STUDIES ON THE ECOLOGY OF FUSARIUM MONILIFORME SHELD. UNDER SORGHUM CULTURE

Fusarium moniliforme populations were determined at monthly intervals over a two-year period by the dilution plate technique. Populations were consistently higher in rhizosphere soil than in soil obtained between grain sorghum (Sorghum bicolor (L.) Moench) rows (furrow soil). Populations remained stable or declined only slightly during the winter but declined rapidly during the spring. The mean number of propagules per gram of soil (oven dry equivalent) varied from 2 at the end of May to 147 in October in furrow soil, and from 12 in May to 1,046 in October in rhizosphere soil. When field soil was artificially infested with F. moniliforme conidia and mycelia, the population declined to zero in 10 and 14 weeks under greenhouse and field conditions, respectively. F. moniliforme failed to colonize pieces of autoclaved sorghum stalks incorporated in infested field soil, indicating that it is a soil invader (root inhabitant) and not a soil inhabitant.;In both field and greenhouse studies, F. moniliforme aggressively invaded and colonized seedling grain sorghum plants and colonization continued until the plants were physiologically mature. Colonization of stalk tissues did not always result in disease symptoms. Overwintered stalks with minimal decay were the major reservoir of F. moniliforme (2 x 10('5) propagules/g stalk). Populations in stalks were negatively (r('2) = 0.957) correlated with stalk decay.;F. moniliforme was detected in 27 of 28 untreated grain sorghum hybrid seed lots from 6 different sources. The propagules were more commonly detected as a surface contaminant than internally, as the percentages of nonsurface contaminant than internally, as the percentages of nonsurface disinfested seeds (100 seeds tested/lot) affected ranged from 0 to 56% while surface disinfested seeds ranged from 0 to 24%.;The herbicides Atrazine, Cyanazine, Terbutryn, Propazine and Propachlor did not reduce F. moniliforme populations when applied at the field rate. Propachlor, applied at 20 times the recommended rate, totally suppressed F. moniliforme growth in culture.;F. moniliforme conidia and mycelia overwintered (1978-79 and 1979-80) in grain sorghum stalks without loss of viability or pathogenicity. No thickened hyphae, chlamydospores or sclerotia were observed in 1979 although chlamydospore-like cells were observed on some hyphae in 1980. No loss in viability of conidia or hyphae was observed when F. moniliforme was stored at -16(DEGREES)C for 6 months.