MSU Extension

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. We now have confirmation that this has occurred in two 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.

In 2008, we sampled eight orchards in the Fruit Ridge area and eight other orchards in eastern Michigan. Our results indicated a widespread strobilurin resistance problem (Table 1).

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

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).

The results in Table 1 tell us several things and let us speculate about the current status of orchards in other locations in Michigan. First, the occurrence of resistance at such high levels on the Ridge suggests that these resistant strains have been developing over the last few years. 2008 was not an especially bad scab year, in fact, conditions were dry on the Ridge throughout much of the primary scab season. We only observed scab problems on the most susceptible variety, McIntosh, and believe that we were fortunate that scab infection pressure was not as high in the year where we discovered problems due to a fungicide control failure. The lower level of resistance observed in the eastern Michigan orchards suggest that these orchard populations are likely a year or two behind those found on the Ridge. However, one strobilurin application in an orchard that currently harbors 50 percent resistant isolates is enough to change the orchard population to 100 percent resistant by killing off the sensitive isolates. This is why it is important to not utilize strobilurins in orchards where resistance is known.

The geographic distribution of orchards with strobilurin resistant apple scab isolates suggests that we may have a wider problem in Michigan. Thus, orchards in Southwest, West Central, and Northwest Michigan may have some population of strobilurin-resistant apple scab fungus. We will be sampling orchards in these locations in 2009 to get a definitive picture of the problem in these other locations.

What are the alternatives to strobilurins for apple scab control? We address these options in the accompanying article. Any use of strobilurins in Michigan apple orchards in 2009 should be at full rates in a tank mix with a broad spectrum protectant. The next fungicide application should follow on a shorter interval (7 day maximum) and include a broad spectrum protectant.

We are facing the loss of the strobilurin fungicide mode of action in Michigan which will put a serious constraint on apple scab disease control. As we lose fungicide modes of action, other methods of control, including reduction of overwintering inoculum, will become more and more important.