May 20, 2011

Fungicide resistance management for cherry leaf spot

Fungicide management programs for cherry leaf spot must be initiated around petal fall when leaves are unfolding. What we are hoping to accomplish with leaf spot sprays is preventing or actually delaying initial infection events. As with all other diseases, leaf spot infection will rise to epidemic proportions under conducive weather conditions. Thus, the purpose of early season sprays is to prevent, or significantly delay, initial infection events. This both provides disease control prior to harvest (severe early season infection can affect fruit ripening) and helps trees hold leaves into September and beyond.

Prior to shuck split, the only logical choice for cherry leaf spot management is chlorothalonil (Bravo and generics). This is a multi-site fungicide that is both excellent for leaf spot control and is not at risk for fungicide resistance development. At least two applications of chlorothalonil should be made before shuck split.

There are five major classes of fungicides and one mixture registered for leaf spot control that could be used as cover sprays (Table 1). Of the five classes, only the SI’s are not recommended for management of cherry leaf spot due to resistance to SI fungicides in the cherry leaf spot fungus; this resistance occurs universally throughout Michigan orchards. Thus, SI fungicides should never be used for leaf spot control but will still have effectiveness against American brown rot if used as a tank mix partner. SI resistance is quantitative, meaning that leaf spot populations exhibit a wide range of susceptibilities to these fungicides, currently moderate to highly resistant. However, eventually the use of SI’s will continually shift orchard populations such that they contain more and more highly resistant individuals. These shifts result in significant control failures across whole orchards, and we have observed such control failures in many orchards over the last few years.

Table 1. Class of chemistry and fungicides registered for cherry leaf spot control.

^a = Because of widespread resistance in Michigan, SI’s should never be used alone and are not recommended for leaf spot control.
^b = Pristine is a mixture of a strobilurin and boscalid, another fungicide (separate chemistry).
^{c =}Captan should only be used in combination with other fungicides because the rate (2.5 lbs/A Captan 80WDG) allowed on cherries is too low for effective disease control.

After shuck split, the use of the strobilurin Gem (3.0 to 3.8 oz./A) or the strobilurin/boscalid Pristine (10.5 to 14.7 oz./A) at the first cover timing is an excellent choice because both of these materials are also excellent powdery mildew materials. Preliminary data has shown early control of powdery mildew will result in better season-long control, so these two materials are optimal for first cover timing. Gem and Pristine are two excellent fungicides, but both are at risk long-term for the development of fungicide resistance. Growers need to think about long-term strategies in order for use of these materials to remain effective for a significant number of years. Overuse of these fungicides now could compromise the effectiveness of these products against leaf spot in the future. We suggest that both Gem and Pristine are not used more than two times per season.

More importantly, limiting the use of Gem and Pristine will provide long-term preservation of these fungicides, but will require more active resistance management strategies. Strobilurin resistance has emerged in the apple scab pathogen Venturia inaequalis and is now completely widespread in Michigan. This emergence happened quite fast, and the first resistant apple scab strains probably developed in 2006 – only seven years after the registration of the strobilurins. Because of the development of resistance in apple scab, we recommend the resistance management strategy for Gem and Pristine use in cherry leaf spot control needs to include the broad spectrum fungicide Captan. Similar to tank mixing fungicides with an EBDC for apple scab control, tank mixes with Captan should be utilized for leaf spot control.

Gem is also available as one component of the fungicide Adament. Adament is a mixture of two fungicides – Gem and the SI Elite. For leaf spot control, the Elite component of Adament would be essentially ineffective. Thus, Adament would be most effective when used at a timing where American brown rot control is also needed. It is also critical to use a rate of Adament that ensures that the Gem component is at a rate sufficient for leaf spot control. Adament at the 6 oz/A rate is equivalent to 3 fl oz Gem + 3.3 oz Elite. Adament at the 8 oz rate is equivalent to 4 fl oz Gem + 4.4 oz Elite. Adament should also be tank-mixed with Captan for resistance management.

Syllit (dodine), at 24 to 27 fl oz per acre, is an excellent leaf spot material. In a survey conducted in 2007, we did not isolate strains of the leaf spot fungus that were resistant to Syllit. However, experience tells us that Syllit is a fungicide that is resistance-prone, so, Syllit should also be tank-mixed with Captan as a resistance management strategy to prevent any buildup of dodine resistance. Also, Syllit is not as effective in controlling powdery mildew or brown rot, so Syllit is best used as a second or third cover sprays in a tank mix with Captan. The current Syllit label indicates an incompatibility with Captan, but these fungicides can be tank-mixed and a revised label indicating this is planned for 2011.

Copper has proven be a highly effective fungicide for cherry leaf spot control. Results from several years of experiments consistently show that cherry leaf spot treatments utilizing one, two or three cover sprays of copper sulfate (1.2 lbs metallic Cu per acre) provided excellent cherry leaf spot control equivalent to or better than standard programs using conventional fungicides, such as strobilurins. Successful efficacy of copper compounds aids both conventional growers, as copper extends the life of traditional fungicides, and organic growers because copper is the only viable option for disease control in tart cherry. The only detriment to copper use is the potential for phytotoxicity effects to tart cherry trees.

When copper compounds are applied to tart cherry trees in advance of hot, dry weather, the trees can exhibit phytotoxicity symptoms such as bronzing on the undersides of leaves, large yellow and brown blotches on the upper surface of a few leaves, or blackening of veins on the undersides of leaves. In severe cases, copper phytotoxicity can also cause leaf defoliation. Thus, the second and third cover timings are good for copper use if temperatures are not projected to remain above 80°F for several days. However, we have been trying to quantify the phytotoxicity of copper use for many years, and we have not been successful in showing the potential impacts of multiple copper sprays in hot conditions. Adding lime to the tank can help minimize phytotoxicity in hot conditions. Growers should also remember that the copper and lime combination in a tank will render Imidan ineffectual due to the lime component.

If leaf spot infections are visible in orchards at harvest or if weather is highly conducive to infection events during harvest, we continue to suggest that chlorothalonil (Bravo) is the only real option for post-harvest cherry leaf spot sprays. At the timings it can be used – before shuck split and post-harvest – Bravo is the resistance management, or prevention, fungicide of choice because using it reduces the number of applications of other at-risk materials.

George W. Sundin and Nikki Rothwell, Michigan State University Extension