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

Nematode parasitism of cotton (Gossypium hirsutum L.) has become incre asingly important because of yield losses and increased distribution a cross 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 resist ance: M-8, susceptible to root-knot; M-78 (M-78RNR), moderately resist ant; and M-315 (M-315RNR), resistant. Responses were characterized by penetration of plant roots by root-knot nematode juveniles and by repr oduction, measured as number of eggs, egg-masses per plant, and eggs p er egg-mass. Susceptible, moderately resistant, and highly resistant c otton genotypes were penetrated about equally by juveniles. Production of egg-masses, eggs per egg-mass, and eggs per plant by nematodes wer e different among the three cotton genotypes. The development of egg m asses was delayed in M-78 and M-315 as compared with M-8, with the gre atest delay and prevention of the formation of significant numbers of egg-masses in the highly resistant M-315. Production of eggs and secon d-stage infective juveniles in the moderately resistant M-78 were suff icient to contribute significantly to the build-up of the next generat ion of root-knot populations in cotton production fields. Reproduction on M-315 was so low that nematode populations would be expected to de crease significantly under continuous culture. M-315 possesses two maj or genes for resistance and is a valuable source of root-knot nematode resistance for cotton improvement programs. These genes in combinatio n do not alter penetration of the root-knot nematode juvenile stage in to cotton roots, but do have major negative effects on survival and re production of the nematodes.