Evaluation Methods, Resistant Germplasm, and Breeding for Resistance to Bacterial Blight in Cotton: A Review

Abstract

Bacterial blight (BB) caused by Xanthomonas citri pv. malvacearum (Xcm) is an important cotton (Gossypium spp.) production problem. In the U.S., BB has been controlled effectively using resistant cultivars and acid-delinted seed since the 1970s; however, resurgence of BB occurred in the early 2010s because of negligence in breeding. This review provides an up-to-date account on the pathogen, resistance evaluation methods, resistant germplasm lines, and breeding methods. Twenty-two Xcm races have been reported worldwide, and race 18 is currently the only one found in production fields in the U.S. To evaluate cotton for BB resistance, a pressure-sprayer-based method with surfactant in the field and a cotyledon-scratching-based method in the greenhouse are most often used. Breeding for BB resistance was highly successful in Sudan between the late 1930s and 1960s, when many resistance genes were transferred to G. barbadense from G. arboreum, G. herbaceum, G. anomalum, and G. hirsutum. Breeding for BB resistance commenced in the U.S. in the 1940s, leading to development of numerous resistant Upland cultivars. Although backcrossing was often used to transfer resistance genes in early years, forward breeding has been the breeding method of choice. Currently, some and possibly all resistant cultivars in the U.S. possess the resistance gene B12, which confers immunity with no water-soaked symptoms. Although B12-based resistance has held for a long time, identification of new resistant sources is needed to prevent an epidemic of BB due to evolution or introduction of possible new virulent Xcm races.