From the Field – Agronomy Notes: Vol. 6, Num. 4

— Written By

April 1, 2021-

As we gear up for transplanting and general field work, we also find ourselves making a number of crop management decisions. Many of the decisions we make revolve around pesticide selection, and these are often conversations that Cooperative Extension is heavily involved with. Sometimes these are easy conversations to have and sometimes they require in-depth dialogue in order to fully understand what pesticides may or may not be acceptable by industry. Last year we posted an article about specific issues that deserved special attention. We’re reposting this article because much of it rings true in 2021 as well. Below are specific items that we’ll be considering again in 2021, and we strongly feel that you should as well:

1.) FOLLOW YOUR CONTRACT! There is usually information in a tobacco contract that outlines specific pesticides that cannot be used to grow a crop. Unfortunately, we find that there are major differences in what various buying companies have deemed acceptable for application, so there is no longer a “one size fits all approach” for our recommendations. This is where it’s important to remember that just because a pesticide is labeled for tobacco does not mean it can be used in each situation. Remember, you as a farmer have direct access to your contract and the representatives from your buying company – we in Extension do not.

2.) Some insecticides are still labeled for use in tobacco production but are no longer approved for use by some purchasers. Two great examples of this are chlorpyrifos (Warhawk and Lorsban Advance) and acephate (Orthene 97, Acephate 90 WSP, and many others). Dr. Burrack recently posted a great article outlining the chemical alternatives to chlorpyrifos, so be sure to give it a look if you contract with a company that has prohibited its use. We are also fortunate to have chemical alternatives that offer similar or better insect control than acephate (see pg. 150 of the 2021 Tobacco Production Guide). As was mentioned in point number one, this why it is so important to read your contract and consult with your buyers for further clarification.

3.) Our options are very limited when it comes to herbicide selection, and it appears that for some growers those options just became even more limited. For the last few years, we have strongly encouraged producers to consider using pendimethalin (Prowl H2O, Prowl 3.3 EC, and many others) at layby as a way to extend residual weed control and reduce the resistance pressure that we are placing on sulfentrazone and clomazone. Back to point one, this is where reading your contract and having a discussion with your buyer(s) can be useful because some growers can make this application and some cannot. We are fortunate that napropamide (Devrinol 2XT and Devrinol DF-XT) is also labeled for a layby application and is a different chemistry than other herbicides that are used. We have been told that napropamide will be a suitable material for layby application where pendimethalin is not. The application rate for napropamide will vary depending on whether or not it was applied before transplanting or over-the-top after transplanting. If it was not, then 3 qts or 3 lbs per acre would be my targeted rate (depending on formulation). You may only apply a maximum of 4 qts or 4 lbs per acre per season, so remember that total if multiple applications will be used.

4.) Remember certain guidelines for pesticide applications. I specifically think about chlorantraniliprole (Coragen) in this situation. Back around 2017, some buyers began dictating the crop stage at which the final application of this insecticide could be made (no applications after layby, for example). Previous guidelines for acephate application had also been outlined (no applications post-topping).

5.) READ EACH PESTICIDE LABEL!! We cannot stress this enough. It’s important to read and understand these labels as it ensures you know what you are applying to the crop. Certainly, this helps to control target organisms in a safe and environmentally sound manner, but it also helps prevent the application of active ingredients that are not labeled for tobacco. One good example is Quadris Flowable (which is labeled for tobacco) and Quadris Top (which is NOT labeled for tobacco) (Fig. 1). Both fungicides contain the active ingredient azoxystrobin, but Quadris Top also contains difenoconazole – which is not approved for application in US tobacco production. Collectively, this means that an application of Quadris Top to tobacco is an illegal activity and can result in your contract being canceled. This specific point has gained traction because difenoconazole has been added to a number of pesticide residue testing lists, which means an honest mistake may wind up costing you big time. This particular issue is not limited to Quadris as it also applies to multiple herbicides and insecticides. So again, please take the time to read each chemical label.

Quadris Flowable and Quadris Top fungicide labels

Figure 1. Quadris Flowable and Quadris Top fungicide labels.

6.) Do not use azoxystrobin (Quadris Flowable) to promote leaf “holdingability.” It does not work in this manner (unless you have foliar diseases present that azoxystrobin will control). In addition to this application being off-label, it often occurs very close to harvest (7-10 days before harvest) thus greatly increasing the chances of high cured leaf residues. If you look at the most recent Quadris Flowable label, the PHI has been extended from 0 to 21 days. If results from field trials indicated that this product would not increase holdingability when applied 7-10 days before harvest, it’s even less likely that it will help at 21 days. One final point about using azoxystrobin for leaf holdingability is that this application is very likely one of the underlying causes behind the current azoxystrobin resistance we are now finding in Frogeye Leaf Spot populations. Just as we reported with glyphosate-resistant Palmer amaranth, repeated applications of the same chemical mode of action over a period of time without any rotation are going to select for and promote resistant populations. Right now, Quadris Flowable is the only chemical we have that offers control of Target Spot and we CANNOT afford to lose this product to resistance or the perception of high residues.

7.) The fungicide mancozeb (Manzate® Pro-StickTM) was recently re-introduced to North Carolina and other tobacco growing states. The 24(c) label contains specific instructions for when and how to use this fungicide in greenhouse and field production. Historically, the use of this active ingredient was focused almost exclusively on blue mold control in field production. The manner and frequency in which it was applied resulted in high cured leaf residues. Certainly, we do not expect that current use patterns will mimic those previously used, but it’s still a good idea to check with your buyer(s) to ensure that this material can be used this year.

In summary, choosing the pesticides that you need to be successful and sustainable continues to become more challenging. We in Extension are here to help you make sound agronomic decisions, but there is an added layer to those considerations and only you as a contract holder can make the final decision. Just remember that the NC Ag Chem Manual and the 2021 Flue-Cured Tobacco Guide are great starting points. We also have a regional Burley and Dark Tobacco Production Guide. Of course, you can also reach out to your local Extension Agent for specific pesticide information when you need it.

DISCLAIMER: The mention of pesticide trade names and active ingredients are only included for reader convenience and are not endorsement or criticisms of those products.

Post Authored By:

Dr. Matthew C. Vann, Tobacco Extension Specialist, Dept. of Crop and Soil Sciences

Dr. Hannah Burrack, Professor and Extension Entomologist, Dept. of Entomology and Plant Pathology