The reniform nematode, Rotylenchulus reniformis, is an emerging problem in U.S. cotton. The impact of this nematode and the extent to which it has and will continue to spread across the U.S. cotton belt are controversial. Long-term changes in cotton production and unique biological attributes of R. reniformis are key factors. Expert opinion surveys indicate that R. reniformis has replaced the root-knot nematode (Meloidogyne incognita) as the major nematode of cotton in Mississippi, Louisiana, and Alabama. In neighboring states the incidence of heavily infested fields has increased during the past 10 years. Estimated annual loss to the U.S. cotton crop is $ 130 M. Crop rotation and nematicides can reduce losses. Introgression of genetic resistance from primitive accessions of other cotton species offers the most promising opportunity to effectively control this pathogen in the long term. Laboratories in several institutions are currently pursuing this goal, with the promise of resistant cultivars adapted to U.S. cotton production regions within three years.

Absence of sources of resistance to the reniform nematode, Rotylenchulus reniformis Linford & Oliveira, 1940, is a major impediment to the production of upland cotton (Gossypium hirsutum L.) in the USA. In this study, two trispecies hybrids of G. hirsutum, G. longicalyx J.B. Hutch. & B.J.S. Lee, and either G. armourianum Kearney or G. herbaceum L. were used as bridges to introgress high resistance to the nematode from G. longicalyx into G. hirsutum Introgression was accomplished by recurrent backcrosses to G. hirsutum with cytogenetic analysis of early backcross generations to assess progress toward the euploid state (2n = 52), selection for nematode resistance at each generation, and examination of self progeny at the first, third, sixth, and seventh backcross to identify and eliminate lineages with undesired recessive traits. Altogether, 689 BC1 progeny were generated from the two male‐sterile hybrids. Introgression was pursued from 28 resistant BC1 plants, each of which was backcrossed four to seven times to G. hirsutum to derive agronomically suitable types. The resistance trait segregated (resistant/susceptible) 1:1 in backcross progeny and 3:1 in self progeny. There was no obvious diminution of the resistance across backcross generations. Advanced backcross plants were indistinguishable from agronomic cotton under greenhouse conditions, and comparisons of 240 homozygous resistant BC6S2 plants with heterozygous, susceptible, and recurrent parent plants in field plantings in 2006 showed normal lint quality and quantity. The upcoming release of seed from this project is expected to provide the cotton industry with a major new tool for managing the reniform nematode in cotton, which costs U.S. producers about $100 million annually.

Reniform nematodes (Rotylenchulus reniformis Linford & Oliveira) decrease U.S. production of Upland cotton (Gossypium hirsutum L., 2n = 52, 2[AD]1) by more than US$100 million yr−1 We report here on the mapping of a gene for extreme resistance that was introgressed from the African species G. longicalyx (Hutch. & Lee, 2n = 2x = 26; 2F1). The responsible allele, designated Renlon, was localized to chromosome 11 by first screening A‐subgenome simple sequence repeat (SSR) marker loci for parental polymorphism and then for association with resistance. The three most strongly coupled SSRs and a G. longicalyx gene conferring green seed fuzz, designated Fzglon, were screened against 984 resistant and susceptible individuals of multiple backcross generations. We used marker data and pedigrees to identify nonrecombinant heterozygous parents and thereby avoid bias from repeated sampling of a recombination event. We constructed linkage maps after progeny testing a small population (147) and after implementing three alternative approaches better suited to larger populations—marker‐assisted genotyping analysis, applying a cut‐off value as population‐wide genotyping criterion, and genotype‐selective sampling. The maps concordantly indicated the order to be Fzglon–Renlon–BNL3279_114–BNL1066_156–BNL836_215, with most Ren‐proximal bilaterally flanking markers within 6 cM of each other. The results will clearly facilitate use of Renlon in breeding, additional mapping, genomics, and prospective cloning.

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