Fungicides


Fungicide Resistance Management

General Information 

Crop management, fungicide chemistry, disease pressure, and application patterns profoundly affect the emergence of pathogen strains resistant to fungicides. Resistance results from prolonged and repeated use of the same or closely related fungicides. The onset of resistance can be delayed by minimizing selection pressure on the pathogen; it is best to begin this process early in the life of the fungicide or fungicide group. Some crop management and cultural practices can lower disease and, therefore, selection pressure 1) avoid planting in high-pressure areas such as ravines or draws where relative humidity may be excessive, 2) properly manage nitrogen fertilization and plant vigor, and 3) practice sound and effective orchard sanitation and maintenance. All of these methods should reduce the reproductive rate of fungal pathogens and therefore reduce selection pressure. The following fungicide use patterns may delay onset of resistance: 1) apply fungicides only when absolutely necessary, 2) use in a protective, rather than eradicative, 3) rotate fungicide chemistries, and 3) use mixtures of chemically unrelated fungicides 4) use formulations that contain two or more fungicide active ingredients against the target organism.

Fungicide classes differ in their potential for resistance. The Fungicide Resistance Action Committee (FRAC) has developed numeric codes for that various fungicide classes in order to better design resistance management programs.  The FRAC class code (or codes in the case of premix fungicide formulations) are normally found at the upper right on the first page of the fungicide label.  Compounds possessing low inherent resistance risk include sulfurs, soaps, carbonates, oils, and biologicals. Fungicide classes having high resistance risk include the strobilurin (QoI; FRAC code 11) and benzimidazole (FRAC code 1) classes.  Fungicide classes having moderate resistance risk include the DMI (FRAC code 3), quinolone (FRAC code 13), anilinopyrimidine (FRAC code 9), SDHI (FRAC code 7) groups. Mix or rotate all moderate and high risk with fungicides having different modes of action. For example, alternating the QoI fungicides pyraclostrobin (FRAC code 11) and trifloxystrobin (FRAC code 11) is virtually the same practice as using either compound continually. Starting resistance management strategies early in the life of the class and compound is important. When using resistance-prone fungicides for powdery mildew control, alternate with oils, soaps, sulfurs, or calcium polysulfide (Do not use cyprodinil for powdery mildew control). When using these chemicals for apple scab control, alternate them with ziram, captan, or mancozeb.  When using premix fungicides for resistance management, it is imperative that both fungicide components are active against the causal organism.

Fungicide resistance in sweet cherry

Fungicide resistance has been documented in the cherry powdery mildew pathosystem in the Pacific Northwest.  The resistance has been documented in all cherry producing areas but it is currently unclear how extensive the resistance is within each area.  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 I are fungicides that are either standalone Group 11 compounds, or premix formulations that contain a Group 11 component.  Cross-resistance also occurs in Group 3 but it is poorly understood because of the rate-dependent nature of the resistance. 

 

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

Abound

azoxystrobin

n

n/a

n/a

Cabrio

pyraclostrobin

n

n/a

n/a

Gem

trifloxystrobin

n

n/a

n/a

Luna Sensation

trifloxystrobin

y

fluopyram

7

Merivon

pyraclostrobihn

y

fluxapyroxad

7

Pristine

pyraclostrobin

y

boscalid

7

Quadris Top

azoxystrobin

y

difenoconazole

3

 

Resistance to Group 3 compounds has also been documented in the cherry mildew system but (given that directional nature of Group 3 resistance) cross-resistance, while present, 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 same restrictions as the Group 11 compounds listed below.  Resistance to Group 11 and 3 compounds has been detected in every WA production area. However, resistance was detected in slightly greater in 50% of orchards sampled but not in every orchard. In rare cases isolates were resistant to both Groups 3 and 11.  There is also some evidence of spatial variability in the distribution of resistant isolates in specific orchards.  

 

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

Rally

myclobutanil

n

n/a

n/a

Procure

triflumizole

n

n/a

n/a

Topguard

flutriafol

n

n/a

n/a

Cevya

mefentrifluconazole

n

n/a

n/a

Unicorn

febuconazole

y

sulfur

M2

Quadris Top

difenconazole

y

azoxystrobin

11

Generics

fenarimol, propiconazole, etc. 

y

 

 

 

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 is this case is to avoid the use of Group 11 and Group 3 compounds for at least one growing season. If Group 11 or Group 3 compounds have already been ordered, or regulatory issues (e.g. MRLs) complicate the mildew management approach, they can be used as a last resortunder the following restrictions:

 

1) Rely more on other chemistries while consideirng MRL Issues

2)Limit Group 3 and 11 applications to one per growing season

3)   Always tank mix a Group 3 or Group 11 compound with a second fungicide that is effective against powdery mildew when applied alone. 

4) Encourage your neighboring growers to follow these guidelines.  

Minus Group 11 and Group 3 compounds, mildew management programs should make use of Group 13 (quinoxyfen), Group 7 (SDHI), Group 19 (zinc salts), U13 (flutianil), M2 (sulfur), U6 (cyflufenamid), U8 (metrafenone), narrow-range petroleum oils, and biologicals. In order to protect our remaining resistance-prone compounds, general resistance management guidelines should be followed when using Groups 7, 13,19, U13, U6, U8, and 50.  Always tank-mix a resistance-prone synthetic compound with a contact fungicide (e.g. sulfur) or different mode of action.  Ensure that each fungicide is effective against powdery mildew.

 

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