Reproductive Potential of Monosporascus cannonballus.

Vine decline of melons caused by Monosporascus cannonballus is a destructive disease worldwide. Ascospores, the only spore stage produced by this soilborne fungus, serve as the primary inoculum. Ascospore production on roots occurs primarily at the end of the cropping season, and high soil temperatures (25 to 30°C) govern, in part, the rate of reproduction of the pathogen. In vitro studies confirm that the optimal temperature for reproduction ranged from 25 to 30°C. Additionally, the root system of a single mature cantaloupe plant is capable of supporting the production of approximately 400,000 ascospores. The latter population, if incorporated uniformly into 0.03 m3 (1 ft3) of soil, would be equivalent to 10 ascospores per gram of soil. Known problem fields contain as few as 2 ascospores per gram of soil. These results offer a possible explanation for field observations that economically significant disease problems can occur after only two consecutive melon crops if environmental conditions are conducive to pathogen reproduction, and they suggest that strategies to inhibit reproduction would be instrumental in disease management.