Monosporascus cannonballus-melon pathosystem: Mechanism of vine decline, phenotypic characterization and mycelial incompatibility, and ascospore germination and sources of resistance

Abstract

Monosporascus cannonballus Pollack & Uecker, a soilborne root-infecting ascomycete, an economically important pathogen of melons (Cucumis melo L.) and watermelons (Citrullus lanatus L.). The fungus causes root rot and/or vine decline in different geographical areas worldwide. Little is known about the biology of this fungus and the mechanism by which the pathogen induces wilting of infected plants. This dissertation focuses on the biology, epidemiology, and etiology, as well as management strategies of both the pathogen and the disease. Microscopic examination of the xylem vessels of wilted plants revealed heavy occlusion by tyloses. Measurement of hydraulic conductivity indicated a 20-fold reduction in flow rate of plugged vessels, suggesting that tyloses contribute to the rapidity of vine decline in infected plants. The onset of symptoms coincide with high soil temperatures (25°C and above) and although vine decline occurs late in the growing season (i.e. generally two weeks before harvest), plants could be infected as early as nine days after planting. This dissertation also reports for the first time the existence of strains of M. cannonballus. Phenotypic differences such as cultural characteristics and growth rate were observed among the isolates and results indicate that genetically isolated strains, based on mycelial incompatibility, exist within M. cannonballus. Furthermore, local field populations of M. cannonballus can be heterogenous according to the number of mycelial compatibility groups identified. Ascospores of M. cannonballus germinate only in the rhizosphere/rhizoplane of melon and watermelon hosts in five soil. This result suggests a very limited host range of ascospore germination of M. cannonballus. Finally, field tolerance against M. cannonballus exists among cantaloupe cultivars and breeding lines. This will facilitate breeding for resistance within the cantaloupe type of melon. The knowledge derived from these studies contribute to our understanding of the biology and epidemiology of M. cannonballus and will serve as the basis for control or management of vine decline in the future.