The effect of two QTLs for resistance to in cotton on nematode egression from roots.

Cotton is widely grown in the southern US and is its most significant pathogen. The germplasm line M-120 RNR is highly resistant to due to two resistance QTLs (quantitative trait loci), and . Both QTLs reduce total egg production, but the QTLs affect development at different life stages. The QTLs do not appear to affect initial penetration of but genotypes containing had fewer nematodes in the roots 8 days after inoculation than near isolines without , which may indicate egression from roots. Three greenhouse trials were conducted using cotton isolines to determine whether and affect egression of juveniles from roots. On each of the five sampling dates (4, 6, 8, 10, and 12 DAI), nematodes that egressed from roots were counted and roots were stained to count nematodes that remained in the roots. The effect of resistance QTLs on egression from the roots differed among the trials. Nematode egression was consistently numerically greater, but inconsistently statistically different, from plants with both QTLs than from plants with neither QTL. Plants with only one QTL generally did not differ from plants with both QTLs, and the effects of and did not differ in any consistent way. In a separate experiment, plants with neither QTL had more eggs per egg mass than did plants with both QTLs, whereas plants with only one QTL had an intermediate number. Root gall size was measured in two trials and no consistent differences in gall size were observed. We conclude that (1) and do not stimulate nematode egression from cotton roots, (2) both and reduce eggs/egg mass, and (3) neither nor affect gall size.