Cherry leaf spot, a common fungal disease caused by Blumeriella jaapii, can significantly impact the health and productivity of cherry trees. Understanding its symptoms and implementing effective management strategies are crucial for protecting your orchard. The disease overwinters on infected leaves and fallen fruit, releasing spores in the spring when temperatures and moisture levels are favorable for infection.
Symptoms of Cherry Leaf Spot
The initial symptoms of cherry leaf spot appear in early summer as small, purple spots on the upper surface of older leaves. These spots gradually enlarge, reaching about 1/4 inch in diameter, and change to a reddish-brown color. As the infection progresses, several spots may merge, forming larger dead areas on the leaf. Yellow halos often surround these spots, and affected leaves begin to yellow, wither, and eventually fall from the tree, typically about a month after the infection begins. In some cases, the diseased tissue within the spots may dry out and fall away, leaving small holes in the leaves.
Managing Cherry Leaf Spot
If your cherry trees are experiencing significant leaf drop due to leaf spot, it is essential to implement aggressive control measures. A comprehensive approach combining sanitation and fungicide applications offers the most effective management of this disease.
Sanitation Practices
Sanitation is a cornerstone of cherry leaf spot control. The fungus responsible for the disease survives the winter months in fallen leaves and debris. Therefore, meticulous cleanup of your orchard is critical. Rake up and compost fallen cherry leaves in the fall, typically in September or October. By removing these infected leaves, you significantly reduce the number of disease spores present in the environment, thereby minimizing the risk of infection in the following spring.
Fungicide Applications
Fungicides play a vital role in protecting cherry trees from leaf spot, especially when trees have a history of dropping a majority of their leaves before September for several consecutive years. Fungicide applications should commence approximately two weeks after bloom, once the leaves have fully unfolded. It is essential to adhere to the application intervals specified on the fungicide label, repeating applications throughout the growing season. A crucial application is also recommended after harvest to protect the tree for the remainder of the season.
Effective Fungicides and Resistance Management
Fungicides containing active ingredients such as myclobutanil or captan have proven effective in protecting leaves from cherry leaf spot when applied correctly. However, to prevent the development of fungicide resistance, particularly with myclobutanil, it is advisable to alternate between different active ingredients during repeated applications. This strategy helps to manage the pathogen's ability to develop resistance over time.
Fungicides with copper as an active ingredient can also offer some level of protection against leaf spot infection. Certain copper-based fungicides are approved for use in organic production systems. It is important to remember that fungicides are most effective when used in conjunction with good sanitation practices.
Safe and Proper Fungicide Use
Always prioritize safety and follow all instructions meticulously when using fungicides. The name of the plant being treated must be clearly listed on the fungicide label for the product to be used legally and safely. Some products are registered for ornamental Prunus species and are not suitable for use on stone fruits intended for consumption. Always read and strictly adhere to all directions provided on the fungicide label.
The post-harvest interval (PHI) is a critical factor to consider. The PHI is the mandatory waiting period between the last fungicide application and when the fruit can be harvested. Ensure you wait the full duration specified on the label to guarantee the safety of the harvested fruit.
Timing of Early Season Applications
Protecting tart cherry trees early in the season is paramount for effective cherry leaf spot management. Historically, it was suggested that the first fungicide application should be made around tart cherry petal fall to adequately protect the first fully-expanded leaves. This timing was based on the understanding that the cherry leaf spot fungus infects leaves through stomata, which are not fully open and functional until the leaves mature.
However, observations from epidemic years, such as 2012, highlighted that this strategy might not account for the very first bract leaves. In such scenarios, severe leaf spot infections were observed by mid- to late-June, even when environmental conditions were not exceptionally conducive. This indicates the importance of considering all emerging foliage.
Understanding Inoculum Spread
Spores originating from lesions on existing leaves within the tree canopy are far more likely to infect new leaf targets compared to spores originating from fallen leaves on the ground. The proximity of leaves on a tree facilitates easier spread of spores from one leaf to an adjacent one. This contrasts with the more challenging trajectory for spores to travel from ground level up to the leaf canopy.
The primary objective of cherry leaf spot spray programs is to prevent or delay the initial infection events. Similar to other plant diseases, cherry leaf spot can escalate to epidemic proportions under favorable weather conditions. By reducing the initial infection early in the season, there will be less inoculum within the tree to manage during periods with more optimal conditions for disease development.
Pre-Shuck Split Fungicide Recommendation
Prior to shuck split, the fungicide chlorothalonil (available under trade names like Bravo and its generics) is recommended for cherry leaf spot management. This fungicide acts as a multi-site protectant, making it highly effective for leaf spot control and posing a low risk for fungicide resistance development.
Applying Copper Fungicide
A note on plant descriptions: While efforts have been made to accurately describe these plants, variations in height, bloom time, and color may occur across different climates. These descriptions are compiled from numerous external resources.