Cherry growers in the Pacific Northwest have multiple fungicides at their disposal for managing powdery mildew. Products in the powdery mildew “toolbox” include members of the DMI, QoI, SDHI, quinolone synthetic compounds and multiple “contact” fungicides from other classes (e.g. sulfur and narrow range petroleum oils). A more inclusive list of fungicides is presented in the table below. The table includes fungicide class information, Fungicide Resistance Action Committee (FRAC) group number or code, and resistance risk.
Site of action, mode of action, and FRAC
A fungicide’s mode of action refers to specific cellular processes inhibited by a given fungicide or fungicides from the same chemical group. A fungicide’s FRAC code refers to the compound’s site of action. For example, group 3, 7, and 11 sites of action are inhibition of demethylation, succinate dehydrogenase, and ubiquinol oxidase pathways, respectively. However, both group 7 and 11 fungicides inhibit cellular respiration and therefore have the same mode of action. This information is helpful when designing a fungicide program that conforms to FRAC resistance management guidelines.
The table below includes members of the fungicide site-of-action groups (or FRAC Groups) known as:
- DMI (demethylation inhibitors, Group 3),
- QoI (quinone outside inhibitors; previously called strobilurins, Group 11),
- quinolines (quinoxyfen, Group 13),
- SDHI (succinate dehyrodgenase inhibitors; group 7),
- sulfur (Group M2),
- various “biological” fungicides (Group 44),
- petroleum derived spray oils, potassium bicarbonate, and various inorganic salts – all listed as “Not Classified” (NC) by FRAC.
Several products are formulations or “premixes” of two different fungicide classes (modes of action) of FRAC groups. Consult product labels for appropriate rates and spray intervals. The resistance risk is product-dependent (see table). Representatives of all of the aforementioned FRAC groups have over the years performed well in efficacy trials at WSU-IAREC.
Resistance management involves the use of multiple fungicide sites of action (via tank mixing, alternation, and rotation) as well as other pest management practices, such as cultural control. It is important to understand that if a pathogen population develops resistance to fungicides within a FRAC group (i.e. compounds with the same site of action, e.g. FRAC Group 11), it is likely to be fully or partially 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.
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.
Alternating modes of action
Resistant management guidelines include limiting the number of applications of individual modes of action per season and limiting 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 3 times per season and no more than twice in sequence. High risk QoI (FRAC Group 11) and SDHI (FRAC Group 7) compounds or premixed formulations containing them (Abound, Adament, Cabrio, Gem, and Pristine) fungicides 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 1/3 of the total number of fungicide applications per season. In programs utilizing tank mixes or pre-mixes 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 1/2 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.
The availability of “premix” or combination fungicide formulations are common 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 security, if there is field resistance to one of the two active ingredients, and help prevent resistance if there is not. However, one has to be careful because resistance to one active ingredient of the mixture can allow the fungus to adapt and develop resistance to the other active material. This has been described for several pathogens.
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.
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.
Gary G. Grove
Washington State University Irrigated Agriculture Research and Extension Center, 24106 N. Bunn Road, Prosser, WA 99350
Fungicide choices for powdery mildew management in Washington cherries, 2019. More information on fungicide resistance can be found at http://www.frac.info.
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