Scouting Cotton for Bacterial Blight

Bacterial blight of cotton. Note the angular appearance of the lesions. The water-soaked appearance around the lesion is a diagnostic characteristic early in the morning.

Last year certainly set the stage when it came to observation of what should be considered to be a disease of rare occurrence in cotton.  Bacterial blight was a cotton disease that dominated the early 1970s and since that time has been observed in rare situations in the occasional field in MS.  Several blog postings from 2011 are still available for informational purposes and more specifically provide information regarding the reaction of specific varieties to inoculation with the bacterial blight organism based on foliar symptom development.  See:https://www.mississippi-crops.com/2011/07/29/2011-cotton-bacterial-blight-trial-ratings-stoneville-ms/

https://www.mississippi-crops.com/2011/07/21/update-bacterial-blight-of-cotton/

https://www.mississippi-crops.com/2011/07/15/alert-bacterial-blight-of-cotton/

Since last Saturday (6/23/12), more than 30 cotton fields in 5 MS counties planted to 6 different varieties have been observed with the characteristic angular leaf spot lesions that are attributed to the bacterial blight bacterium, Xanthomonas axonopodis pv. malvacearum.  Samples were confirmed to have bacterial streaming in the diagnostic laboratory (Ms. Clarissa Balbalian) and the MSU bacteriologist (Dr. Shien Lu) has samples to make isolations.

Systemic bacterial blight infection on the upper surface of a cotton leaf.

Bacterial blight can oftentimes be referred to as angular leaf spot.  The angular nature of the leaf spots is associated with the small veins in the leaf that limit the movement of the organism in the leaf tissue.  The leaf spots are generally small, less than 1/8 inch in size and depending upon the environmental conditions and time of day when observed can have water-soaked lesions around the lesion (see photo).  The water-soaking will appear dark green on the underside of the leaf especially in the presence of dew.  However, without dew lesions will appear almost black in color.  If the disease becomes systemic, groups of lesions will coalesce along the veins.  In addition, bacterial blight can also cause a lesion to occur on the petiole; however, rarely were those particular symptoms observed during 2011.  The lesions on the petiole will not be angular, will appear elongated, and will be black and have a sooty appearance.  This particular stage of the disease is referred to as the “black arm” phase.  However, one of the more important things to keep in mind, not ALL lesions that could appear angular are bacterial blight.  In some rare cases, other foliar diseases caused by fungi can produce a lesion that will appear angular.

When scouting for bacterial blight, keep in mind that the best method (in the field) to confirm the presence of the disease is to try to find leaves where the bacterium has gone systemic in the veins of the leaf.  More often than not, leaves that have the systemic infection will be present in the upper canopy while leaves infected with the angular leaf spots will be present in the low to mid-canopy.  However, there are exceptions to this rule as several consultants have indicated finding angular leaf spots throughout the canopy this season.  In addition, the lesions present on bolls will typically not be misdiagnosed with other problems especially if the initial water-soaked lesions are present.

Management alternatives

Put most simply, once the disease is detected there are no management alternatives that could be applied to address the disease.  Farmers should be aware that once the disease has been detected in the field that the application of a fungicide will NOT provide control of the disease.  The disease is caused by a bacterium.  In addition, the foliar application of products for nutritional purposes (e.g. foliar potash) will also not provide control of bacterial blight and should not be applied to manage plant diseases.

Note the angular appearance of bacterial blight associated with the three lesions in view.

At present, the best management situation we can hope for would be continued hot and dry environmental conditions that will not be conducive for further development and spread of the disease either within a particular field or to an adjacent field.  Essentially, any situation that limits free moisture in the form of dew will likely reduce the presence of the disease.  However, in cases where overhead irrigation is present the disease will likely continue to spread with the simulated rain splash associated with pivot irrigation.  Attempting to conduct irrigation during nighttime hours if at all possible may reduce the spread of the disease since dew would likely already be present.