Written by Prashant Swamy and Gary Grove, Washington State University. April 3, 2021
Powdery mildew remains one of the most important diseases of cherries in the Pacific Northwest (PNW). We have fortunately had over the last two decades many fungicide products available for disease management. However, our choices were at one time far more limited; for example, in 1986 only sulfur was recommended for regional disease management. Registrations for FRAC (Fungicide Resistance Action Committee) Group 3 (DMI), Group 11 (QoI), and Group 13 (quinoxyfen) became available during the late ’80s, mid-’90s, and early 2000s, respectively. Group 7 (SDHI) compounds appeared a few years later. Given our perennial challenges with powdery mildew, repeated applications of some of these fungicide groups for up to several decades enforced a very high selection pressure on the pathogen to develop resistance. Some manufacturers introduced “premix” fungicide formulations (containing two FRAC groups). This development was in part designed to incorporate resistance management (through the use of two sites of action) into single product formulation. The cherry toolbox contains several premix product types: Pristine (pyraclostrobin + boscalid), and Unicorn (tebuconazole + sulfur), Luna Sensation (fluopyram + trifloxystrobin) are a few examples. Both active ingredients in these compounds have activity against powdery mildew when first introduced. 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 none of the two active ingredients. 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, which is in some cases the situation now facing the PNW cherry industry.
Recent research in our laboratory has identified resistance to Group 11 (QoI or strobilurin) and Group 3 (DMI) fungicides in the cherry powdery mildew pathosystem in the PNW. The resistance has been documented in all cherry-producing areas, but it is currently unclear how extensive the resistance is within each area. Almost half of the mildew isolates tested in our lab were resistant to Group 11 compounds. Unfortunately, cross-resistance is prevalent in Group 11 meaning that if resistance develops to one compound in the group, the mildew fungus will also be resistant to other Group 11 compounds. Products such as Abound, Gem, Cabrio, Pristine, Quadris Top, Merivon, and Luna Sensation are Group 11 compounds or (in the case of premix compounds) contain a Group 11 component as part of a fungicide mixture. Presented in Table 1 are fungicides that are either standalone Group 11 compounds or premix formulations that contain a Group 11 component.
Table 1. Cherry mildew fungicides containing Group 11 compounds.
|Trade Name||Group 11 Component||Multiple fungicides (y/n)||Other Fungicide Components||Other FRAC Group Number|
Resistance to Group 3 compounds was detected in 28% of powdery mildew isolates tested in the laboratory. Cross-resistance to Group 3 (Table 2) compounds has also been documented but (given the complexity of Group 3 resistance) is less understood in this group of compounds. These compounds can also be sparingly used in PNW cherry production but they should be used under the restrictions outlined below.
Table 2. Cherry mildew fungicides containing Group 3 compounds.
|Trade Name||Group 3 Component||Multiple fungicides (y/n)||Other Fungicide Components||Other FRAC Group Number|
|Generics||tebuconazole, propiconazole, etc.||n/s|
Until a commercial test is available to identify resistance in individual orchards, one should (particularly if control failures have become common in your area or orchard) assume that resistance to Groups 11 and Group 3 is present. The proper response, in this case, is to avoid the use of Group 11 and Group 3 compounds for at least two growing seasons. If Group 11 or Group 3 compounds have already been ordered, or regulatory issues (e.g. MRLs) complicate the mildew management approach, they should be used as a last resort under the following restrictions:
- Rely more on other chemistries but be cognizant of potential MRL Issues with alternative chemistries.
- Limit Group 3 or 11 applications to one each per growing season.
- Avoid application of Group 3 or 11 in sequence in a growing season.
- Always tank mix a Group 3 or Group 11 compound with a contact fungicide (eg. sulfur) as allowed per label instructions.
- Encourage your neighboring growers to follow these guidelines, too.
