COTTON RESISTANCE TO ROOT-KNOT NEMATODE .2. POST-PENETRATION DEVELOPMENT

Root-knot nematode (RKN), Meloidogyne incognita (Kofoid and White) Chi twood, damages cotton, Gossypium hirsutum L., roots, affecting water a nd nutrient uptake and predisposing the plants to other soilborne path ogens. Post-penetration development of RKN was compared on three genot ypes. Susceptible M-8, moderately resistant M-78, and highly resistant M-315, were grown in a greenhouse in a Wickham sandy soil (a fine, lo amy, mixed thermic, Typic Hapludult) and inoculated with second-stage juveniles. RKN in each of 7 developmental stages were counted every 2 d for 44 d. At 18 DAI, 70, 45, and 6% of RKN present were adults in M- 8, M-78, and M-315, respectively. In addition to slower development of RKN in M-315, resistance is expressed by significantly fewer developi ng third and fourth stage juveniles at 8 DAI and fewer developing to m ature females at about 24 DAI. Most RKN that penetrated M-315 failed t o establish and/or maintain giant cells. However, a few females establ ished giant cells. At 44 DAI, the numbers of egg-laying females on M-8 , M-78, and M-315 were 299, 144, and 5, respectively. There were signi ficantly fewer root galls on M-315 than on M-78 or M-8 beginning 10 DA I, and significantly fewer on M-78 than or M-8 at 40 DAI. Galls were s ignificantly smaller on A M-315 than on M-78 or Mg beginning 8 DAI, an d significantly smaller on M-78 than on M-8 beginning 18 DAI. Thus, th e post-penetration development of root-knot nematode was slower, fewer developed to adult females, and root galls were fewer and smaller on the resistant genotypes than on the susceptible M-8.