Controlling cherry leaf spot in tart cherry orchards with reduced crop
Cherry leaf spot must still be managed in tart cherry orchards in 2012. Without a crop, we suggest that growers focus on using broad-spectrum fungicides for long-term resistance management of other resistance-risk fungicides.
Cherry leaf spot is the most important disease of tart cherry. Early defoliation caused by cherry leaf spot infection (August, early September) reduces the ability of trees to store photosynthate in roots, leading to an overall loss of vigor and leaving trees more susceptible to killing by winter injury. Early-defoliated trees also typically exhibit reduced flower bud formation and often set less fruit the following season.
In a year with no crop, cherry leaf spot management remains important because if left uncontrolled, the major effects of this disease are on overall tree health and the potential for winter injury and tree death.
We believe that the focus in 2012 should be on using broad-spectrum fungicides that do not have fungicide resistance concerns. These fungicides are chlorothalonil, captan, and copper. Since American brown rot is not an issue this year, single-site fungicides such as Indar that target this disease will not be needed. Growers should also consider leaving other resistance-risk fungicides such as Pristine, Gem, and even the new fungicides Luna Sensation and Fontelis in the shed to protect these modes of action for the long-term.
The only situation in which resistance-risk fungicides may be needed would be in blocks where powdery mildew is a problem (see Controlling powdery mildew in tart cherry orchards with reduced crop). A single application of a fungicide such as Pristine or Gem at the first cover timing after shuck split will go a long way in keeping mildew under control. This application will also control cherry leaf spot, but the fungicide should be tank-mixed with captan for fungicide resistance management.
Broad-spectrum fungicides for cherry leaf spot
Chlorothalonil is the fungicide of choice for the first spray timings targeting cherry leaf spot starting about petal fall and again at shuck split. In a normal season, chlorothalonil cannot be used between shuck split and harvest. We do have a Section 24(c) label now in Michigan extending chlorothalonil use past shuck split under certain cropping conditions (see Tart cherries receive 24 (c) for use of Bravo Weather Stik (chlorothalonil) beyond shuck split), but even timing limits from this label become moot with no crop. This is because label restrictions are there due to residue issues on fruit. Without a crop, the only label restriction for chlorothalonil of importance is the seasonal limit – 20.5 pints per acre for Bravo Weather Stik, 18.8 lbs per acre for Bravo Ultrex. These seasonal restrictions will enable approximately five applications per season.
Captan is an excellent cherry leaf spot fungicide that is mainly limited by the maximum labeled use rate of 4 lbs per acre for Captan 50WP, 2.5 lbs per acre for Captan 80WDG, or 2 QTS per acre for Captec 4L. Seasonal limits are based on actual amounts of a.i. of captan used. The seasonal limits are 28 lbs per acre if using Captan 50WP, 17.5 lbs per acre for Captan 80WDG, and 14 QTS per acre for Captec 4L or 7 total captan applications.
Copper is a highly effective fungicide for cherry leaf spot control when used at a rate of 1.2 lbs metallic Cu per acre. 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.
Dr. Sundin’s work is funded in part by MSU‘s AgBioResearch.