BREEDING FOR RESISTANCE TO BACTERIAL BLIGHT OF COTTON IN RELATION TO RACES OF THE PATHOGEN
Mar 10, 2017

Breeding & Genetic improvement WCRC Breeding-NorthAmerica WCRC1
Abstract                                                                         Back to Table of contents

Bacterial blight of cotton incited by Xanthomonas campestris pv malvacearum (Xcm) occurs in most cotton producing regions of the world.  Bacterial blight causes a 1% yield loss annually in the USA, and more than 50% loss during epidemics in Africa.  Currently 19 races of Xcm are recognized in the USA.  A shift in races of the pathogen has recently occurred in Texas, from USA races 1, 2 and 7 to the most virulent race 18.  In Africa, new virulent isolates (HVS) of the pathogen have evolved, with HV1 being highly virulent.  At least 22 major genes (B) for resistance to Xcm have been reported.  The majority of these genes are partially to completely dominant for resistance.  Additive and digenic interactions were also reported.  Single resistance genes confer resistance to a few races (vertical resistance), but are vulnerable to the other races of the pathogen.  Thus, different combinations of single B genes and modifiers are important to obtain a stable source of horizontal resistance.  Immunity to all USA races of the pathogen conferred by the B2B3BSm gene combination has been stable for 22 years in the USA. Resistance to the HV1 isolate has been obtained in Africa.   Selection must be made utilizing a compatible race mixture of the pathogen, including virulent races to identify gene combinations that give broad spectrum resistance to many races.  Recurrent selection and the backcross methods have been used to develop horizontal resistance to all races of the pathogen.  Resistant cultivars will reduce disease incidence and severity, and control bacterial blight.

Conclusions

Cotton pathologists and breeders have made considerable advances in breeding for resistance to the bacterial blight pathogen.  High resistance is available in several cultivars throughout the world.  For example, Siokras in Australia; Barakat, Bar, and Baras in the Sudan; Allen, Uk74, UK71, Albar, Reba B50 in East and Central Africa; BJR-734 in India, and the Tamcot cultivars and MAR germplasm in the USA have been developed for resistance to the bacterial blight pathogen.

Continued focus and progress should be made worldwide  to develop cotton cultivars with high and stable resistance to the virulent races of Xanthomonas campestris pv malvacearum.  We agree with Painter's (1951) view that resistant cultivars are not a panacea for all pest problems. The use of resistant cultivars alone should not be expected to control pests under all conditions or in all locations where the crop may be grown.  Resistant cultivars should be used in concert in integrated pest management  (IPM) programs, which may also include minimal use of pesticides, cultural management, and biological control.  Cotton cultivars with resistance to insects and pathogens, high yielding ability, earliness and improved fiber quality provide the cornerstone for a successful and profitable production system.

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