Cherry leaf spot: Balancing the need to prevent early infections with bee safety

Recommendations to balance a successful, early-season cherry leaf spot control and be mindful of honey bee colonies.

Cherry leaf spot bract leaf. Photo credit: George Sundin, MSU
Cherry leaf spot bract leaf. Photo credit: George Sundin, MSU

Cherry leaf spot is caused by the fungal pathogen Blumeriella jaapii and is the most important disease of tart cherries. Early season management of this disease is critical to protect orchards against a potential fungal epidemic. Once cherry leaf spot infection occurs in the tree, the fungus will produce thousands of spores from lesions established on the leaves, and in most cases, there are more spores developing from these established lesions than ascospores that are coming up from leaves overwintering on the ground.

Secondly, 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 on a tree 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. An early infection that can move from leaf-to-leaf can result in a major infection event.

Bract leaves of tart cherry can play a critical role in the cherry leaf spot disease. Bract leaves are out early in the season, often around bloom time in tart cherry (see photo). Just like full-size leaves, bract leaves have stomata that open once the leaves unfold, and the cherry leaf spot fungus infects all leaves through these stomata. While stomates of vegetative leaves on trees are not open and functional until around petal fall, bract leaves can be infected during bloom if a cherry leaf spot infection period occurs.

Infections of bract leaves can contribute to an early infection that can lead to a cherry leaf spot epidemic. We observed this type of epidemic in 2012 when bract leaves were infected early in Northwest Michigan and many orchards had severe leaf spot infection by mid- to late June even when environmental conditions were not exceptionally conducive for leaf spot infection. Thus, bract leaf infections can jumpstart a cherry leaf spot epidemic, and in a year with disease-conducive environmental conditions, growers may be faced with challenges to effectively control cherry leaf spot.

First fungicide applications for cherry leaf spot control were recommended at or around tart cherry petal fall to ensure we are adequately protecting the first fully expanded leaves from fungal infection. However, after the 2012 incident, our goal is to prevent or delay initial infection events with well-timed early cherry leaf spot sprays; with less infection early, there will be less inoculum that growers will have to control during times with more optimal conditions for cherry leaf spot development. Hence, the purpose of early season sprays is to prevent or significantly delay initial infection events.

Again, this early season control will help with disease control prior to harvest (severe, early season infection can affect fruit ripening) and help trees hold leaves into September. Therefore, Michigan State University Extension is recommending that at the bract leaf stage, tart cherry trees must be covered if MSU Enviro-weather is predicting a cherry leaf spot infection event. If no cherry leaf spot infection period is predicted and conditions are not favorable for fungal development, growers can wait until post-bloom to spray for cherry leaf spot.

Prior to shuck split, the recommended fungicide for cherry leaf spot management has been chlorothalonil (Bravo and generics). This fungicide is a multi-site protectant and is excellent for cherry leaf spot control and is not at risk for fungicide resistance development. Chlorothalonil has been the traditional workhorse to control cherry leaf spot at this time during the season. However, a recent and preliminary initial study has shown that chlorothalonil may have some negative impacts on honey bees, particularly when hives have been treated with insecticides for mite infestations.

Until further studies can confirm chlorothalonil’s impacts on honey bee colonies, we are recommending that if growers need to spray at or during bloom to protect open bract leaves, they use one of the new SDHI fungicides (Luna Sensation or Merivon). This recommendation is intended to minimize early season cherry leaf spot infection while protecting our valued pollinators. Further studies on the interactions of fungicide applications and honey bee health will be initiated in 2014.

See also: Minimizing pesticide exposure to bees in fruit crops

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

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