Two diseases should be at the top of tobacco transplant growers’ minds as setting time approaches: blue mold and target spot. Early identification and management of blue mold and target spot will not only reduce losses on the farm, but has implications for the tobacco-growing community in Kentucky and beyond.
Here are the symptoms to look for and how to treat the crop once disease is confirmed.
Blue mold is caused by Peronospora tabacina, a water mold pathogen that does not overwinter in Kentucky. The pathogen is moved northward on weather patterns from areas to our south. Earlier this year, tobacco transplants were confirmed to be systemically infected in Georgia greenhouses. While symptomatic plants were not set in the field, this early recognition of disease indicates spores of the blue mold pathogen are present and active in the U.S. Growers, Extension agents, and specialists should be on the lookout for blue mold, particularly after our recent rainy, cool weather.
Systemically infected tobacco transplants display symptoms different from field plants affected by blue mold. Systemically infected transplants will have partially or fully necrotic leaves with darkened veins (Figure 1). Unlike blue mold in the field, circular lesions and/or sporulation on the undersides of leaves may not be apparent on these transplants.
To provide some protection, Manzate ProStick may be applied every 7 to 10 days to dime-sized plants or larger. Quadris, which may be applied once during transplant production, will also manage the disease if applied preventatively. However, if you suspect blue mold after inspecting your plants, call your local Extension agent as soon as possible for confirmation. Transplants from greenhouses with blue mold should be destroyed immediately after confirmation to protect the remainder of the crop, as well as neighbors’ crops.
Target spot, caused by Rhizoctonia solani (Thanatephorus cucumeris), is a very common disease of tobacco both in transplant production and in the field. In rainy years, severe losses can result from lower leaf loss due to target spot. While it can persist from year to year in poorly cleaned transplant trays, the target spot pathogen is also very common in the environment and can be introduced into production systems from outside houses or beds.
Target spot initiates on transplants as small lesions with clear centers (Figure 2); lesions gradually enlarge as the leaves expand (Figure 3). Look for target spot by inspecting plants along structure sidewalls, which will be under the highest pressure from spores discharged outside the bed. Symptomatic plants may also develop in inner trays in the bed, which often are most apparent around an ‘epicenter’ of a damped-off or nearly completely rotten plant. Removal of this rotten plant as well as the plants around it will help protect the rest of the crop.
Similar to the recommendations for blue mold, Manzate Prostick is the major fungicide applied to manage target spot in transplant systems. The single application of Quadris is recommended to be applied immediately following first or second clipping. Following Quadris, Manzate applications may be resumed on a 7 to 10 day schedule to continue to protect new leaves until setting.
For additional information on blue mold and target spot, please see:
- Burley and Dark Tobacco Production Guide (ID-160)
- Fungicide Guide for Burley and Dark Tobacco (PPFS-AG-T-08)
- Maintaining the Efficacy of Foliar Fungicides for Tobacco Disease Management (PPFS-AG-T-05)
- Managing Target Spot and Rhizoctonia Damping-Off in the Float System (PPFS-AG-T-02)
By Emily Pfeufer, Extension Plant Pathologist