Response of commercial cotton varieties to Xanthomonas citri pv. malvacearum at early developmental stages in south Texas

Authors: KODALI, SRINADH - Texas A&M University; Medrano, Enrique; SHUSTER, GRETA - Texas A&M University

Submitted to: Research Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/2021
Publication Date: 9/24/2021
Citation: Kodali, S., Medrano, E.G., Shuster, G. 2021. Response of commercial cotton varieties to Xanthomonas citri pv. malvacearum at early developmental stages in south Texas. Research Journal of Plant Pathology. 4(4):01.

Interpretive Summary: Xanthomonas citri pv. malvacearum (Xcm) is a bacterial pathogen that is responsible for the cotton disease known as bacterial blight. Historically, this disease has been managed by breeding and using cotton varieties that are resistant to the pathogen. However, a resurgence of the disease has been reported throughout the U.S. Cotton Belt, but the cause of this resurgence is unknown. We isolated Xcm from field-infected plants and inoculated commercial cotton varieties that were considered to be susceptible, partially resistant, or resistant to bacterial blight infection. Expectantly, the susceptible plants became diseased and the resistant plants showed negligible disease symptoms. However, the infection level of cotton varieties that were considered to be partially resistant was nearly double than the expected infection rate. These results suggest that the resurgence of bacterial blight in the U.S. may be caused by a new strain of Xcm that has increased virulence and/or plants in the field did not contain the full suite of resistance genes. Regardless, our research demonstrates the importance of continuous evaluation of commercial cotton varieties to ensure plants contain the appropriate resistance genes to manage the bacterial blight disease in cotton.

Technical Abstract: 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 & DP 1725), and four partially resistant cotton (NG 3729, DP 1646, DP 1845 & DP 1948). All varieties were inoculated with Xcm at match head, candle, and pink flower stages at a concentration of 106 cfu 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 than 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.