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A variety of microorganisms, including common plant-associated saprophytes, plant parasites, and other members of the microbial community, reside on and in the tissues of hydrilla (Hydrilla verticillata). Among fungi commonly found on hydrilla are species of Fusarium that are capable of attacking this plant by pathogenic or phytotoxic modes. Yet, despite the presence of rich microbial biodiversity on hydrilla, no practical microbial herbicide has been developed for use against hydrilla. One reason for this is the lack of understanding of the epidemiological principles involved in underwater diseases.
In our laboratory, we have studied the epidemiology of a disease of hydrilla caused by an isolate of Fusarium culmorum, which was originally isolated from Stratiotes aloides (Hydrocharitaceae). The deposition and attachment macroconidia and chlamydospores of F. culmorum in aquatic systems was investigated by using assays with glass slides and hydrilla shoots. The number of spores deposited on leaves along the length of hydrilla shoots in assay tubes did not vary significantly with water depth. Macroconidia and chlamydospores deposited on the leaf surface did not become attached to the leaf for several hours. Two-week-old macroconidia required 3 h for attachment and 6-wk-old chlamydospores required 7 h. Pretreatment of macroconidia in hydrilla extract induced production of an adhesive substance from the tips of the macroconidia which attached to glass slides and hydrilla leaves. The adhesive substance was induced after incubation for 1 min in hydrilla extract, and the adhesive capacity declined after 20 min of incubation. Macroconidia that adhered to glass slides began to break free after 3 h. Co-incubation of macroconidia in hydrilla extract and the lectin Concanavalin A prevented adhesion of the macroconidia. The process of deposition and attachment of spores in the hydrilla-F. culmorum pathosystem is therefore complex and must be understood in relation to spore dispersal in water.
The dispersal of spores of F. culmorum, was investigated in aquatic microcosms. Macroconidia and chlamydospores, applied to the surface of the liquid, settled rapidly in deionized (DI) water, tap water, 5% Hoagland's solution, natural spring water, or river water held in glass containers. The rate of fall, as measured for 50% of the spores, was determined to be 9 cm h-1. Rapid lateral dispersal of spores from a point source occurred in still water. This initial spore movement occurred at a rate of >9 m hr-1 (15 cm min-1), approximately 100 times faster than the rate of settlement. The spores attained an even lateral distribution in a still, closed system. Spores dispersed rapidly in moving water and were transported with the water current. Spores carried a positive electrostatic charge as shown by their migration toward the negative pole during electrophoresis. Thus, the physical components of dispersal of F. culmorum spores in a still aquatic system were defined to consist of rapid lateral dispersal and sinking due to gravity. In moving water, the dynamics of water movement was superimposed over the other two factors, thus complicating the movement dynamics of spores.
Several Fusarium spp. have been field-tested for control of hydrilla, and some have been found to be highly effective.
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Copyright ©2002, UF Department of Plant Pathology