Fungicide considerations for cherry leaf spot control at first cover
The first cover spray timing after shuck split is a critical disease timing in tart cherry orchards, and fungicide sprays must target both cherry leaf spot and powdery mildew diseases.
Cherry leaf spot is the most important fungal disease of tart cherries 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 causing leaf yellowing and drop is variable. Sweet cherries can tolerate quite a few lesions before leaf drop occurs; however, Montmorency tart cherries will drop with only a few lesions, signifying the importance of proper leaf spot management.
Ascospore discharge from the cherry leaf spot pathogen is highest over a wide temperature range (60 to 85 degrees Fahrenheit) and lowest at 41 to 46 F. Ascospores are usually discharged starting at petal fall and continuing for the next four to six weeks. The optimum conditions for lesion development are temperatures of 6 to 68 F with rainfall or fog. After lesions appear on upper leaf surfaces, examination of the underside of leaves reveals a proliferation of white spore masses. These spores are dispersed by rain and wind within trees and to adjacent trees; such secondary cycles can continue repeatedly under favorable conditions through autumn.
Prior to shuck split, the fungicide of choice for cherry leaf spot management is chlorothalonil because this is a broad-spectrum fungicide with excellent efficacy against cherry leaf spot. After shuck split, Bravo WeatherStik is the only chlorothalonil fungicide that can be used for cherry leaf spot, and must be used under conditions of the Section 24 (c) label.
The first cover spray timing after shuck split is a critical disease timing in tart cherry orchards. This is not only because warming temperatures favor cherry leaf spot spore discharge and infection, but also because other diseases such as powdery mildew become active (see the Michigan State University Extension article “Controlling powdery mildew in tart cherry orchards”). At first cover, fungicide applications need to target both cherry leaf spot and powdery mildew.
There are three currently-registered fungicides that effectively control both cherry leaf spot and powdery mildew. These are the new SDHI premixes Luna Sensation and Merivon and the strobilurin fungicide Gem. Luna Sensation and Merivon both provide excellent control of cherry leaf spot and powdery mildew. These new or second-generation SDHI premixes are replacements for Pristine. Pristine contains the older or first-generation SDHI compound boscalid, and we have detected resistance to boscalid in cherry leaf spot populations in northwest and west central Michigan orchards. Our field trial results show that both Luna Sensation and Merivon effectively control boscalid-resistant cherry leaf spot. For resistance management, these two practices are absolutely critical:
- Use Luna Sensation and Merivon at high-label rates! Suggested rates are 5.6 fl oz/A for Luna Sensation and 5.5-6.5 fl oz/A for Merivon.
- Both of these premixes should also be tank-mixed with Captan. A minimum rate of Captan 80 WDG to be included would be 1.75 to 2 lbs per acre.
Gem remains highly effective against both cherry leaf spot and powdery mildew. This fungicide is also prone to resistance development and we have observed this happen with apple scab. Because of the length of time in years we have been using Gem on tart cherry, this fungicide definitely should be tank-mixed with 2 lbs Captan 80 WDG per acre.
- Video: Evaluation of SDHIs for control of cherry leaf spot, powdery mildew, and American brown rot
- Video: SDHI fungicide premixes: resistance management strategies for the SDHIs
- Video: Identifying cherry leaf spot in tart cherry
Dr. Sundin's work is funded in part by MSU's AgBioResearch.