Current status of sterol inhibitor fungicides for control of American brown rot in Michigan

Recent studies show that Michigan populations of American brown rot still remain sensitive to sterol inhibitors.

The sterol inhibitor (SI) class of fungicides is the most effective fungicide group used to control American brown rot (ABR), caused by the fungus Monilinia fructicola. However, there has been increasing concern in Michigan of the possibility of development of strains of the ABR fungus with resistance to SI fungicides. To gain a better understanding of the status of M. fructicola isolates in Michigan, a total of 71 cherry and peach orchards were sampled in 2008-2010.

In 2008, a survey of 21 sweet and nine tart cherry orchards (566 total isolates) throughout Michigan detected no SI-resistant strains, although results indicated that some of these orchard populations may be shifting toward reduced SI-sensitivity.

Results from isolates collected in 2009 from five peach orchards in southwest Michigan (50 isolates) and one peach orchard (27 isolates) in northwest Michigan indicated only one SI-resistant isolate in southwest and that all other orchard populations were sensitive to SI fungicides. Although there was an initial concern among growers of chemical efficacy in the field during 2009, our observations show that orchard populations remain SI-sensitive.

During 2010, a more extensive survey of isolates from cherry and peach orchards was performed in southwest (13 orchards), west central (two orchards), and northwest Michigan (20 orchards); 172 total isolates. Our results indicated that all isolates examined were sensitive to SI applications in 2010 and that only two of these 35 orchard populations were shifted for resistance. Both of these orchards were located in southwest Michigan.

Reduced sensitivity or resistance of a pathogen to SI fungicides is quantitative. This means that a population with decreased sensitivity to SI’s can be controlled by applying higher rates of the fungicide. In 2009, Michigan obtained a Section 24(c) label for Indar that allows the application and use of increased fungicide rates of up to 12 fl. oz. per acre per application; the current field label rate is 6 fl. oz.  per acre (maximum seasonal allowance of 48 fl. oz. per acre).

The current rate of 6 fl. oz. per acre should provide optimal ABR control – if fruit are effectively covered during the current and upcoming warm, wet conditions – conducive for ABR infection and proliferation. Areas concerned with possible population shifts, particularly southwest Michigan, could increase the rate of Indar applied to 8 fl. oz. per acre. This 33 percent increase in fungicide rate should be effective in controlling any fungal strains exhibiting reduced sensitivity. It is critical that the fungicide be applied in a protective mode prior to rain events such that fruit are protected when fungal spores arrive.

Additional strategies to reduce the onset of SI resistance include rotation with other modes of action such as Pristine (shown to be effective in ABR control) and the use of Rovral during bloom. The most important factors to limit further shifting in ABR populations are to effectively eliminate the pathogen by minimizing its chances of growth and initiating new infections, thereby decreasing the chances of pathogen mutations for SI resistance.

Our research in the past three years has shown that Michigan populations of M. fructicola remain sensitive to SI fungicides. Although resistance to SI’s is quantitative and higher chemical rates of SI’s may control less sensitive isolates, this is not a long-term solution – the risk of resistance and complete loss of SI fungicides is too great. We are actively researching the efficacy of other fungicide modes of action, but unfortunately, none are commercially available at this time.

Related article: Time to scout and control American brown rot in cherries

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