| Direct observation of soil-borne fungal pathogens is hampered by the opacity of soil and lack of techniques for examining subterranean events. To overcome these constraints, three methods utilizing root observation boxes, nylon mesh, and glass slides were used to observe directly microsclerotial germination and behavior of Macrophomina phaseolina in soil and in the rhizosphere of soybean. The glass slide technique was the best of the three. This method employed glass microscope slides, which were dipped into a water agar suspension of microsclerotia of M. phaseolina. After the agar had solidified, the slides were placed in soil at a 45 degree angle with the soil covering 7 cm of the slide. Soybean seeds were then planted 1 cm deep directly on the agar-coated slide so that the roots would grow down and across the agar film. After various incubation periods, slides were removed from soil, washed, stained, and examined microscopically. Microsclerotial germination was greater if roots were grown in sterile soil and rhizosphere soil. The effects of soil temperature and soil water potential on microsclerotial germination were also studied using the glass method. Microsclerotial germination in response to soil water potential was greater in the Sarpy sandy loam soil than in the Mexico silt loam soil. The optimum soil temperature for microsclerotial germination was between 25-33 C. No apparent positive tropism of mycelium from germinating microsclerotia toward roots was observed. Competitive growth of other soil microorganisms around and on the surface of microsclerotia may have contributed to reduced germination of microsclerotia in nonsterile soil. |
