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Response of Commercial Cotton Varieties to Xanthomonas citri pv. malvacearum at Early Developmental Stages in South Texas

Cotton is the most profitable non-food crop in the world. Xanthomonas citri pv. malvacearum (Xcm) is a Gram-negative prokaryotic pathogen that causes bacterial blight of cotton. Bacterial blight has been controlled for more than 50 years by breeding for host plant resistance. However, recent sporadic blight outbreaks in the US commercially grown varieties have raised concerns among growers and scientists about the possible development of resistant Xcm strains. The objective of the study was to evaluate seven commercial cotton varieties for resistance towards Xcm when inoculated at three cotton growth stages. The varieties consisted of one resistant (NG 5711), two susceptible (NG 3406 and DP 1725), and four partially resistant cotton (NG 3729, DP 1646, DP 1845 and DP 1948). All varieties were inoculated with Xcm at match head, candle, and pink flower stages at a concentration of 106 colony forming units per ml. The inoculation mixture consisted of Xcm, Silwet L-77 (0.25% v/v), and deionized water. Disease incidence and severity data were collected seven and fourteen days post-inoculation. A statistically significant difference was observed among the varieties both for disease incidence and severity. As predicted, susceptible varieties had significantly greater disease expression than the resistant varieties (P<0.05). Between partially resistant varieties, disease expression was greater in variety NG 3729 and lowest in DP 1948. Notably, varieties considered partially resistant were found to be nearly twice (40%) as susceptible to infection as labeled (25%). Amongst the possible reasons may be due to a higher Xcm virulence from the recently field-isolated strain used, the plants harbored an incomplete suite of resistance genes and/or environmental conditions that were conducive for infections. In any event, the importance of maintaining a full resistance gene package to minimize disease was shown. The data demonstrated the vitality of a continuous evaluation of commercial cotton varieties for resistance towards bacterial blight to identify and control the spread of epidemics.

Author(s): Enrique G. Medrano, Greta L. Schustera and Srinadh Kodalia

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