Introgression of Resistance to Nematode Rotylenchulus reniformis into Upland Cotton ( Gossypium hirsutum ) from Gossypium longicalyx

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 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 BC 1 progeny were generated from the two male-sterile hybrids. Introgression was pursued from 28 resistant BC 1 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 BC 6 S 2 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.