Chapter 25 - Hypovirulence to Control Fungal Pathogenesis

Abstract

The cytoplasmic factor associated with hypovirulence in chestnut blight is found to be double-stranded RNA (dsRNA) molecules. While mitochondrial hypovirulence appears to play a role in the biological control of chestnut blight in specific stands of American chestnut trees in Michigan and Ontario, the preponderance of the hypovirulent strains found in surviving chestnut stands in Europe and North America appears to be dsRNA associated. The most significant aspect of the natural development of hypovirulence is that it has allowed researchers to focus on the biological constraints of fungal pathogens in a living substrate. Hypovirulence, initially found as a naturally occurring biological control of chestnut blight, has now been a part of experimental and practical application. To treat chestnut trees, hypovirulent strains traditionally have been placed around the margins of cankers caused by virulent strains. The hypovirulent inoculum is placed in the hole and taped to prevent the rapid desiccation of the introduced strain. If the hypovirus or mitochondria transfer to the thallus of the virulent strain inciting the canker, a layer of wound periderm or callus will develop at the margins, walling off the fungus. Effective treatments generally lead to the eventual closing of the canker when periderm completely covers the exposed wood within 2 or 3 years.