June 10, 2014 Tobacco Scouting and Crop Report
Scattered rainfall has provided some heat and drought stress relief for many. However, it should be noted that rainfall total with the county for the past 10 days ranges from 0.3 inches to slightly over 2.0 inches. With variable rainfall, variable planting dates and variable soil types, tobacco stage ranges from just 10-12 leaves to approaching button stage. Having thus said, tobacco color is greatly improved and is growing rapidly.
No insect scouting report will be provided this week. The primary reason no data is provided is that 50% of the fields scheduled for scouting received an insect application within the last 7 days. Having thus said, fields that were scouted showed no insect pests warranting treatment.
SCOUTING FOR INSECTS
The image to the left shows the result of hornworm feeding. It is no surprise to anyone that a large hormworm can cause severe damage, especially to a small plant. Isolated plants with hornworms can be found in fields if one spends a bit of timing specifically looking for them. However, this is not proper scouting. Discovery of these isolated cases are evident in some fields but not all fields. This type of damage is discouraging to see, but not economical to treat. Economic thresholds are established BELOW levels for which yield loss occurs. The threshold for hornworms is 10%. Put in another perspective, typically we plant between 5,000-6,000 plants per acre. Even if we find 20 plants in one field with damage such as shown, this represents only 0.4% of the plants within one acre. Treating damage at levels this low is not economical.
Proper scouting needs to be objective and random. Typically when scouting, I enter the field 50-60 feet and then determine that I will begin my first random area to check after 50 steps. I plan ahead to zigzag through the field in a similar manner with a predetermined number of steps to take before stopping to examine plants. I simply take the steps and then begin checking the next 5 plants. This is repeated 8-12 times depending upon field size. This ensures completely random check areas. Conversely, if one specifically hunts damaged plants, one can always find insects. The bottom line is that data supports the manner of scouting described and shows that no economic loss occurs when following appropriate insect thresholds. If scouting differently, one may erroneously decide to treat when in fact, no economic damage has occurred.
Tobacco producers often discuss with me the desire to include an insecticide when making a trip across the field to apply some other product. As example, discovery of Target Spot has resulted in some producers applying a preventative fungicide. Typically, growers discuss adding Belt® or Coragen® insecticides to potentially avoid making another future trip across the field (within the next 10 days or so). If this sounds familiar, consider the point from another perspective. Assume one makes an application of an insecticide now and does not have insect pressure beyond threshold for the next thirty days. One assumption might be that the product performed as promoted. However, another reasonable explanation is simply that there are not major insect pest present. Craven County may have an early June population of hornworms, but then again, this pest may not arrive until mid-July. Unless there are fields or sections of a field left untreated with an insecticide, how does one know whether the product worked or that we simply did not have a high enough insect population to warrant treatment in the first place? The goal of saving money by reducing a trip across the field may actually result in less profit!
Another troubling point of making a preventative type insecticide treatment is that one is assuming that conditions within the fields are exactly as those conducted by companies and researchers evaluating these products. With tremendous respect toward the companies involved (They do stand behind the effectiveness of the product) one needs to consider that the rates, pressure, total volume of the application, weather conditions, plant growth stage, insect life cycle, insect size or other factors may alter residual control of a pesticide. Thus, using these products in hope that it will eliminate future trips across the fields is placing a lot of faith in many variables and does not follow proper IPM standards.
One of word of caution merits discussion. Insecticide products that offer longer residual control normally need to contact the leaf area in order to provide such control. As example, if one applies an insecticide to plants with 12-15 leaves, these leaves would be protected. Within the following 10-15 days, the plant may add another 10-12 leaves. These new leaves will be unprotected since they do not have an insecticide treatment. Thus, insects migrating may still infest the new growth and reach an economic threshold mandating another insecticide treatment. In this case, this is not an insecticide failure. Rather the failure is due to an improperly timed application of the insecticide.
Lastly, while there is no evidence of resistance to these products that I am aware, many pests have shown remarkable ability to quickly develop resistance to pesticides if that pesticide is used continually. As such, it simply makes great sense to protect these two relatively new insecticides. Continued use of pesticides with the same mode of action has historically proven to produce resistant pest. As such, we need to be mindful of this and alter subsequent applications with a product with a different mode of action.
TOMATO SPOTTED WILT VIRUS (TSWV)
By far, the most common call from tobacco producers this week has been seeking assistance to reduce TSWV. Regrettably, at this point there is nothing that can be done. All decisions to control this disease must be made in advance of any major thrip population migration or thrip population increase.
There are a few trends noteworthy to mention for the situation this year. Generally, scouting has revealed that producers applying Actigard® as a greenhouse floatwater treatment (even at the lowest rate) have TSWV incidence ranging from 4-6%. Those without such treatments have TSWV incidence ranging from 8-12%. Higher TSWV incidence is found in fields transplanted early (mid-April). Field applications of Actigard® made between May 7-10th appear to have reduced TSWV compared to no treatment but not as effective as the greenhouse floatwater treatment alone (Transplanting date, field location and historical TSWV may contribute to this observation). Field application of Actigard® made after May 10th were applied too late to be effective. (Note this agrees with data from the TSWV and Thrip Forecasting Tool predicting the major migration of the 4th generation of thrips to occur around May 15th). Lastly, essentially all fields have evident spots of higher TSWV than the average. This is commonly found along field edges with heavy weed cover or near weedy fields that were terminated after tobacco transplanting.
Evaluation of loss of yield for TSWV should be evaluated just as when scouting for insects. Random areas should be selected to count the number of TSWV infected plants per 100 plants checked. Notes should include the scattered spots with greater incidence than the average. However, one should also be aware that just as with insects, the loss of an entire plant is normally not an economical loss. Adjacent plants normally compensate with greater growth to make up for an occasional missing plant. Only when several adjacent plants within one row are diseased or missing do we actually lose yield.
The image shows variance of size tobacco plants within a field due to soil type. Typically this type of variance can be found within a field with very deep sandy soils that simply do not have the water holding or nutrient holding capacity desired. Too, heavy rainfalls easily leach some nutrients from these types of soils (Note that we have not had any leaching rainfalls to date). As such, it can be difficult to manage regardless of efforts.
If one has spots within a field such as this, consider an application of a product supplying additional potassium (K) and sulfur (S). Tissue, soil and nematode tests should be submitted to the NCDA & CS Agronomic Division to confirm application and eliminate any other possible reason for poor growth. However, historical examination of such problem areas (soil and tissue test results) typically show low K & S, not nitrogen (N) for such uneven growth. Consider that producers relate a historical range of total N application from 50-85 lbs/ac. Even those reporting the use of the lower range of N produce 2500+ lbs/ac of tobacco. Additionally, NCSU presentations over the last few years show data supporting no yield loss with the lower N rates but also report an increase in problems with low K.
Now is a great time to map areas of uneven growth and document with appropriate testing. If equipment can enter a field without damaging taller tobacco plants, application of a product such as 0-0-22 will supply additional K & S. If this is not possible, utilize the data for the next tobacco rotation. Apply greater K & S to these areas with GPS variable rate fertilizer application prior to transplant.
Belt® is a registered product of Bayer Crop Science
Coragen® is a registered product of DuPont
Actigard® is a registered product of Sygenta
The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the N.C. Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conform to the product label. Be sure to examine a current product label before applying any product.