Black shank is the most serious disease faced by burley tobacco growers. Many varieties now available have very good to excellent resistance to it. Fungicides on the market for control of black shank do a good job, too, but need to be applied correctly to get best results and investment return.
The number of fungicides out there for black shank in tobacco is pretty small: Ridomil Gold SL, Ultra Flourish, and MetaStar 2E AG.
Ridomil Gold and Ultra Flourish contain the active ingredient mefenoxam, while MetaStar contains metalaxyl and is similar to the original Ridomil 2E that was introduced back in the 1980’s. Mefenoxam is a purified form of metalaxyl, meaning that Ridomil Gold, Ultra Flourish, and MetaStar all work the same way against the black shank pathogen (called Phytophthora nicotianae). These products perform similarly against black shank as long as they are used correctly. Each fungicide is formulated differently. The actual rate of the product per acre will be different.
Ridomil Gold is the brand-name product and the cost per acre for a treatment is over $100 (based on a 1 pt/A application), while Ultra Flourish and MetaStar are often less expensive. If choosing to use either Ultra Flourish or MetaStar, keep in mind that the use rates are higher because these products are less concentrated. When comparing prices between these two products and Ridomil Gold, figure out the actual cost per acre based on the planned use rate; don’t focus on the price of the container.
Fungicides usually give the best results against black shank when they are applied to resistant varieties (4 or better on the rating scale for race 0 and race 1) planted on properly rotated ground. Ridomil Gold (as well as Ultra Flourish and MetaStar) will not provide acceptable control on varieties with little or no resistance planted in locations that have a history of moderate-to-severe black shank. Good soil moisture is needed when these fungicides are applied so that enough material is taken up by plant roots.
Where black shank has been a problem in the past, consider making a pre-plant application of Ridomil Gold at 1-2 pt/A. This would be the same as 1-2 qt of Ultra Flourish or 4-6 qt/A of MetaStar. Apply fungicides in a volume of water or fertilizer that is sufficient for good soil coverage and incorporate into the top 2-4 inches of soil by disking or irrigation.
For light-to-moderate disease pressure, the first application of fungicide can be delayed up to 1 to 2 weeks after transplanting. For extended control of black shank, make a supplemental fungicide application (1 pt of Ridomil, 1 qt of Ultra Flourish, or 2 qt of MetaStar) at layby (around the time the last cultivation is made) OR at first cultivation and again at layby. Sprays made after transplanting should be directed toward the soil and incorporated immediately by cultivation or irrigation (an inch or two of rain will also incorporate these products).
Know the label; know the law
Avoid “over-the-top” applications of fungicide after the last cultivation because of the difficulties in getting chemical down to the soil surface as plants begin to spread out. Any chemical that contacts tobacco leaves will not be taken up by the roots, and will not be available to protect against black shank. Do not make supplemental applications if more than 1 pt of Ridomil Gold, 1 qt of UltraFlourish, or 2 qt of MetaStar was used at planting. Do not exceed the equivalent of 1.5 lb a.i./A of mefenoxam per season (3 pt of Ridomil Gold or, 3 qt of Ultra Flourish, or 6 qt of MetaStar).
Over the years, growers have wondered if it would be possible to add Ridomil Gold to the setter barrel and apply the fungicide in transplant water. Until recently, adding Ridomil tobacco setter water was not supported by the product label, although many probably ‘experimented’ with this off-label method.
There are some obvious advantages to using Ridomil in the setter barrel – fungicide is placed directly into the root zone of newly-set plants, where it can be taken up quickly. Also, the cost of application can be reduced because less chemical is used compared to broadcast treatments and treatments can be applied during setting. This saves trips across the field to apply and incorporate Ridomil before transplanting.
As mentioned earlier, this type of application was not legal; however, in March of 2011, several states (Georgia, Kentucky, Indiana, Ohio, North Carolina, Tennessee, and Virginia) approved the use of Ridomil Gold SL as a setter-water treatment for the control of black shank on burley, dark and flue-cured tobacco. The special local-need label, or 24(c), was granted after several years of testing in these states.
Setter-water treatments with Ridomil will work best for growers using sound management practices (sanitation and crop rotation) along with one of the newer resistant varieties, and who are looking for additional protection from black shank. In these cases, it will be possible to get adequate control of disease and also reduce fungicide costs by approximately $70 per acre.
Where disease pressure is high, particularly if limited crop rotation is being practiced, or where varieties with little or no resistance are being planted in fields at risk to black shank, broadcast-applied Ridomil at 1-2 pt/A would be a more effective treatment, based on our research.
To get the best results from Ridomil Gold SL when applying in transplant water:
- Use good management practices and plant a variety with moderate to high resistance to race 0 and race 1 black shank.
- Use only Ridomil Gold SL in setter water. Ridomil Gold EC and Ridomil 2EC contain high levels of solvents and can injure or kill tobacco seedlings. Likewise, avoid generic versions of mefenoxam or metalaxyl. Only Ridomil Gold SL is covered by the Special Local Need Label.
- Add 4 to 8 fl oz/A of Ridomil Gold SL to no less than 200 gal/A of transplant water to avoid injury to tobacco. Use the high rate in areas with a greater risk of black shank.
- Make 1-2 supplemental, soil-directed applications at 1 pt/A if needed to provide control of disease.
- Do not apply Ridomil Gold SL to stressed or weak seedlings, or during hot, dry conditions to avoid serious plant injury.
- Mix thoroughly before transplanting. Using a separate tank to pre-mix Ridomil Gold SL will help ensure thorough mixing and a consistent concentration of fungicide during transplanting, resulting in less injury potential and better control of disease.
- The user must have a copy of the Special Local Need label in their possession at the time of application.