal fungi were primarily due to the agricultural system of monoculture.