Limited Supplies of Chlorothalonil? Options for Managing Leaf Spots

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Many of you have probably heard that chlorothalonil may be in short supply this year. Chlorothalonil (whether Bravo, Echo, Equus, or other generics) is very effective against leaf spots while also being relatively inexpensive. It also is our best defense against fungicide resistance in leaf spots. Here I will discuss various options for managing leaf spots if chlorothalonil supplies are limited while keeping the risk of fungicide resistance low.

First, some good news: almost everyone will be getting off to a good start with rotation patterns and cultivars that will help to make leaf spot control easier. In the V-C area most rotations are excellent, easily meeting the recommended minimum of two years away from peanut to reduce leaf spot pressure; longer rotations reduce pressure ever more. Likewise, the popular cultivar Bailey holds up very well against leaf spots and the new high-oleic cultivar Sullivan looks even better than Bailey in our studies so far.

Growing Bailey or Sullivan under excellent rotation means that most growers should be able to manage leaf spots with only four calendar sprays. On susceptible cultivars, expect to apply a leaf spot fungicide five times per season, assuming calendar sprays applied at two-week intervals. The number of sprays could be lower if we have spells of weather unfavorable for leaf spot, allowing growers to lengthen intervals or skip sprays entirely by using the peanut leaf spot advisory. I do not recommend stretching intervals if advisories indicate that weather favors leaf spot, or if using a calendar spray schedule. However, a few products allow for longer intervals in some situations. Read the label carefully and contact the company representative to be sure that they will stand behind the product if it is used this way.

In the V-C area, we typically recommend that growers use chlorothalonil alone or in mixture for their first and final sprays, mixed with stem rot fungicides such as tebuconazole, Artisan, or Convoy, and in alternation with the resistance prone fungicides in groups 3, 7, and 11.

Below are some alternatives to chlorothalonil for growers to consider. Some of these alternative may cost more than chlorothalonil, but excellent disease control is a wise investment. It is much easier and more economical to keep leaf spots under control than to try to recover from a situation where they have gotten out of hand.

The first spray: an easy alternative to chlorothalonil in this and other situations is Tilt/Bravo and its generic equivalents (products with propiconazole + chlorothalonil). This product is not expected to be in short supply and is a good choice for the first spray. It can be used in place of chlorothalonil at other times in the season as well. Other fungicides can be used for the first spray, but this can limit options for the rest of the season.

Mixtures: It is a bit more difficult to come up with alternatives to chlorothalonil as a mixing partner with tebuconazole, particularly when targeting stem rot as well as leaf spot. I don’t recommend using tebuconazole alone because it may not control some leaf spot populations. One alternative is to reduce the chlorothalonil in tebuconazole mixtures to 1 pint per acre instead of the usual 1.5 pint per acre rate. Tebuconazole can also be mixed with Tilt/Bravo, thiophanate methyl (Topsin M and other products; group 1) or mancozeb (Koverall and others). As always, test new tank mix combinations for compatibility before using.

Fungicides from the different fungicide resistance groups also tend to have somewhat different activities against peanut diseases. Thus, mixing products from different activity/resistance groups can broaden their efficacy profiles. For example, Alto (group 3) can be mixed with a group 11 fungicide such as azoxystrobin, or it could be used in combination with Artisan or Convoy (group 7). Premixes of different groups also are becoming more common and include products such as Priaxor (11 + 7), Elatus (11 + 7), and Stratego (3 + 11), as well as Custodia (tebuconazole + azoxystrobin; 3 + 11) and various generics. See the label for the maximum number and amounts of applications per season for these products.

Alternation: Unmixed group 11 fungicides such as azoxystrobin (Abound) or pyraclostrobin (Headline) can be alternated with fungicides from different groups, including Provost (group 3) or Fontelis (group 7). If using unmixed products, be sure to follow the resistance management guidelines on the label.

Last spray: If you can get it, chlorothalonil (or Tilt/Bravo) is still your best alternative for the last spray of the season. This broad spectrum fungicide can kill any fungicide resistant members of the leaf spot population so that they do not carry over into later years. If you must use a product other than chlorothalonil, mixing fungicides from two different activity groups is strongly recommended. If possible, one of these fungicides should be from a group not previously used during the season.

Most growers should be able to work around chlorothalonil shortages with a little thought and planning. See Tables 6-4 and 6-5 in 2015 Peanut Information for more details about peanut fungicides and resistance management. For all products mentioned, see the 2015 North Carolina Agricultural Chemicals Manual, 2015 Peanut Information, and product labels for rates and application details.

Article first appeared as North Carolina Peanut Note (PNNC-2015-026)