THE SURVIVAL OF FUNGAL SCLEROTIA UNDER ADVERSE ENVIRONMENTAL CONDITIONS

1. Sclerotia have been found to survive under adverse environmental conditions for long periods. Their viability is retained at low and subzero temperatures, while it is progressively reduced at temperatures above about 15° C. Dry conditions and lack of competition from other organisms favour survival. Depletion of food reserves results in loss of resistance to environmental factors. 2. The majority of sclerotia consist of an outer layer of thickened, close-fitting, often pigmented cells that form a rind over a tissue of closely interwoven hyphae. A cortex and medulla is often discernible. 3. Features of sclerotia that are important in overcoming the harmful effects of desiccation and extremes of temperature include: the presence of a rind and sometimes an additional covering of either fungal or host tissue; the compact nature of the sclerotium; miscellaneous protectants on the surface of and in the hyphae; melanized hyphal walls; high intracellular osmotic concentrations; slow air-drying to lower moisture content. Also the buffering action of the soil and the protection afforded by vegetation and plant debris are important. 4. The active exudation of water, the accumulation and secretion of soluble carbohydrates during sclerotium development and maturation, and the formation of large amounts of mucilage are of significance in resistance to desiccation, extremes of temperature and radiations. 5. The nature and the pigmentation of the rind together with the site of sclerotium development may give protection against harmful radiations. 6. The loss of sclerotium viability caused by toxic chemicals in the soil and atmosphere or by the enzymic activities of other organisms is reduced by the rind and the melanization of hyphal walls. 7. Survival under starvation conditions is achieved by the accumulation of abundant food reserves (lipids and/or carbohydrates) and a low level of metabolic activity. In the presence of a compatible host exogenous sources of energy are sometimes used. 8. Although the sclerotium is highly adapted to survive adverse conditions for long periods, sometimes micro-environmental conditions may be so severe that only a few sclerotia can retain their viability. However, even a few such comparatively large bodies, rich in food reserves, have a considerable inoculum potential.