Post harvest sprays for cherry leaf spot
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.
Cherry leaf spot is the most important fungal disease of tart cherry in Michigan. The leaf spot fungus Blumeriella jaapii infects leaves with symptoms first appearing on upper leaf surfaces as small purple spots. As spots accumulate on leaves, the leaves turn yellow and fall. The amount of lesions required to cause leaf yellowing and drop is variable. Late summer (August, early September) defoliation 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.
As harvest will be winding down for cherries, many growers will be considering post-harvest applications for cherry leaf spot. Considering the shortage of rain this season, the disease pressure has been much lower compared to seasons past. We have also seen very few cherry leaf spot lesions on cherry leaves in the region for these two reasons plus saving a spray, growers may want to opt out of a post-harvest spray. However, there are a few other factors to reflect on this season when deciding if a post-harvest spray is warranted: 1) Ethrel damage from the past two years, 2) drought-like conditions this summer, 3) mite numbers, and 4) predicting the precipitation for the rest of July and into August. All of these factors have caused stress to trees, and trees under stress can be more prone to winter damage.
As many know, Ethrel (ethephon) damage has been observed in blocks all across the northwest, both in sweet and tart cherries. This damage occurred in the past when ethephon was applied during hot and dry weather conditions during 2006 and in many blocks in 2005. Trees under stress, particularly drought stress, become more susceptible to ethephon damage. Damaged trees exhibit excessive gumming, and branches lose their leaves. Growers with trees stressed by Ethrel damage in 2005 or 2006 (or 2007 if we should have any) should consider a post-harvest spray to ensure that excessive leaf loss from cherry leaf spot does not predispose trees to winter damage.
Drought is another condition that will cause tremendous stress to trees. So far this season, we are approximately 2.5 inches below our normal precipitation levels (based on the past 26 year average). Trees that have been under stress due to lack of moisture may also be less likely to withstand severe winter conditions, so again, growers should prevent leaf loss due to cherry leaf spot. Mites are another stressor growers may not have considered when making a decision about a post-harvest disease application. Mites can cause early defoliation within the inner canopy of trees, and if that defoliation is combined with defoliation from cherry leaf spot, trees could head into the winter at a severe disadvantage.
Lastly, we cannot guess as to the weather for the remainder of July and into August. Unfortunately, growers will have to gamble on the weather to decide if they will need a post-harvest application. Often growers do not notice cherry leaf spot later in the season until they observe lesions forming on the leaves, and at that point, it is often too late for control. However, at this time, growers should be scouting the tops of trees for yellow leaves. This indicator is usually the first sign of cherry leaf spot infections that occur at this time. The tops of trees tend to be more infected at this time because of reduced spray coverage reaching these high areas. Growers also need to keep in mind that even a relatively low level of infection now could spread rapidly if sufficient rain events occur in the coming weeks.
Leaves typically defoliate a few weeks after they begin to show disease symptoms. Thus, the goal of a cherry leaf spot management program is to maintain a healthy canopy on trees at least through the end of September. This recommendation would ensure an adequate amount of leaves on trees into late October and beyond. The fungicide of choice for leaf spot control after harvest is chlorothalonil (Bravo). Bravo 82.5 WDG at a rate of 3 lbs./A provides excellent leaf spot control. In addition, Bravo is a broad spectrum fungicide and thus is not subject to fungicide resistance concerns.