One of the biggest threats to tobacco that we see in float beds during warm weather is bacterial soft rot, or blackleg. Temperatures have been summer-like in recent days, and a few cases of this disease have begun to show up around Kentucky. Blackleg is a fast-moving and devastating disease that can result in severe plant losses in a short period of time. Careful management of tobacco float beds can reduce the risk posed by blackleg and will prevent serious losses or delays in setting.
Warm, humid conditions in the float bed are the ideal environment for Erwinia carotovora subsp. carotovora and other bacterial species that cause blackleg. Initially, leaf clippings (debris) or wounded tissue are colonized by the blackleg pathogen. Debris and leaves infected by Erwinia appear necrotic and slimy (Figure 1).
Systemic infections, which arise when Erwinia moves from debris or wounded tissues into healthy plants, result in darkening of the stem that tends to primarily move up one side of the seedling, hence the name “blackleg” (Figure 2).
Affected areas of the stem may also show splitting, and in advanced stages, seedlings will collapse. Under favorable conditions, blackleg will spread rapidly, causing significant loss of useable transplants in as few as 1 to 2 days (Figure 3).
The bacteria that cause blackleg are essentially parasites of wounded or stressed tissue, and are plentiful in soil and on leaf surfaces. Because the pathogens are always present, development of disease is dependent on a favorable environment and plentiful food (in the form of plant debris or wounded/stressed tissue). Factors that may lead to outbreaks of blackleg include: high nitrogen levels (greater than 150 ppm), warm temperatures (greater than 75° F), high humidity, long periods of leaf wetness, and plant injury (stress and wounding). The latter occurs routinely during clipping and can lead to rapid spread of bacterial soft rot if clipping is carried out when plants are wet.
Cultural practices are the most important ways to prevent bacterial diseases. Provide adequate ventilation to shorten the length of time that foliage stays wet – this may be the most important of all management practices to reduce the incidence of blackleg. Most outbreaks we see in Kentucky are associated with warm temperatures and excessive moisture on float plants. Avoid over-fertilizing, a practice referred to as “pushing” seedlings, as this leads to dense, lush growth that is more susceptible to disease and takes longer to dry. Clip and handle plants only after they have been allowed to dry properly. Leaf debris left behind after clipping can serve as a starting point for the pathogens that cause blackleg and should be removed promptly. Along with maintaining good airflow in the float system, keeping as much leaf debris out of the beds as possible is a key to holding blackleg in check.
Chemical options for control of blackleg are limited. Agricultural streptomycin can be used in outdoor plant beds to suppress bacterial diseases, but is not specifically labeled for use in transplant facilities. Because the use of agricultural streptomycin is not expressly prohibited in transplant production, however, EPA rules allow its use in the float system. Streptomycin provides only moderate suppression of blackleg, though, and growers who choose to apply the material in the greenhouse must accept all liability. Apply 3 to 5 gallons of a 100 to 200 ppm solution of streptomycin to 1000 square feet of float bed. This use rate translates to 0.5 to 1 pounds per 100 gallons of water, or 1 to 2 teaspoons per gallon. Apply streptomycin before symptoms appear for best results, using the lowest rate. Use the 200-ppm rate immediately after the appearance of symptoms of blackleg. Some plant injury may be observed when applying the higher rate. Refer to the product label and the 2013-2014 Kentucky-Tennessee Tobacco Production Guide (ID-160) for more information. The guide can be found online at http://www.uky.edu/Ag/TobaccoProd/pubs/id160.pdf.
By Kenny Seebold, Extension Plant Pathologist