Evaluation of Gossypium species for resistance to cotton leaf curl Burewala virus

Cotton leaf curl disease (CLCuD), caused by cotton leaf curl Burewala virus (CLCuBV), has emerged as a major threat to cotton production in Pakistan. Resistance to CLCuBV was evaluated in cultivated and wild cotton genotypes representing six Gossypium species by visual symptom scoring and virus assessment using PCR tests. Considerable variation in responses was observed when using whitefly and graft transmission to inoculate Gossypium genotypes with CLCuBV in field and greenhouse experiments. Under field evaluation, all cultivated genotypes of Gossypium hirsutum and three genotypes of G. barbadense were susceptible. Eleven genotypes that represented six wild and cultivated Gossypium species were considered to be highly resistant as they were free from infection. Similar results were obtained when these genotypes were tested using whitefly transmission. To verify these findings, 132 cultivated and wild genotypes were tested by graft inoculation. All G. hirsutum genotypes (116 cultivated, 1 wild, 1 transgenic Coker-312 and 1 non-transgenic Coker-312), three G. barbadense genotypes and one G. thurberi genotype were highly susceptible and exhibited symptoms 9-12 days after grafting. Four genotypes of G. arboreum and one genotype of G. anomalum did not express symptoms but had a detectable level of virus. One genotype of G. herbaceum and three wild genotypes of G. hirsutum showed mild symptoms (severity indexes of 1-2) and exhibited delayed disease development. These genotypes were classified as moderately resistant to resistant. Resistant genotypes that were identified in this study will be useful sources for exploitation of breeding programmes aimed at developing CLCuBV-resistant varieties and increasing genetic diversity.