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Powdery Mildew of Cherry: New Fungicide Tools and Resistance Management Guidelines

Tuesday May 16, 2017 4:04pm

As of spring 2011 cherry growers in Eastern Washington 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). A more inclusive list of fungicides is presented in the table below. The table includes fungicide class information and Fungicide Resistance Action Committee (FRAC) 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, of FRAC groups. Consult product labels for appropriate rates and spray intervals. The resistance risk is product-dependent (see table). All of the aforementioned "new" products have performed well in efficacy trials at WSU-IAREC.

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.

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.

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 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) 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.

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.

Gary G. Grove and Mark E. Nelson
Washington State University Irrigated Agriculture Research and Extension Center, 24106 N. Bunn Road, Prosser, WA 99350

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.

Fungicide choices for powdery mildew management in Washington cherries, 2011.

Trade Name1

Chemical Name

Class

FRAC Group2

Resistance Risk

Abound

azoxystrobin

QoI

11

High

Adament3

tebuconazole +

DMI

3

Medium

trifloxystrobin

QoI

11

High

Armicarb

potassium bicarbonate

bicarbonate

Not classified

Low

Cabrio

pyraclostrobin

QoI

11

High

Elite

tebunconazole

DMI

3

Medium

Gem

trifloxystrobin

QoI

11

High

JMS Stylet Oil

light summer oil

PDSO

Not classified

Low

Kaligreen

potassium bicarbonate

bicarbonate

Not classified

Low

Pristine3

pyraclostrobin +

QoI

11

High

boscalid

carboximide

7

Medium

Procure

triflumizole

DMI

3

Medium

Quash

metconazole

DMI

3

Medium

Quintec

quinoxyfen

Quinoline

13

Medium

Rally

myclobutanil

DMI

3

Medium

Regalia

extract of Reynoutria sachalinensis

Plant host inducer

P

Low

Rubigan4

fenarimol

DMI

3

Medium

Serenade Max

Bacillus subtilis

Biological

44

Low

Sonata

Bacillus pumilis

Biological

44

Low

Sulfur

sulfur

sulfur

M2

Low

Unicorn3

tebuconazole +

DMI

3

Medium

sulfur

sulfur

M2

Low

1Trade names in bold/italics are new to the industry in 2011
2Fungicide Resistance Action Committee
3Premix (formulation of two modes of action or FRAC groups)
4Also available under different trade names (e.g. Focus and Vintage)Download Table 1. Fungicide choices for powdery mildew management in Washington cherries, 2011


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