Controlling cherry leaf spot in orchards with early infections

Cherry leaf spot infection has already occurred in many Northwest Michigan tart cherry orchards. It will be critical to protect new leaf growth with fungicide applications with tightened spray intervals to prevent a wide-scale epidemic.

With this slow start to the season, we have observed early growth of tart cherry bract leaves that were open prior to bloom. These leaves can be a critical starting point for cherry leaf spot infection because the stomates on bract leaves are open and can be infected by the cherry leaf spot fungus. We have observed widespread bract leaf infection and infection of the first true leaves in tart cherry orchards throughout Northwest Michigan. It is critical to realize the significance of these early infections and their potential detrimental impact on cherry leaf spot this season.

Once infection occurs on new leaves in the tree, visible cherry leaf spot lesions will produce tremendous spore numbers that can readily infect new tissue. In most cases, there are more spores developing from these lesions than ascospores that are shot up into the developing canopy from leaves overwintering on the ground. In addition, spores from lesions on the leaves are much more likely to find new leaf targets within the tree than spores coming up from the ground – the distance from one leaf to another leaf is minimal, and the potential for spores to infect by moving from one leaf to an adjacent leaf is “easier” than for spores shot from ground level to hit the leaf target up in the tree canopy. The relative ease with which the fungus spreads from infected leaves within trees can lead to significant disease spread even under marginal cherry leaf spot infection conditions.

With cherry leaf spot infections already present in trees, it is extremely important to protect new leaf growth. The goal is to minimize cherry leaf spot infection to keep a full canopy of leaves on the trees to adequately ripen the crop. Fruit will not ripen evenly on trees with significant infection and resulting defoliation that happens before harvest. If cherry leaf spot lesions are present on bract leaves or new leaves at this time, trees will require more frequent fungicide applications (tighter spray intervals), and it will be critical for growers to adequately cover foliage prior to rain events.

Prior to shuck split, the recommended fungicide for cherry leaf spot management is chlorothalonil (Bravo and generics). This fungicide is a multi-site protectant and is excellent for leaf spot control and is not at risk for fungicide resistance development. However, according to the label, chlorothalonil applications must be spaced a minimum of 10 days apart. This 10-day spray window may be too long if rains and cherry leaf spot infection conditions occur within this interval because new leaves are growing rapidly and this tissue will be unprotected. With the cherry leaf spot fungus already present in trees, fungal spores can readily infect this unprotected tissue.

If growers did apply a full cover of chlorothalonil and need to spray within the required 10-day interval to cover for the rains predicted this Sunday, June 8, a good fungicide option is Captan 80WDG at 2.5 pounds per acre. Michigan State University Extension researchers have observed excellent control with Captan in cherry leaf spot trials conducted at the Northwest Michigan Horticultural Research Center over the past several years. As with chlorothalonil, Captan residues must be present on leaves prior to arrival of spores of the cherry leaf spot fungus. After the Captan application, an additional chlorothalonil application can be put on within seven to eight days at shuck split.

Unfortunately, this season seems to be shaping up to be a significant disease year. Therefore, it will be especially critical to utilize the best possible cherry leaf spot fungicide combination at the first cover timing. The recommended cover would be either Luna Sensation (5 fluid ounces per acre) + Captan (2.5 pounds per acre) or Merivon (5.5 fluid ounces per acre) + Captan (2.5 pounds per acre). The advantages of using Luna Sensation or Merivon are that these fungicides have the best activity against cherry leaf spot and also they are partially systemic and more persistent in controlling cherry leaf spot. However, the risk of fungicide resistance development is possible with these fungicides, so these chemistries should not be overused. Therefore, the best timings for Luna Sensation or Merivon are first cover and pre-harvest. In addition, at the first cover spray timing, these fungicides will also control powdery mildew, and at the pre-harvest timing, these fungicides will also control American brown rot.

Drs. Sundin and Rothwell's work is funded in part by MSU's AgBioResearch.

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