Control of plant pathogens with viruses and related agents

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Abstract
Cytoplasmically transmissible agents causing diseases of plant pathogenic fungi characterized by reductions in pathogenicity, ability to form sexual and asexual spores, spore viability and growth rate, are often associated with the presence of one or more specific segments of virus-like double-stranded RNA (dsRNA). In Italy, hypovirulent dsRNA-containing strains of the chestnut blight fungus,Endothia(Cryphonectria)parasitica, have become predominant in many areas where blight is no longer a serious problem. dsRNA-containing strains of other pathogens, with various degrees of debilitation, survive in natural populations but have not become predominant or resulted in any great reduction in disease. Examples include the Dutch elm disease fungus,Ophiostoma(Ceratocystis)ulmi, and the wheat take-all fungus,Gaeumannomyces graminisvar.tritici. Successful biological control of such pathogens could probably be achieved, however, if methods could be developed to suppress the loss of dsRNA that occurs during the sexual and other stages of their life cycles, and to suppress the vegetative incompatibility reactions that reduce the cytoplasmic transmission of dsRNA. Systemic infection with attenuated strains of plant viruses can protect plants from later infection by virulent strains of the same or closely related viruses. Despite some notable successes, e.g. control of citrus tristeza and tomato mosaic viruses, such ‘cross-protection’ has not been widely applied because of the cost and difficulty of application, and caution about the widespread distribution of infectious agents in the environment. These problems could be overcome if cross-protection could be achieved by the expression of a single viral gene rather than infection with intact virus, and consideration of possible mechanisms of cross protection suggests novel ways of producing virus-resistant plants.