Producing Top-Quality Transplants Depends on Managing Diseases

by Kenny Seebold, Extension Plant Pathologist, University of Kentucky

Few could argue that quality tobacco transplants are part of the foundation of a successful crop.  Producing transplants is a tough job, and requires intensive management on the part of the grower if he or she is to get the best plants possible.  Among the challenges faced by transplant growers are plant diseases.  The vast majority of tobacco seedlings are raised in float systems, either in greenhouses or outdoor beds, which are highly conducive to the development and spread of disease.  High plant populations, high humidity, and float water itself all contribute to increased threats from plant pathogens.  Diseases that occur during transplant production can result in direct loss of usable plants, delays in production, and plants that are less thrifty and more susceptible to diseases (like black shank) in the field.  Getting good control of diseases in float system can help increase tobacco yields and thus income.

            The major diseases encountered in production of transplants in the float system are Pythium root rot, target spot, Sclerotinia collar rot, blue mold and black leg (bacterial soft rot). Less common are anthracnose, damping-off, Botrytis gray mold, angular leaf spot, and virus diseases (such as tobacco mosaic).

Prevention is the most important part of disease management in tobacco float beds, mainly because of the disease-conducive environment and the relative lack of fungicide tools that we can use to prevent disease or slow disease spread once it begins.  Here are a few tips to get ahead of transplant diseases in 2009.

Exclude Pathogens from Transplant Facilities

Avoid the introduction of plant pathogens into the float system.   Water from ponds, creeks, and rivers should not be used, since these are sources of fungi like Pythium.  Keep soil out of float beds, since several tobacco pathogens are harbored in soil and will flourish once in the system.  If you by plants, such as plugs or even finished seedlings, inspect them carefully for disease before floating.  It may be necessary to reject diseased plants or destroy them to avoid outbreaks later on, particularly if blue mold is found.  Controlling weeds around greenhouses and outdoor float beds will reduce the potential introduction of certain pathogens and the insects that spread them.

Make Sanitation a Routine Practice

Old trays are one of the main sources of pathogens like Pythium or Rhizoctonia (which causes target spot and damping-off).  Sanitize old trays as recommended, or use new trays. New trays will all but eliminate carrying diseases over between crops of transplants. Re-used trays pose more of a risk, and this risk increases as trays age.  Consider replacing trays that are more than 3-4 years old.  There’s no foolproof way to sanitize used trays; however, soaking them or spraying them with a 10% solution of commercial bleach, followed by a rinse with clean water, can be helpful.

 Dispose of diseased or unused plants by burying them or placing them in cull piles located at least 100 yards from float beds or tobacco fields. This is necessary to avoid problems with Sclerotinia collar rot. Remove clippings and debris to prevent buildup of material that can favor development of collar rot or black leg (bacterial soft rot). Don’t mow plants if diseases such as black leg are active, as this will spread disease from infected to healthy seedlings. If blue mold is found in float beds, destroy all plants immediately even if only a small number actually show symptoms of disease. Given time, disease will develop on these plants as well. Sanitize equipment, tools, and other items (shoes, hands) that will come into contact with plants or float water. Use a 10% bleach solution for equipment and shoes, and antimicrobial soap for hands to prevent the introduction and spread of pathogens.

Create an Unfavorable Environment for Plant Pathogens

Maintain good air movement through the use of side vents and fans, and keep water levels high enough for float trays to clear the side boards of the bays, which allows for better movement of air (water level may be kept low initially when plants are small to prevent cold injury and raised as plants grow). Good airflow promotes rapid drying of foliage, creating less favorable conditions for diseases such as target spot, collar rot, blue mold, and black leg. Minimize the potential for water to splash between trays. Avoid overhead irrigation, fix leaks in roofs, and apply fungicides early in the day so that foliage dries quickly. Temperature control is critical - excess heat can lead to problems with target spot and black leg, while cooler temperatures favor collar rot and blue mold.

Minimize Plant Stress

            Keep your transplants as stress-free as possible. Avoid temperature extremes and keep fertilizer at recommended levels. Plants that are under- or over-fertilized are more susceptible to diseases in general. For example, target spot is much more of a problem when nitrogen levels are below 50 ppm for extended periods of time, while black leg is generally seen when nitrogen is consistently above 150 ppm. Excess nitrogen also leads to rapid, rank growth of transplants. New, succulent growth is more disease-prone, and also takes longer to dry out.

Clip properly to minimize stress and also the volume of clippings, and use a well-sharpened blade to promote rapid healing of wounds. Make sure that plants dry quickly after mowing.

Apply Fungicides and Bactericides Wisely

Relatively few fungicide products are labeled for use on tobacco in the float system, and only Pythium root rot, blue mold, and target spot/damping-off are targeted by these materials. The remaining diseases can be managed only by cultural practices.

            Terramaster 4EC, applied to float water in a timely way, is very effective against Pythium root rot, while products that contain mancozeb (Dithane DF, Manzate ProStick, or Penncozeb) can help with Rhizoctonia damping-off, target spot, and blue mold.  Mancozeb products have a “special local need”, or SLN registration for most tobacco states – check to find out which ones can be used in your area.  Agricultural streptomycin, sold under several trade names, is useful in the suppression of certain bacterial diseases.  Disease-control products for tobacco transplants are best used preventively – see product labels for rates, timings, and method of application.

Develop an Integrated Plan to Manage Diseases

            Disease-free transplants pay dividends down the road because they are more vigorous and less prone to attack by pathogens in the field. Follow a control program that integrates management of the environment, sanitation, and fungicides to get the best possible control of diseases in the float system and produce the highest quality transplants that you can. While it may not be possible to avoid diseases completely, integrated management practices will reduce the impact of diseases in the float system greatly.

Figure 1.  Old trays can harbor pathogens that cause diseases in the float system.

Figure 2.  Pythium root rot is the most common disease seen in the float system and can spread quickly if not managed properly.

Figure 3.  Target spot on tobacco seedlings.