Multiple sources are reporting that there will be a shortage of the fungicide chlorothalonil, which is widely used in peanut and vegetable crops, according to Austin Hagan, Auburn University Extension plant pathologist.

The fungicide is marketed under numerous trade names such as but not limited to Bravo WeatherStik, Bravo Ultrex, Echo 720, Equus 720, Chloronil, Chlorothalonil 720 and others. “Issues with product formulation are the likely cause of chlorothalonil shortage but I’m not sure it refers to U.S. or foreign sourced material,” says Hagan. “There are also chlorothalonil combination products such as Muscle ADV and Echo Eminent Co-Pack that are available for use in peanuts, but supplies of these products are also likely to be limited.”

In peanuts, says Hagan, chlorothalonil is a critical broad-spectrum anchor in nearly all fungicide programs due its good activity at relatively low cost against early and late leaf spot diseases as well as peanut rust. It also has value as a resistance management tool with strobilurin and triazole fungicides.

“Due to the existing risk of resistance-related control failures for triazole fungicides like tebuconazole, metconazole (Quash), and propiconazole (Bumper, Propimax, Tilt) as well as potential for risk for resistance-related control failures with the strobilurin fungicides azoxystrobin (Abound 2SC and Azaka), pyraclostrobin (Headline 2.09SC), and fluoxastrobin (Evito), total application numbers of these fungicides is limited to half or less of the total number of fungicide applications in a peanut disease control program,” says Hagan.

With the limitations to the use of the above systemic, single-site fungicides, there are not a lot of broad-spectrum alternatives to chlorothalonil in peanuts or other crops with a similarly high efficacy level, says Hagan, but there are some options for stretching chlorothalonil supplies.

“Reducing chlorothalonil application rates from 1 ½  to 1 pints per acre is a possibility. Field trials over the past few years have shown that there’s little drop off in leaf spot control at chlorothalonil application rates of 1 pint versus 1 ½ pints per acre (0.9 vs. 1.4 pounds per acre for Bravo Ultrex).

“Going with reduced chlorothalonil rates season-long will be much riskier where peanuts are cropped every year or every second year, and irrigated peanuts regardless of location, as well as high rainfall areas such as Baldwin, Mobile and Escambia counties. A tropical storm also can trigger a leaf spot or rust control failure when using reduced rates or numbers of applications of chlorothalonil,” says Hagan.

Other options available

Other options, he says, include combining a reduced rate of chlorothalonil with a three-spray block program with Fontelis at 12 to 24 ounces per acre, four-spray block with Provost 433SC at 8 to 10.7 ounces per acre, or a two-spray program with Headline 2.08SC at 9 or 12 fl oz/A, two sprays of Abound 2SC (and generic azoxystrobin) at 18.2 to 24 ounces per acre (or 18.2 ounces of Abound 2SC plus 5.5 ounces per acre of Alto), a two or three-spray program with Priaxor at 4 to 8 ounces per acre, or possibly, Custodia at 15 ounces per acre. Producers should go with the higher rate of leaf spot/white mold fungicides in at-risk settings, he says.

Growers can replace one or two early season chlorothalonil applications with 3.5 ounces per acre of Absolute 500SC or 1.5 to 2.25 pints per acre of Tilt/Bravo and then switch to a recommended rate of Provost 433SC, Headline 2.08SC, Abound 2SC (or generic azoxystrobin Azaka) alone or plus Alto, Fontelis or Priaxor, and finish with a single chlorothalonil application, he recommends.

“It’s difficult to say what impact the chlorothalonil shortage will have on combination products like Tilt/Bravo, Echo Eminent Co-Pack, and Muscle ADV. All of the above-metioned fungicides are direct replacements for chlorothalonil, but the Muscle ADV would be more useful for mid to late-season white mold control and could be alternated with Abound 2SC (or generics) alone or tank-mixed with Alto or Priaxor for extra punch against leaf spot, rust and white mold.”

Another possibility, says Hagan, is a copper fungicide such as 1.25 pounds per acre of Kocide 3000 or 2 pounds per acre of Cuprofix UltraDisperss.

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“Copper fungicides were widely used in peanuts for leaf spot control but were quickly replaced with the more effective fungicides chlorothalonil and benomyl. Since then, copper fungicides have not being screened for leaf spot control in Alabama fungicide trials and are not listed in the Alabama peanut leaf spot recommendations.

“Copper fungicides probably fit best on rotated dryland fields in a two or three year-out rotation in southeast Alabama, but they may not give acceptable leaf spot control in high leaf spot pressure situations such as continuous peanuts. While it’s not been tested, tank mixing a copper fungicide with a generic tebuconazole may provide acceptable leaf spot and white mold control. Be advised that phytotoxicity or incompatibility issues may pop up anytime multiple pesticides are mixed with antifoaming agents and other adjuvants.”

