Tobacco Disease Management in Greenhouses

Tobacco Disease Note No. 5
Thomas A. Melton, Philip Morris Professor and Department Extension Leader



Most of the tobacco transplant production in North Carolina is produced under the float tray system in greenhouses. In the float tray system, cells of styrofoam trays are filled with a soilless (1:1 peat:vermiculite) medium, seeded with a pelletized seed, and floated in a shallow water reservoir. The cells in the styrofoam trays are perforated in the bottom to allow for water and nutrient uptake. Trays are typically 26'' x 13'' with 200 to 394 cells per tray, which allow the production of more transplants of uniform size in a smaller area compared to seed beds.

The high plant density and high moisture conditions present in the greenhouses provide extended periods of leaf wetness and promote succulent plant tissue for the development and spread of seedling diseases. In addition, certain diseases that may not be a problem in outdoor soilbeds become a problem in greenhouses. The most common diseases observed in tobacco greenhouses are Rhizoctonia stem rot or damping off, Rhizoctonia target spot, Pythium root rot, and Sclerotinia collar rot. Some other diseases with less incidence are bacterial soft rot, Botrytis gray mold, and blue mold.

The pathogens that cause disease may enter through vents, in contaminated soil, on workers' hands, in water, on flats, on tools, etc. The key to managing diseases in greenhouses is to keep the pathogen out of the greenhouse and manage environmental conditions to keep humidity low and float water temperature cool. 

Disease Management

Disease management in tobacco greenhouses can be accomplished through the use of adequate sanitation practices that prevent the introduction of pathogens into greenhouses. Also, controlling the greenhouse environment by reducing excessive moisture reduces or delays disease infection and spread.  Frequent monitoring of plants for disease, good production practices and solarization (were suitable), can also help reduce losses to disease. 

1. Sanitation - Helps to ensure that pathogens do not have an opportunity to come in contact with plants and cause disease. All precautions should be taken to ensure that field soil or used media does not enter greenhouses. 
- Polystyrene trays should be sanitized using 3 lb/1000 cu ft. methyl bromide for 24 hours and allowed to air dry for 48 hours. 

- Most importantly, always use new media and never reuse media. Never use field soil or attempt to sterilize field soil. 

- The black shank and Pythium root rot organisms have been brought into greenhouses by the use of contaminated irrigation water, primarily from  ponds. Pond water should not be used for either water beds or overhead irrigation.

- Mowers that are used to clip plants should have their under-carriages washed and sanitized with a 50% bleach solution at least between every clipping and preferably more frequently. 

- If disease does appear to be developing, remove trays that show any symptoms of disease. Bury or burn the contents of those trays and store the trays in an enclosed area away from the greenhouse. 

- Never dump old trays, old media, infected plants, and clippings around greenhouses. 

- No tobacco products should be used or allowed in greenhouses. 

-  Workers who need to step into water beds should first wash and sanitize their boots to prevent the black shank and Granville wilt organisms from being carried into the water.

- Walkways should be constructed of gravel, asphalt, or concrete to be easily washed.

- It is also important to clean work areas (flat filling, seeding, etc.) daily. Storage areas for trays should be kept clean. 

-Finally, keep stray animals out of the greenhouse. 

2. Control of Environment. Proper ventilation and air circulation are key in preventing foliar diseases such as target spot, collar rot, gray mold, and soft rot. Most tobacco greenhouses use a passive (no fans) ventilation system in which vents are on the end of the house or on the sides (full length of the house). End vents alone are usually not adequate to reduce moisture in the leaf canopy. Where side vents are used, they are not usually adequate whenever they are in a wide open position. Disease problems begin when conditions too cool to open vents persist for several days. 

A supplement air circulation system called horizontal air flow (HAF) has been adopted by most tobacco producers and is very common in the floriculture industry. HAF uses large diameter fans that move air slowly around the greenhouse in a circular fashion just above the plant canopy. This system helps to reduce condensation, keeps foliage drier, and helps to eliminate cold spots in the greenhouse. The system is relatively inexpensive and may be very beneficial during periods when side curtains cannot be opened.

Another supplemental system, the polytube, has been adopted by only a few transplant producers but is widely used in the vegetable transplant industry. This system is slightly more expensive than the HAF but probably provides benefits for crops other than tobacco that remain in the greenhouse for longer periods of time. With the polytube system, the plastic tube, ranging in diameter from 2 feet up, is hung from the ceiling of the greenhouse and is stretched from one end of the greenhouse to the other. A pressurizing fan is at one end of the tube and an exhaust fan is located somewhere else in the house. These fans insure that the tube stays inflated and that air is pushed through the tube. Air exits the tube through two rows of holes, approximately 45 degrees from bottom vertical. Polytubes have the advantage of allowing cool air to be brought in through the polytube and warmed before the air comes in contact with plants. Therefore, fresh, dry air, may be brought into the house during cool periods when vents would not otherwise be opened. The polytube system also provides for excellent heat circulation and reduces condensation and foliage moisture. Using heat in combination with either supplemental air circulation system also helps to reduce relative humidity. 

3. Frequent Monitoring - Monitoring is very important because of how rapidly diseases can spread in a greenhouse.  When disease development is noted, remove the diseased plants immediately. Afterwards, get the disease identified. 

4. Follow Good Production Practices - Read and follow recommendations provided by the N. C. Cooperative Extension Service and keep close watch on pH and salts. Be sure not to over- water or over-fertilize. Another factor that may have an effect on disease severity is media texture and water holding capacity. The design and size of trays and cells may also influence disease. Insuring adequate media drainage is essential to preventing root disease. Clipping practices that reduce debris left on plants should be used, because debris can be the starting point for diseases such as collar rot. Mantain moderate temperatures. It is better to be too cool than too warm.

5. Solarization - Solarization is using the heat from the sun to kill plant pathogens in the greenhouse during periods when plants are not present. Solarization is very effective in controlling many pathogens but may have the disadvantage of damaging certain components within the greenhouse structure, including PVC pipe and polystyrene trays. For proper solarization, heat sensitive items should be removed and the gravel should be moist. All doors and vents should be closed during July or August for a period of at least 7 days (bright, sunny), 8 hours a day, to allow temperatures to reach 140 F. Another form of solarization is practiced when at the end of the transplant growing season, the black plastic or black ground cloth is left on the ground. This helps to prevent weed growth and keeps soil temperatures high enough to kill disease causing organisms close to the soil surface. Solarization will kill most of the disease-causing bacteria, fungi, and nematodes down to about 1/2 inch in the soil. 

Diseases that affect tobacco in greenhouses

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Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Reformatted June 2001 by Plant Disease and Insect Clinic