Ascospores of Monosporascus cannonballus: Germination and distribution in cultivated and desert soils in Arizona

Our objectives were to provide an explanation for the observed uniform distribution of vine decline of cantaloupe in commercial fields and determine the role of ascospores of Monosporascus cannonballus in the etiology of this disease. Population densities of ascospores in cultivated soils were uniform in distribution, both vertically and horizontally. Additionally, ascospores of the fungus were recovered from all soil samples collected both vertically and horizontally from two native desert sites, and perithecia of the fungus were observed on roots of a native plant, Lepidium lasiocarpum, growing in a native habitat. These results indicate that the fungus is an indigenous soilborne fungus, which provides an explanation for the field-wide occurrence of the disease in commercial cantaloupe fields. Ascospores, both native and culturally produced, germinated readily in the rhizosphere and penetrated the roots of susceptible melon seedlings growing in field soil. Up to three germ tubes per ascospore were produced, and germ tubes up to 800 mu m in length were observed. Our results confirm the suspected, but previously undocumented, role of ascospores as primary inoculum.