REPLACEMENT SERIES - A TOOL FOR CHARACTERIZING COMPETITION BETWEEN PHYTOPARASITIC NEMATODES

The replacement series approach was used to detect and define competit ion between Meloidogyne incognita (Mi) and Rotylenchulus reniformis (R r) on soybean. In three greenhouse tests, soybean cv. Davis seedlings were inoculated with 1,000 vermiform nematodes in the following Mi:Rr ratios: 0:0, 100:0, 75:25, 50:50, 25:75, and 0:100. After 86 days, rel ative nematode-yield values (number of each species in mixed culture d ivided by number in nonmixed culture) were calculated based on nematod es in soil per gram of dry root tissue. Calculated values were plotted and the resulting line compared with a reference line representing eq ual inter- and intraspecific competition predicted by the replacement series. Relative yields for Mi were higher than predicted at all ratio s where Mi and Rr occurred together (lack-of-fit regression, F = 5.940 1, P = 0.0008), indicating increased reproduction in the presence of R r. Relative yields for Rr did not differ from predicted yields (lack-o f-fit regression, F = 0.7565, P = 0.5203), indicating no effect of Mi on Rr. These relationships were not detected using analysis of varianc e. The relationship between Mi and Rr was independent of host coloniza tion by Diaporthe phaseolorum var. caulivora, the stem canker fungus.