Several new fungicide sites-of-action have become available during the last several years (Table 3). Without Group 11 and 3 compounds, mildew management programs should make use of 13 (quinoxyfen), 7 (SDHI), 19 (polyoxins), U13 (flutianil), M2 (sulfur), U6 (cyflufenamid), 50 (metrafenone), narrow-range petroleum oils, potassium bicarbonate, and biologicals. In order to protect our remaining resistance-prone compounds (Table 3), general resistance management guidelines should be followed when using remaining Groups 7, 13,19, U13, U6, and 50. Always tank-mix a resistance-prone synthetic compound with a contact fungicide (e.g. sulfur) as allowed per label instructions. General principles of fungicide resistance management are available at http://treefruit.wsu.edu/article/current-status-of-the-powdery-mildew-fungicide-toolbox-in-cherries/ and https://www.frac.info/fungicide-resistance-management.
Table 3. Newer fungicides (and in some cases sites-of-action) registered for use on Washington cherries.
|Trade Name||Common Nam||FRAC Group||Other Fungicide Components||Preharvest Interval (PHI)||Reentry Interval (REI)|
|Gatten||flutianil||U13||No||3 days||12 hours|
|Luna Privilege||fluopyram||7||No||0 days||12 hours|
|Miravis||pydiflumetofen||7||No||0 days||4 hours|
|Fontelis||penthiopyrad||7||No||0 days||12 hours|
|Cevya||mefentrifluconazole||3||No||0 days||12 hours|
|Vivando||metrafenone||50||No||7 days||12 hours|
|Torino||cyflufenamid||U6||No||6 days||4 hours|
Developing a fungicide program that minimizes the use of Group 3 and 11 compounds is complicated by several factors. First, given the availability of newer products and remaining contact fungicides (e.g. sulfur, zinc salts, horticultural oils, potassium bicarbonate), multiple fungicide combinations could be employed to manage mildew through harvest. Second, the grower needs to manage what are essentially two phases of the mildew epidemic: one phase of the epidemic on foliage and the second a phase of the epidemic on fruit. The two epidemics are related because spores produced on infected foliage are responsible for the initiation of fruit infections. Therefore, management of the foliar phase is essential and needs to begin early in the growing season in order to keep foliar disease incidence and severity low. Fruit are susceptible during a narrow (roughly two weeks) window just before harvest; fungicide applications within this window should include fungicides listed in Table 3. Third, there may be MRL issues associated with these compounds. Fortunately, essential local cherry MRL information is available online thanks to the Northwest Horticultural Council (https://nwhort.org/export-manual/comparisonmrls/cherry-mrls/) and Washington Tree Fruit Research Commission (https://treefruitresearch.org/wp-content/uploads/2020/09/2012-2020-Cherry-Residue-Summary.pdf). Both sources should be consulted when developing a fungicide program. In addition, the most recent FRAC codes and resistance information are available at www.frac.info/publications. The growers and orchard managers should be cognizant of PHI and REI restrictions for all fungicides.
Fortunately in the context of the disheartening news about Group 3 and Group 11 compounds, we have alternatives (albeit with caveats) for mildew management. However, our “margins for errors” are not what they once were. Therefore, it is imperative to minimize the odds in the growers favor through the use of effective fungicides, proper sprayer calibration, good spray coverage, proper spray intervals, employing cultural practices that reduce disease pressure (e.g. vigor management and suckering), and the application of good fungicide resistance management techniques.
1(see 2020 article for background information: http://treefruit.wsu.edu/article/current-status-of-the-powdery-mildew-fungicide-toolbox-in-cherries/)
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. Always make applications under good conditions.
Use pesticides with care. Apply them only to plants, animals, or sites listed on the labels. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.
YOU ARE REQUIRED BY LAW TO FOLLOW THE LABEL. It is a legal document. Always read the label before using any pesticide. You, the grower, are responsible for safe pesticide use. Trade (brand) names are provided for your reference only. No discrimination is intended, and other pesticides with the same active ingredient may be suitable. No endorsement is implied.
The authors appreciate financial support of this research by the Washington Tree Fruit Research Commission, Oregon Sweet Cherry Commission, Washington State Commission on Pesticide Registration, and Northwest Nursery Improvement Institute.
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