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A disease of muskmelon (Cucumis melo L.) characterized by a vine decline and a cortical root rot was first observed in the Lower Rio Grande Valley of Texas in 1986. In 1990, isolations from diseased plants collected from four commercial production fields yielded the fungus Monosporascus cannonballus. Pathogenicity tests with eight isolates confirmed Koch's postulates; however, there were differences in aggressiveness observed among isolates. M. cannonballus is an ascomycete fungus that typically produces only one (rarely two), round, jet-black ascospore per ascus. There is no known asexual stage. Temperature optimum of one isolate was 35 C. The optimum pH for growth was 6-7, but it grew well up to pH 9. M. cannonballus was first reported on muskmelon in 1970 from Arizona and recently was found in Japan under glass house culture. The presence of this fungus in Texas marks only the third report of this species worldwide, although a similar species (M. eutypoides) is the cause of a collapse of melon plants in Israel.
Twenty isolates of Fusarium oxysporum recovered from diseased sugar beet and spinach (Chenopodiaceae) or red-root pigweed (Amaranthaceae) were examined using pathogenicity, isozyme, and mtDNA RFLP markers to determine genetic similarity among isolates from different hosts. Pathogenicity tests defined several levels of host specificity. Most isolates were specific to their original host; however, a few primarily were pathogenic to their original host but also caused some wilt on other hosts. Two isolates were pathogenic on all three hosts and six were not pathogenic to any of the hosts. Differences in isozymes and mtDNA RFLPs corresponded with differences in pathogenicity. Three main polymorphic groups based on host specificity were identified along with three sub-groups corresponding to aggressiveness of the isolates. These data suggest that while most isolates display a high degree of host specificity isolates exist within the population that lack such specificity and cross over to other species.