Update on resistance to strobilurin fungicides in the apple scab fungus in Michigan

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Apple scab, caused by the fungus Venturia inaequalis, is an important constraint on apple production in Michigan. The primary inoculum for apple scab develops in the spring in infected leaves on the orchard floor from the previous season. If primary scab infection is not controlled, significant levels of leaf and fruit infection can be expected. Infection periods for apple scab occur every year in Michigan orchards. Although apple varieties with resistance to scab do exist, these varieties are not widely planted; thus, scab must be actively controlled each year through intensive usage of fungicides. This intensive fungicide usage has led to the development of resistance in V. inaequalis to some classes of fungicides such as dodine and the benzimidazoles in Michigan.

The strobilurin class of fungicides was first used in Michigan in 1999 when Sovran was registered. A second strobilurin fungicide, Flint, was registered soon after that. Both of these fungicides provided excellent control of apple scab. However, since these fungicides have a single site mode of action, there was always a risk for the development of resistance in the pathogen population. Heading into the 2010 season, we now have confirmation that strobilurin resistance is widespread through ALL growing regions of Michigan.

We use two tests to identify strobilurin resistance. The first is a genetic test for the G143A mutation, which is known to confer resistance to strobilurins in many different fungal pathogens. The second is a spore germination test that compares germination of fungal isolates on a growth medium amended with a high rate of strobilurin with one that does not contain any fungicide. If the spores can germinate on both media, that fungal isolate is resistant. We found that the results of the spore germination test correlated with the G143A genetic test in our studies.

Isolates that carry the G143A mutation are immune to the stobilurins and cross resistance is observed between Sovran and Flint. Thus, these fungicides are no longer effective for scab control. Note: we do not have information on the status of susceptibility of other fungal pathogens in which strobilurins are effective for control (e.g. powdery mildew, black rot, summer diseases). We expect that the strobilurins are still effective against these diseases.

In 2009, we sampled 44 orchards in Southwest Michigan, the Fruit Ridge area, Oceana County, Northwest Michigan and Eastern Michigan. Our results indicated a widespread strobilurin resistance problem (Table 1).

Table 1. Strobilurin resistance results (percentage resistance) in analyses of apple scab fungal isolates from Michigan orchards in 2009.

Fruit Ridge  % Resistance              Eastern Michigan         % Resistance
Orchard 1                80                               Orchard 1                                45
Orchard 2                100                             Orchard 2                                30
Orchard 3                60                               Orchard 3                                90
Orchard 4                100                             Orchard 4                                60
Orchard 5                100                             Orchard 5                                0
Orchard 6                100                             Orchard 6                                0
Orchard 7                84                               Orchard 7                                0
Orchard 8                100                             Orchard 8                                0
Orchard 9                40
Orchard 10              52
Orchard 11              59
Orchard 12              0
Oceana County % Resistance                     SW Michigan               % Resistance
Orchard 1                76                               Orchard 1                                0
Orchard 2                100                             Orchard 2                                70
Orchard 3                100                             Orchard 3                                0
Orchard 4                63                               Orchard 4                                0
Orchard 5                84                               Orchard 5                                0
Orchard 6                100                             Orchard 6                                0
Orchard 7                100                             Orchard 7                                16
Orchard 8                100                             Orchard 8                                48
Orchard 9                100
Orchard 10              100
Orchard 11              100
Orchard 12              100
Orchard 13              100
NW Michigan % Resistance
Orchard 1                100
Orchard 2                25
Orchard 3                60

The strobilurin resistance results illustrate that resistance is almost completely spread throughout Michigan orchards. There are only a few orchards where we did not find resistance. Although the resistance percentages appear low in orchards from Southwest and Northwest Michigan, we want to stress that we did not sample a large number of orchards in those regions. We will be sampling more heavily in those regions in 2010, but do predict that resistance will be present at similar levels in those regions. Because strobilurin resistance confers complete immunity to these fungicides, even in orchards with relatively low levels of resistance (less than 30 percent), a single application of Sovran or Flint would cause the resistance percentage to quickly rise to 100 percent. Thus, the strobilurin fungicides should not be used for apple scab control in Michigan.

Any use of strobilurins in Michigan apple orchards in 2010 for diseases other than apple scab should be at full rates in a tank mix with a broad spectrum protectant. The next fungicide application should follow on a shorter interval (seven-day maximum) and include a broad spectrum protectant.

See also the Apple scab control 2010 article for related information.

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