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New powdery mildew fungicides for cherries

Following resistance management guidelines will help preserve the use of new materials.

As of spring 2011, cherry growers in eastern Washington State have several new fungicides at their disposal for managing powdery mildew. New products included in the powdery mildew toolbox include Adament (tebuconazole + trifloxystrobin), Quash (metconazole), and Unicorn (tebuconazole + sulfur). All of the aforementioned “new” products have performed well in efficacy trials at Washington State University’s Irrigated Agriculture Research and Extension Center in Prosser. A more inclusive list of fungicides is presented in the accompanying table.

The table includes all fungicides registered for cherry powdery mildew in Washington and shows the fungicide class information and Fungicide Resistance Action Committee group number, or code. The FRAC code represents the mode of action of the fungicide. This information is helpful when designing a fungicide program that conforms to FRAC resistance management guidelines. It is important to remember that if a pathogen population develops resistance to fungicides within a FRAC group, it is likely to be resistant to all members of that group. Resistance is more likely to develop if the pathogen is frequently treated with one or multiple fungicides within a given FRAC group.

Included in the table are members of the fungicide classes (or FRAC groups) known as DMI (demethylation inhibitors, Group 3), QoI (quinone outside inhibitors; previously called strobilurins, Group 11), quinolines (quinoxyfen, Group 13), sulfur (Group M2), various “biological” fungicides (Group 44), petroleum-derived spray oils, and potassium bicarbonate. Petroleum spray oils and potassium bicarbonate are listed as “Not Classified” (NC) by FRAC. Several products are formulations or “premixes” of two different fungicide classes, modes of action, or FRAC groups. Consult product labels for appropriate rates and spray intervals. The resistance risk is product-dependent.

Premix trend

The availability of “premix” or combination fungicide formulations is a relatively recent trend in agriculture. The cherry toolbox contains several of these product types: Adament (tebuconazole + trifloxystrobin), Pristine (pyraclostrobin + boscalid), and Unicorn (tebuconazole + sulfur). Both active ingredients in these compounds have activity against powdery mildew. When both modes of action have activity against the target organism, some level of resistance management is built into the products, provided that they are used rationally. The use of “premix” types of products can provide better disease control, provide disease control security if there is field resistance to one of the two active ingredients, and help prevent resistance if there is not.

No resistant populations yet

QoI (Group 11) or QoI-containing fungicide products (Abound, Cabrio, Gem, and Pristine) are part of the cherry industry’s first line of defense against powdery mildew. The resistance risk of these Group 11 fungicides (formerly known as strobilurins) is high, while the risk of other important classes (DMI and quinolines) is considered medium. The resistance risk of contact fungicides sulfur, narrow-range petroleum oil, and potassium bicarbonate is low. We have no evidence of fungicide-resistant mildew populations in eastern Washington, but this could change rapidly, given the nature of powdery mildew and the resistance history of Group 11 and Group 3 fungicides. Therefore, it is imperative that resistance management guidelines be followed beginning with the introduction of the group.

Resistance management guidelines

General resistance management guidelines include the incorporation of cultural practices that lower disease pressure. Cultural practices such as vigor management and effective pruning both serve to lower disease pressure and improve spray penetration. The incorporation of these practices serves to lower selection pressure on pathogen populations. Always use fungicides in a protective, rather than reactive, manner: It is far easier to prevent powdery mildew than to cure it. Additional guidelines include:

• Limit the number of applications of individual modes of action per season, and limit sequential applications.
• Do not tank-mix or alternate fungicides with the same FRAC number in a spray program. Medium-risk compounds such as DMI (Group 3) and quinoline compounds (Group 13) should be applied no more than three times per season and no more than twice in sequence.
• High-risk QoI (FRAC Group 11) compounds or premixed formulations containing them (Abound, Adament, Cabrio, Gem, and Pristine)  should be preferably alternated 1:1 with other modes of action or groups. It is preferable to make only one application of any resistance-prone compound and then switch to a fungicide from a different class or FRAC group, but the cost of this approach can be expensive in eastern Washington.
• Never exceed more than two QoI applications in sequence. If two sequential applications of a QoI fungicide are made, this “block” should be alternated with at least two applications of one or more fungicides of a different mode of action or FRAC group.
• When Group 11 compounds are used as a solo product (Abound, Cabrio, and Gem), the number of applications should be no greater than a third of the total number of fungicide applications per season. In programs utilizing tank mixes or premixes of a Group 11 fungicide with a fungicide of another group (e.g., Adament or Pristine), the number of Group 11 fungicide (QoI)-containing applications should be no more than half of the total number of fungicide applications per season.
• It also helps to tank-mix fungicides from different groups that are both effective against powdery mildew. Sulfur is a relatively inexpensive and effective companion product for mixing with medium- or high-risk compounds. Try to include it in every spray tank aimed at powdery mildew, if permitted according to usage instructions on product labels and applying it is not detrimental to overall IPM objectives.

Always follow label instructions pertaining to application rates and intervals, and always use a properly calibrated sprayer and sufficient spray volume to provide good coverage.
Read the Fungicide choices in 2011 for powdery mildew management in Washington cherries table for the full list of fungicide choices and FRAC codes.