Thiophanate-methyl fungicides such as T-Methyl and Topsin M are partial replacements for chlorothalonil, says Hagan.

“There is an issue with their use in peanuts as this active ingredient is in the same fungicide class as Benlate (benomyl), which failed miserably in the mid-1970s. While a sizable portion of the population of leaf spot fungi are sensitive to thiophanate-methyl, the rest are still highly resistant after 40 years to thiophanate-methyl and now departed benomyl. As a result, thiophanate-methyl cannot be applied to peanuts for leaf spot control without a tank-mix partner.”

Leaf spot and rust outbreaks

One widely recommended tank-mix partner is chlorothalonil,  says Hagan, which can be combined at a rate of 1 pint per acre with 10 ounces per acre of the 4.5F formulation of T-Methyl or Topsin M or 0.5 pound per acre of the 70 WSB (WDG) formulations of these same fungicides.

“While it has not been tried, tank-mixing thiophanate-methyl with a copper fungicide might work as would a tank-mixture of full rate of a triazole fungicide like tebuconazole, metconazole (Quash), propiconazole (Tilt, Bumper, Propimax), flutriafol (Topguard) plus the above rates of the 4.5F or 70WSB formulations of either T-Methyl or Topsin M.”

Artisan, says Hagan, is a mixture of propiconazole plus flutolanil (Convoy) that has activity against leaf spot diseases and white mold.

“This product is more widely used on peanut in Georgia than Alabama. To enhance leaf spot control, it has been recommended that Artisan be tank-mixed with 1 pint per acre of a chlorothalonil fungicide. If two or three applications of Artisan were applied in mid- to late-summer on peanuts, possible tank-mix partners include 10 ounces per acre of T-Methyl or Topsin M, 6 to 9 ounces per acre of Headline 2.09SC, Abound 2SC, or Azaka, Evito or Evito T, Koverall (mancozeb), Elast at 15 ounces per acre, and either Kocide 3000 or Curpofix UltraDisperss copper fungicides.”

Elast (dodine) at 15 ounces per acre has been screened for leaf spot control in Alabama, says Hagan. This fungicide alone is not quite as effective as chlorothalonil but would be a possible tank- mix partner with tebuconazole, Quash or Topguard as a direct replacement for chlorothalonil when included in a Fontelis, Provost 433SC, Headline 2.08SC, Abound 2SC (and generic azoxystrobin [Azaka]), Evito/Evito T, Custodia, or Priaxor program.

“Peanut Rx programs have been promoted by nearly all brand-name fungicide distributors as a means of managing fungicide program costs by eliminating two or more applications of chlorothalonil. These programs work best in rotated fields on peanut varieties such as Florida-07 and possibly Georgia-06G that have improved leaf spot tolerance as compared with the more leaf spot-susceptible cultivar Georgia-09B.”

Another option, notes Hagan, is making an application of 9 ounces per acre of Headline 2.09SC at 45 days after planting, thereby eliminating the 30 DAP applications of chlorothalonil, he says. Producers also have the option of applying 9 ounces per acre of Headline 2.09SC and waiting 21 days to make the next fungicide application. Two applications of Headline 2.09SC split by an application of another fungicide on the above schedule will save one fungicide application, he adds.

“Last but not least, a mancozeb fungicide such as Koverall or Manzate Flowable may be substituted as a tank mix partner with a thiophanate-methyl or triazole fungicide (i.e. generic tebuconazole) in place of chlorothalonil. The application rate for Koverall is 1 to 2 pounds per acre and Manzate Flowable at 0.8 to 1.6 quarts per acre, but the higher rate of either fungicide would have better activity against leaf spot diseases and rust. Efficacy and residual activity of mancozeb is limited when compared with chlorothalonil and it should not be applied alone for foliar disease control on peanut.”

Regardless of the program chosen, Hagan advises growers to scout their peanuts weekly for leaf spot diseases and to shorten the spray schedule, increase fungicide rates, and if necessary, insert an extra application of Headline 2.09SC at a minimum of 9 ounces per acre.

“Producers also need to be advised to scout their peanuts starting in mid- to late-July for leaf spot diseases and rust. Should leaf spot or rust appear, they are advised to shorten the time interval between fungicide applications or switch to a more effective fungicide.”

It has been four or more years since Alabama has been visited by a tropical storm and the state is due, says Hagan.

“Many growers have probably forgotten about the leaf spot and rust outbreaks that usually follow a tropical storm. If peanuts have received a fungicide application within five to seven days of a tropical storm strike, then they should be okay. If more than seven days have passed, then growers are advised to make another fungicide application before the storm is scheduled to hit. When a pre-storm application is needed, I’d suggest an application of 9 ounces per acre of Headline 2.09SC, as this fungicide has the best leaf spot and residual activity of all the products on the market.”