Bacterial blight, caused by Xanthomonas axonopodis pv. malvacearum, has been reported from most all cotton production regions around the world. The bacterium is capable of surviving saprophytically on infested crop residue. Dry arid conditions facilitate survival in soil from year to year.
Cotton plants are susceptible to infection at all growth stages; however, leaves and bolls are most commonly infected later in the growing season. Conditions that favor disease development consist of moderate temperatures and high humidity. Wounding of leaves by blowing sand or hail may lead to an increase in incidence of the disease. Sprinkler irrigation can increase spread of the pathogen.
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The population structure of Xanthomonas axonopodis pv. malvacearum is complicated with numerous races being present in cotton around the world. In the United States, race 18 has been predominant for the past several decades. The identification of multiple resistance genes and deployment has led to resistance or immunity in many upland varieties. As a result, bacterial blight epidemics have been sporadic causing negligible losses. Outbreaks of the disease have recently occurred in parts of the Mid-South raising questions about the reemergence of the disease in that region.
Various symptoms are associated with the disease. Initial symptoms consist of small, pinpoint lesions on foliage.
- As the disease progresses, lesions take on a blocky, angular shape as the bacterium is not capable of crossing veins found within leaves.
- Following systemic infections, veins may become necrotic.
- Petiole infections result in severe necrosis, which may progress down the limb or branch resulting in a symptom referred to as Blackarm.
- Premature defoliation and fruit abortion are often associated with the aforementioned symptoms. Later in the season, the bacterium may infect developing bolls causing a boll rot.
- The appearance of these symptoms differs from foliar symptoms. Such lesions have a circular appearance, as there are no veins within the boll to limit growth of the bacterium.
These symptoms are characteristic of what has been associated with infections caused by Xanthomonas axonopodis pv. malvacearum, race 18 over the past several decades.
During the 2015 growing season, subtle differences in symptom expression have been observed when the disease occurs on varieties that were previously documented as being resistant. While angular lesions still occur, the middle potion of the lesion has a distinctly different appearance. The center of the lesion appears more degraded and falls out, giving the infected tissue a ‘shot hole’ appearance (Fig. 5). Furthermore, infected leaves of resistant varieties tend to turn chlorotic more readily (Fig. 6). Recent field observations have shown that most varieties evaluated exhibited some level of disease (Table 1). Additional ratings can be obtained as they become available.
In order to quantify the frequency, distribution and impact of this disease, detailed sampling is needed. We propose that County Extension Agriculture and Natural Resources Agents communicate with producers, consultants and other members of the cotton community to identify fields exhibiting bacterial blight and bacterial blight-like symptoms.
Bacterial isolates obtained from symptomatic tissues will be tested against varieties with a known reaction to Xanthomonas axonopodis pv. malvacearum, race 18 to determine what changes if any have occurred to populations of the bacterium.
An emphasis will be placed to isolates collected from formerly resistant varieties (such as Partially resistant: DP 1359B2RF, PHY 339WRF; Resistant: DP 1410B2RF, FM 2007GLT and Immune: FM 1830GLT, FM 2334GLT, FM 2484B2F and ST 5289GLT).
Samples of leaves expressing bacterial blight symptoms should be placed in a plastic bag and shipped to the Texas A&M AgriLife Research and Extension Center, 1102 East FM 1294 Lubbock, TX 79403. The following information should accompany samples: 1) variety name, 2) estimated level of severity (i.e., low, moderate or high), as well as 3) GPS coordinates of the field (to allow for further monitoring or subsequent sampling).
Additional laboratory and greenhouse tests will compare and contrast bacterial isolates. If you have any questions regarding the appearance of the disease or need additional information about the sampling please contact either Dr. Wheeler (806-746-6101; firstname.lastname@example.org) or myself (806-632-0762; email@example.com).
Blight incidence ratings in a variety trial for a field near Plains, TX suspected of having a new bacterial problem, or a possible mixture of a new bacterium plus our standard Bacterial blight pathogenz
zDifferences between varieties were not significant at P=0.05.