| Summary: | Microplot studies were conducted to evaluate the effects of cotton (LA. 887), soybean (Pioneer 96B21), and three endemic weed species, pitted morningglory (Ipomoea lacunosa), hemp sesbania (Sesbania exaltata), and johnsongrass (Sorghum halepense), on reproduction of the reniform nematode, (Rotylenchulus reniformis). Over two microplot trials the co-culture of cotton with any of the three weeds suppressed numbers of reniform nematode juveniles in soil. When grown singly, reniform nematode reproductive values after 60 days on cotton averaged 69.0, while those for morningglory, hemp sesbania, and johnsongrass averaged 42.0, 23.5, and 18.0, respectively. Reproductive values for cotton co-cultured with morningglory averaged 38.7. Those for the cotton-hemp sesbania and cotton-johnsongrass combinations averaged 23.5 and 26.2, respectively. Reniform reproduction data for soybean cultured alone or with the three weeds in two trials showed reduced reproduction of reniform nematode only in the presence of johnsongrass. Suppression of reniform nematode reproduction likely resulted from the secretion of allelopathic compounds by weed roots and from crowding due to the increased amount of biomass present in microplots containing two plant species. Data from subsequent greenhouse experiments conducted with cotton and soybean and leachates from each of the three weed species supported the allelopathy hypothesis. Reniform reproduction on cotton and soybean plants irrigated with leachates from the roots of morningglory, hemp sesebania and johnsongrass was significantly reduced compared to soybean irrigated with water. Laboratory experiments conducted in which reniform nematode eggs were exposed to leachates from roots of morningglory, hemp sesbania and johnsongrass, nonfiltered and filtered through a .45 μm and a .80um filter unit resulted in suppression of hatch and delayed development of reniform eggs in the nonfiltered portions of both filter units and the filtered portion of the .80 μm filter.
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