Genotypic and phenotypic evaluation of wild cotton accessions previously identified as resistant to root-knot (Meloidogyne incognita) or reniform nematode (Rotylenchulus reniformis)

The root-knot nematode (RKN; Meloidogyne incognita) and reniform nematode (RN; Rotylenchulus reniformis) reduce Upland cotton (Gossypium hirsutum) yields nearly 5% annually throughout the United States. Recent work has proved successful in transferring the chromosome 11 (Chr11) and 14 RKN resistance QTL and the chromosome 21 Renbarb2 RN resistance QTL into elite Upland cultivars. The deployment of a limited number of resistance QTL risks the creation of virulent nematode populations; therefore, we set out to phenotypically and genotypically re-evaluate wild G. hirsutum and G. barbadense accessions that had previously been labeled as resistant to RKN and/or RN. Twenty-six (26) G. hirsutum accessions were evaluated for RKN resistance and were genotyped using SSR and SNP markers that corresponded to the Chr11 and Chr14 QTL. Accessions showed a range of RKN resistance phenotypes as measured by eggs g−1 root, gall score, and reproductive factor; however, genotypic analyses revealed that all resistant accessions carried either the Chr11 or Chr14 resistance QTL. For RN resistance, G. hirsutum and G. barbadense accessions were investigated. Genotyping of sixteen (16) G. barbadense accessions with the SSR marker BNL3279 that is linked to the Renbarb2 QTL revealed that resistance in these accessions was due to an allele of Renbarb2. Twenty-one (21) G. hirsutum accessions were evaluated for RN resistance as measured by eggs g−1 root, vermiform nematodes g−1 soil, and reproductive factor. None of the G. hirsutum accessions showed a consistent resistant phenotype across multiple experiments. Our findings emphasize the importance of screening new genetically diverse cotton populations in order to find novel sources of RKN and RN resistance.