Submitted to: Mycopathologia
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/29/2006
Publication Date: 10/1/2006
Citation: Pratt, R.G. 2006. A direct observation technique for evaluating sclerotium germination by Macrophomina phaseolina and effects of biocontrol materials on survival of sclerotia in soil. Mycopathologia. 162:121-131. Interpretive Summary: Disposal of waste materials produced by large numbers of swine, poultry, and cattle that are raised in indoor confinement at single locations is a major environmental and economic problem. One approach to safe and economical disposal of large quantities of animal wastes is to find positive uses which will convert wastes from liabilities into assets of production. Use of animal wastes as soil amendments to provide biological control of fungal plant diseases in soil is one possible positive use. This use first requires demonstrating that animal wastes exhibit biocontrol activity against fungal plant pathogens when incorporated into soil. In this study, a technique was developed to add survival structures, “sclerotia”, of a major soilborne fungal plant pathogen, Macrophomina phaseolina, to soil with and without ground poultry litter, to retrieve sclerotia from soil, and to assay their survival by direct microscopic observation. Parameters that affect use of the technique, such as drying of sclerotia and soil moisture levels, were evaluated. When ground poultry litter was added to soil with sclerotia at 5% by weight, their survival was significantly reduced, and when litter was added at 10%, survival of sclerotia was nearly eliminated. These results demonstrate that the direct observation technique is useful and effective for evaluating survival of sclerotia of M. phaseolina in soil, and that incorporating poultry litter into soil reduces or eliminates their survival. Therefore, poultry litter exhibits biocontrol potential against sclerotia of this fungus. This technique may also be used to evaluate other animal wastes and agricultural by-products for biocontrol potentials against sclerotia of M. phaseolina in soil.
Technical Abstract: Germination of sclerotia of Macrophomina phaseolina was quantified by direct microscopic observation following application of experimental treatments in vitro and incubation of sclerotia in soil. Survival in soil was evaluated at different moisture levels and with and without poultry litter amendments. Formation of sclerotia in cornmeal agar was greatly increased by most isolates beneath pieces of index card applied to the agar surface. To assay germination, pieces of agar containing sclerotia were excised; macerated in dilute, liquid cornmeal agar on glass slides; thinly spread; and incubated in a saturated atmosphere for 18-22 h. Germinated sclerotia then were identified at X100 by morphological features of germ hyphae. Frequencies of germination were similar in dilute cornmeal, potato dextrose, and Czapek’s agars but less in water agar. Germination was not affected by air drying of sclerotia in agar for 2 weeks, but it was significantly reduced by air drying for 4 weeks and greatly reduced or eliminated by air drying for 6 or 8 weeks. Survival of sclerotia for 14 days in nylon membranes in a sandy loam soil was greatest at 50, 75, and 100% moisture holding capacity (MHC), progressively less at 0 and 25% MHC, and least at 125% MHC (flooded soil). Enhanced mycoparasitism appears to have accounted for reduced survival of sclerotia at 25% MHC in comparison to 0 and 50% MHC. Incorporation of ground poultry litter into soil at 5% by weight significantly reduced survival of sclerotia after 13 days, and incorporation of litter at 10% nearly eliminated it. These results indicate that the direct observation technique may be used to evaluate animal wastes and other by-products for biocontrol activity against sclerotia of M. phaseolina in soil.