Root Galling and Reproduction of Meloidogyne incognita Isolates from Texas on Resistant Cotton Genotypes.

Several cotton genotypes with resistance to Meloidogyne incognita have been released in recent years. To estimate the durability of this resistance, galling severity on these resistant genotypes by M. incognita was measured. Nematode isolates (115 total) were collected from cotton fields in 14 Texas counties in August and September 1996 and 1997. Four additional isolates from Maryland, Mississippi, and North Carolina were also tested. The isolates were evaluated in 12 greenhouse experiments for their ability to gall roots of the resistant cotton genotypes M315, Acala NemX, and Stoneville LA887 and the susceptible cultivar Deltapine 90. Numbers of galls on each genotype by each isolate were counted 60 days after inoculation with 10,000 eggs/plant. M315 consistently had the fewest galls for each nematode isolate, whereas Deltapine 90 had the greatest number of galls. Numbers of galls on NemX and LA887 were usually intermediate and more variable. For each separate experiment, analysis of variance indicated that the effects of nematode isolates, cotton genotypes, and isolate-genotype interaction were significant (P < 0.05). In two of the experiments, nematode reproduction was also measured and galling was positively correlated (r = 0.68 and 0.86) with egg production by M. incognita. Nematode isolates from one field exhibited higher root galling and reproduction (P < 0.05) on resistant genotypes than other isolates, suggesting a need for gene deployment systems that will enhance the durability of resistance.