MSU Extension Fruit & Nuts

Prior to the weekend, May 16-17, 2020, there was little concern for cherry leaf spot infection as there were very few green leaves present on Montmorency tart cherries. However, with warmer temperatures and a good amount of precipitation, these conditions helped cherry buds to move along and new leaves became exposed during the long wetting period that began mid-day on May 17. As of 9 a.m. on May 18, there were exposed and vulnerable bract leaves in some blocks at the Northwest Michigan Horticulture Research Center (Figure 1).

We observed that younger Montmorency trees and Balaton trees were further along in development compared to older Montmorency blocks; there were some, albeit fewer, open leaves present in the older Montmorency blocks. We have received similar reports about open tart cherry bract leaves from growers and consultants in the region. This year, George Sundin’s Tree Fruit Pathology Lab at Michigan State University observed open stomata on bract leaves as they were emerging from buds that may be susceptible to the cherry leaf spot pathogen, and we are working to further refine the timing when cherry leaf spot infections could get a foothold under optimal infection conditions.

The cherry leaf spot fungus infects through open stomata on cherry leaves. For terminal leaves on growing shoots, as a general indicator to when stomata are open and leaves are susceptible, we have used the terminology “open and fully expanded.” That is, cherry leaf spot infection can occur when these leaves are open and fully expanded, a timing that typically coincides with petal fall. However, stomates on bract leaves appear to be open as soon as these leaves emerge from buds.

In early May, the Tree Fruit Pathology Lab photographed open stomata in early Montmorency bud development (Figure 2). The images show that even before leaves are fully exposed, they have open stomata and are vulnerable to cherry leaf spot infection under wet conditions. This observation demonstrates that we to better understand the timing of when this pathogen can begin infecting leaves.

If Montmorency bud development in an orchard was similar to the development shown in Figures 1 and 2 during the wetting period, some of the leaves in the orchard were likely susceptible to infection. However, because tree development occurred during rainy weather, it is difficult to discern what amount of leaves could have been exposed during about a 30-40-plus hour wetting event. You will need to determine whether the development of Montmorency cherries on your farms will warrant a fungicide application to minimize cherry leaf spot fungal growth or if you will wait until later this week to cover before pending rains in the forecast.

Drier weather is in the forecast. Unlike 2019, weather conditions will dry and warm for a few days between now and the next forecasted rain, and this break in the rain will slow the potential for cherry leaf spot infections. Some growers have had success using a tank mix of Syllit plus Captan to minimize leaf spot development. You should also recognize that these materials are not translaminar, and they are susceptible to wash off. We have also observed phytotoxicity with Syllit and Captan on sweet cherries and Balaton as this variety is half sweet cherry.

Data from the 2019 cherry leaf spot trial conducted at the Northwest Michigan Horticulture Research Center showed that treatment 14, Captan used early (i.e., the first two sprays after petal fall followed by four applications of Flint Extra) instead of Bravo Weather Stik, resulted in a lower incidence of bract leaf infection compared to the untreated check; bract leaf rating was conducted on June 18 (Table 1). Treatment 14 had the lowest for leaf spot incidence and defoliation at the end of the season in part due to reduced bract leaf infection early in the season. In comparison, treatment 13, Bravo Weather Stik applied as the first two sprays after petal fall and followed by four applications of Flint Extra, did not perform as well as treatment 14. We hypothesize that chlorothalonil, the active ingredient in Bravo Weather Stik, could be more readily washed off from leaves compared to other products like Captan. Both chlorothalonil and Captan are multi-site protectants and you should not be concerned about resistance in the cherry leaf spot pathogen to these chemistries.

More data is needed to determine the cherry leaf spot efficacy of Captan alone applied following a wetting event. Note that these are experimental treatments/spray programs that may not be in accordance with pesticide labels, and we are not suggesting you follow the presented spray programs for the entire growing season. Please review all pesticide labels prior to applications.

Lastly, leaves will continue to grow and develop during this week’s warm temperatures. Therefore, a spray applied soon after this recent wetting event may not be adequate coverage for the new growth that will occur between the spray and rain predicted for later this week. Hence, an additional cover may be needed before the next chance for rainy weather.