Endemic diseases such as black shank, bacterial wilt and root-knot nematodes always cause significant disease losses in South Carolina (2005-2008). These important and potentially devastating diseases of tobacco can best be managed through a combination of control methods. It is urged that growers identify disease problems in their fields and follow disease management suggestions based on rotation, variety selection, sanitation and chemical treatments. A sound disease management strategy cannot be developed without the proper identification of the disease problems in your fields. Disease development is a dynamic process and can change over time. Low disease losses in your fields in the recent past does not assure disease losses will remain low! New varieties with high resistance to black shank need to be monitored for the development of new strains of the pathogen. Your disease control program should be based on the assumption that changes in pathogen populations and disease pressures will occur. Changes in the tobacco program has made crop productivity and leaf quality essential for economic success. Good disease control will be the cornerstone of a successful farm operation.
Tobacco diseases accounted for an estimated loss of $3.6 million in 2008 to South Carolina farmers (see table). Disease losses were greater in 2008 than the previous year with significant losses still occurring to bacterial wilt, black shank and tomato spotted wilt. Bacterial wilt continues to be our major disease in South Carolina. The widespread losses and disease patterns within affected fields strongly suggest the bacterium was moved during mechanical topping and/or leaf removal. In spite of lower than normal rainfall early in the season, significant losses still occurred in many fields following later season rainfall.
The introduction of newer varieties with immunity to the common race of black shank (race 0) had reduced losses to this disease in past years. However, a new race of the pathogen has emerged (race 1) which can develop on these new varieties and has resulted in a resurgence of black shank in some areas. Farmers should assume that race 1 of the fungus occurs on their farm. Consult the variety table for varieties that contain resistance to race 1 of black shank and consider the use of soil-applied fungicide such as Ridomil Gold if a history of black shank is present on your farm. For additional control information, consult the black shank management section of the 2009 South Carolina Tobacco Growers' Guide.
Tomato spotted wilt (TSW) was greater in 2008 than 2007, especially in the western portion of the tobacco production area. Although statewide losses were low, some fields in the west experienced 30-40% stand reduction. These severe losses alert us to the potential of TSW to cause severe crop loss. Losses to TSW will most likely be severe in the future if the weather is favorable for the thrips vector, virus transmission and disease expression. Sporadic epidemics of TSW have occurred in Georgia and will most likely occur in South Carolina’s tobacco in the future.
Disease losses affect tobacco yields, quality and profitability. Disease control options can be expensive to use and costly especially if the wrong control option is chosen. Great care needs to be exercised to assure a return on your control investment.
Rotation: The best defense against most diseases and the least expensive is a good, well-planned rotation. However, the diseases must be correctly identified within particular fields to develop a sound rotation plan. Any rotation is better than no rotation, but certain crops will do a better job of suppressing certain diseases. While some growers take a chance and do not rotate, sooner or later they will get caught with unexpected losses. Some diseases, such as bacterial wilt or black shank, may destroy entire fields! Also, some diseases such as mosaic and nematodes may be causing more damage than realized through observation because the plant may not completely die. Losses to these diseases are easily masked in a year in which rainfall was plentiful. Although difficult to see, these losses substantially reduce farm income! Losses to the three major diseases in South Carolina, that consistently reduce yields from year to year, can be reduced through a planned rotation program. Study the results of on-farm rotation studies for particular diseases in the following pages.
Host Resistance: Selection of resistant varieties provides a highly effective and inexpensive method of reducing losses to disease. Varieties differ in resistance to black shank, bacterial wilt, tobacco mosaic, Fusarium wilt and root-knot nematodes, so any one variety will not be the best choice in all fields. Study the disease ratings (see tobacco variety test under the Tobacco Production Section) and agronomic characteristics of varieties and select varieties resistant to disease causing organisms found in your fields. Proper identification and record of disease pressure is the key to successful variety selection. Study the results of on-farm variety trials for diseases found on your farm.
Chemical Treatments: Selection of chemical treatments should be your LAST CONSIDERATION in a disease control strategy. Rotation, variety selection and proper sanitation reduce populations of pathogenic organisms to levels that can be controlled by chemical applications. Choose your chemicals to match the disease pressure in your fields. Study the results of on-farm chemical studies for particular diseases in the following pages.
Record keeping: Soil borne pathogens are impossible to remove from a field with applied chemicals or cultural controls. A detailed record of disease incidence and severity is a valuable management tool. Survey tobacco fields for disease pressure and record incidence and severity of disease. Endemic soil borne diseases primarily affect below ground portions of the plant. An excellent management tool is to access the health of your crop’s root system at the end of the season. Tobacco roots at the end of the season should remain white with little root necrosis. Remove roots from soil with a fork and access the root system for necrosis. Sample your fields in a zigzag pattern and record the incidence, severity and location of damaged root systems. Proper disease identification is essential. If you are unsure of the disease identification consult your county extension agent.
There are several potentially important disease problems that may occur in greenhouse transplant production systems. These include target spot (Rhizoctonia solani), white mold or stem rot (Sclerotinia spp.), damping-off caused by Pythium spp. or Rhizoctonia spp., blue mold (Peronospora tabacina), gray mold (Botrytis cinerea), soft rot (Erwinia spp.) and tobacco mosaic virus. The potential also exists for diseases most often associated with field-grown tobacco to occur, and include bacterial wilt (Ralstonia solanacearum) and black shank (Phytophthora parasitica var. nicotianae).
There are few fungicides labeled for greenhouse tobacco transplant production. A label for Dithane DF has been obtained for greenhouse and plant bed use but the potential for phytotoxicity exists (see table). It is imperative that producers take extra precautions to prevent pathogens from entering the greenhouse and to minimize environmental conditions within the greenhouse that might encourage disease development. Thus, ventilation, sanitation, monitoring, and use of good production practices are important disease management factors.