Beware of bacterial soft rot on transplants due to recent rain

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The tremendous amount of rain that has fallen over much of Kentucky during the past month, along with extended periods of cool and overcast weather, is really starting to push outbreaks of diseases in tobacco float beds. We received almost thirteen inches of rainfall, alone in April and more than four inches of rainfall in early May. There have been reports of target spot, Sclerotinia collar rot, and Pythium root rot across the state.

Temperatures are bound to increase over the next couple of weeks, making conditions favorable for blackleg as you attempt to hold plants until it’s dry enough to set. Kenny Seebold, UK Extension Plant Pathologist, provides some helpful disease management advice to protect your float plants.
Warm, humid conditions in the float bed are the ideal environment for Erwinia carotovora subsp.carotovora, and other bacterial species that cause blackleg. Initially, organic matter in trays or wounded tissues, are colonized by the blackleg pathogen. Debris and leaf tissue infected by Erwinia appear necrotic and “slimy.” Systemic infections, which arise when Erwinia moves from debris or wounded tissues into healthy plants, result in darkening of the stem that tends to move up one side of the seedling primarily, hence the name “blackleg.” 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 1-2 days. 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 (>150 ppm), warm temperatures (>75 degrees 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 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 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. However, because the use of agricultural streptomycin is not expressly prohibited in transplant production, 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 3-5 gallons of a 100-200 ppm solution of streptomycin to 1000 sq. feet of float bed. This use rate translates to 0.5-1 lb per 100 gallons of water, or 1-2 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 maybe observed when applying the higher rate. Refer to the product label and the “2011-2012 Kentucky-Tennessee Tobacco Production Guide” (ID-160) for more information. The guide can be found online at www.uky.edu/Ag/TobaccoProd/pubs/id160.pdf.

As you attempt to hold float plants until soils are dry enough for setting, you will need to lower nitrogen levels to 50-75 ppm to harden them off. The danger, here, is that they will be much more susceptible to disease attack. Be very sure to keep a good disease prevention spray program in place until the plants are set.

For more information, call the Trimble County Extension Office at 502-255-7188.
Source: Kenny Seebold, University of Kentucky, College of Agriculture.

Michael Pyles is Trimble County’s Cooperative Extension agent for agriculture.