alternative approach to multigenic resistance within a single cultivar, is use of multiple cultivars with different R-genes, that can be utilized in rotation. This approach has been employed in soybean and potato (Bakker et al., 1993; Niblack et al., 2002).
In sum, all evidence available indicates that the best approach is to work toward the development of reniform nematode resistance in Upland cotton simultaneously through introgression of resistance from multiple known sources. Failure to develop multiple sources of resistance could lead to a situation wherein the only source available carries unacceptable vulnerabilities or is of short duration due to development of resistance-breaking nematode populations.
ONGOING PROJECTS IN THE UNITED STATES
G. hirsutum. About 20 G. hirsutum accessions with weak to moderate levels of resistance to the reniform nematode have been reported. Resistance within G. hirsutum appears highly sensitive to environment and nematode population. In the following three cases, weak to moderate levels of resistance that were reproducible in one environment were not observed when tested in a different environment or against a different population of nematodes: 1) The accessions TX 20 (PI 163608), TX 69 (PI 153964), TX 709 (PI 265146), TX 834 (PI 529833), TX 874 (PI 529852), TX 893 (PI 529860), and TX 903 (PI 529864) that were scored by Yik and Birchfield (1984) in replicated experiments as moderately resistant were later scored by Robinson et al. (1997) as susceptible, because in the latter study they supported 17- to 64-fold increases in nematode populations in pots within a 7-week period. 2) Of 6 primitive G. hirsutum accessions that were scored by Robinson et al. (2004) as moderately resistant, only TX 1828 (PI 530459) and TX 1586 (PI 530217) were classified by Weaver et al. (2007) as resistant, or possibly resistant. 3) Of seven accessions [TX 245 (PI 165358), TX 378 (PI 165321), TX 500 (PI 209305,), TX 1419 (PI 530110), TX 1472 (PI 530151), TX 1565 (PI 530196), and TX 1765 (PI 530396)] observed by Weaver et al. (2007) to consistently support lower nematode populations than the control, all but one (TX-500) had been tested previously (Robinson et al., 2004), and only TX 1565 had been scored as possibly resistant. Its nematode population value in the 2004 study was 21% of the susceptible control, contrasted with 39 to 133% for the other five accessions.
In some cases, moderately to highly resistant primitive accessions of G. hirsutum from the USDA Cotton Collection have been found to have flower and leaf traits like G. barbadense [for example, TX 110, TX 500 (reclassified by A. E. Percival as GB 1032), TX 502 (PI 153878), TX-1347 (PI 530075, reassigned by A. E. Percival as GB 1042), TX-1348 (PI 530076, with origin near that of TX 1347), TX-2468 (PI 607773), and TX-2469 (PI 607774)] and to have been collected in areas (Guatemala: TX 110 and TX 502; Veracruz State of Mexico: TX 1347 and TX 1348; Brazil: TX 2467 and TX 2469) where introgression between G. hirsutum and G. barbadense occurs naturally. Thus, the question remains as to whether some of the resistant accessions that have been identified in the TX collection are G. hirsutum or G. barbadense. Comparison of reniform nematode reproduction on 850 accessions of G. barbadense and 1,419 of G. hirsutum (Robinson et al., 2004) clearly showed that although there is great variation and a wide range in the ability of accessions in both species to support reniform nematode reproduction, susceptible G. barbadense accessions on average support less reproduction than do typical G. hirsutum accessions, and resistance is more common in G. barbadense. In G. barbadense, the incidence of accessions detected with significantly less than 1/3 the reniform nematode reproduction measured on susceptible cv. Deltapine 16 was 2.1%, compared with 0.4% for G. hirsutum (Robinson et al., 2004).