COTTON RESISTANCE TO ROOT-KNOT NEMATODE .1. PENETRATION AND REPRODUCTION

Nematode parasitism of cotton (Gossypium hirsutum L.) has become increasingly important because of yield losses and increased distribution across cotton production regions. Penetration and reproduction of root-knot nematodes, Meloidogyne incognita (Kofoid & White) Chitwood, Race 3, was compared on three cotton genotypes differing in level of resistance: M-8, susceptible to root-knot; M-78 (M-78RNR), moderately resistant; and M-315 (M-315RNR), resistant. Responses were characterized by penetration of plant roots by root-knot nematode juveniles and by reproduction, measured as number of eggs, egg-masses per plant, and eggs per egg-mass. Susceptible, moderately resistant, and highly resistant cotton genotypes were penetrated about equally by juveniles. Production of egg-masses, eggs per egg-mass, and eggs per plant by nematodes were different among the three cotton genotypes. The development of egg masses was delayed in M-78 and M-315 as compared with M-8, with the greatest delay and prevention of the formation of significant numbers of egg-masses in the highly resistant M-315. Production of eggs and second-stage infective juveniles in the moderately resistant M-78 were sufficient to contribute significantly to the build-up of the next generation of root-knot populations in cotton production fields. Reproduction on M-315 was so low that nematode populations would be expected to decrease significantly under continuous culture. M-315 possesses two major genes for resistance and is a valuable source of root-knot nematode resistance for cotton improvement programs. These genes in combination do not alter penetration of the root-knot nematode juvenile stage into cotton roots, but do have major negative effects on survival and reproduction of the nematodes.