Tobacco Disease Management in Greenhouses
TB05 - Tobacco Disease Note 5
Thomas A. Melton, Philip Morris Professor and Department Extension Leader

Introduction

Among the diseases most likely to be observed in greenhouses are Rhizoctonia target spot, Rhizoctonia stem rot, Sclerotinia stem rot (collar rot), Pythium root rot, bacterial soft rot, and tobacco mosaic virus. Other diseases that may be encountered include Botrytis gray mold, black root rot, anthracnose, black shank, and Granville wilt. Soil-borne diseases such as Granville wilt and black shank that result from contaminated media or water are a threat to transplants and field plants, because the disease may be carried on transplants to previously unaffected fields. Target spot, gray mold, and soft rot have been most common in houses using over-head watering where foliage is not allowed to dry sufficiently between waterings. Increased plant density, high humidity, high soil moisture, poor air movement, condensation, and presence of young succulent plants are ideal factors for disease development in greenhouses. 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. Rhizoctonia stem rot or damping-off is a relatively dry, brown to reddish brown rot. The fungus may be visible and resembles fragile spider webs at the base of the plant. Sclerotina stem rot (white mold, collar rot) is a brown, wet rot, which usually occurs in small areas throughout the greenhouse. A dense white mold, resembling cotton, often is present on plants with advanced symptoms. White to black sclerotia may also be present at the soil line or on the stem. They usually range in size from 1/16 in. to 1/4 in. and may be round or oblong.

Disease Management

Disease management in tobacco greenhouses can be accomplished through the use of adequate ventilation, sanitation, frequent monitoring of plants for disease, good production practices, and where suitable, solarization.

1. Ventilation - Proper ventilation and air circulation are key in preventing foliar diseases such as target spot, 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 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 circulation system called horizontal air flow (HAF) has been adopted by some tobacco producers and is very common in the floricultural industry. HAF uses large diameter fans that move air slowly around the greenhouse in a circular fashion (Figs. 1 and 2) 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 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.

2. Sanitation - Increased ventilation, and air movement, as outlined in the previous section, promotes an environment unfavorable for disease development. Sanitation, on the other hand, helps to insure that pathogens do not have an opportunity to come in contact with plants and cause disease. All precautions should be taken to insure that field soil or nonsterilized 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. 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. Most importantly, always use sterilized media, and never reuse media. Never use field soil or attempt to sterilize field soil. 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 or media round greenhouses. Also, as in plant beds, no tobacco products should be used or allowed in greenhouses. The black shank organism has been brought into greenhouses by the use of contaminated irrigation water, primarily from contaminated ponds. Pond water should not be used for either water beds or overhead irrigation. In addition, workers who need to step into water beds, either when putting in new plastic liner or when water is in the 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.

3. Frequent Monitoring - "Just Do It!" It is very important, because of how rapidly diseases can spread in a greenhouse, to closely monitor greenhouse plants for disease development. 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.

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. 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.

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Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies. Last printed _10/90/1000

Reformatted Dec. 2000 by Plant Disease and Insect Clinic