A 2-year field study was conducted to determine the influence of planting method, i.e., transplanting or direct seeding, black plastic mulch, and soil fumigation on the vine growth, yield, and root structure of diploid hybrid watermelon. The experiment was a split-plot design with fumigation as the main plot and there were four replications. Methyl bromide (337 L·ha−1) was applied to the soil, which was then tarped. Black plastic mulch, 0.61 m wide × 2 mil (Visqueen 4020™) was applied to appropriate rows. Vine growth was measured during the season and yield was determined by the number and weight of fruit from each treatment. After fruit harvest, plant roots were excavated so that root structure was maintained with minimal damage and roots were photographed. Root systems were scored for tap root dominance and overall root distribution. Direct-seeded watermelon had more vine growth and higher yields in both years than transplanted watermelon. The advantage of direct seeding was likely the result of the growth and root expansion that occurred for these plants while the transplants were still in the greenhouse. Direct-seeded plants also displayed greater tap root dominance in each year than transplanted watermelon. Roots of both direct-seeded plants and transplants had a greater range in size distribution in both years under plastic mulch than those grown on bare ground. In late-planted watermelon, direct-seeded plants had more favorable vine growth and yield without the aberrant roots systems produced by transplants.
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.
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.