The sudden yellowing of cherry tree foliage, known as chlorosis, signals a deeper health issue. Chlorosis is the loss of the green pigment chlorophyll, which is necessary for photosynthesis. It is a symptom, not a diagnosis, indicating the tree is struggling to convert sunlight into energy. Identifying the specific pattern and context of the yellowing is the first step toward pinpointing the root cause and implementing an effective solution.
Environmental and Cultural Causes of Yellowing
Cherry trees are sensitive to their immediate physical environment, and abiotic stressors frequently trigger chlorosis. Improper soil moisture is a leading cause. Insufficient watering (drought stress) causes leaves to yellow as the tree conserves water. Excessive watering is more damaging, suffocating roots and leading to root rot that prevents nutrient uptake.
Soil conditions beyond moisture content also play a significant role. Cherry trees require well-drained soil; poor drainage or compaction limits oxygen exchange, impairing nutrient absorption. Extreme temperature fluctuations, such as sudden heat waves or cold snaps, can shock the tree, resulting in yellowing and premature leaf drop. Additionally, planting the tree too deep or having girdling roots restricts the flow of water and nutrients, causing generalized canopy yellowing.
Nutritional Deficiencies and Chlorosis Patterns
When environmental conditions are stable, chlorosis patterns often point to a specific mineral deficiency. Nitrogen deficiency results in a generalized yellowing of the entire leaf, starting with older, lower leaves. Because nitrogen is mobile, the tree moves this nutrient to new growth, causing older foliage to show symptoms first.
Deficiencies of micronutrients like iron and magnesium display distinct patterns of interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins remain green. Iron deficiency appears primarily on the newest leaves because iron is immobile within the plant. Magnesium deficiency also causes interveinal chlorosis but typically begins on the older leaves near the margins. These micronutrient deficiencies are often not due to a lack of the mineral in the soil itself, but rather the tree’s inability to absorb them. This is frequently caused by highly alkaline soil with a high pH.
Pests and Diseases Causing Leaf Discoloration
Biological agents, including fungi, bacteria, and insects, can also lead to leaf yellowing, often with specific visual cues. Cherry Leaf Spot, caused by the fungus Blumeriella jaapii, initially manifests as small purple spots on the upper leaf surface. The tissue around these spots turns golden yellow before the leaf drops prematurely, leading to significant mid-season defoliation.
Insect infestations, such as aphids, cause yellowing by piercing the leaf tissue and sucking out plant sap. Aphid presence is often confirmed by the sticky, sugary residue they excrete, known as “honeydew,” which attracts sooty mold. Viral diseases, such as Prunus Necrotic Ringspot Virus, cause leaves to develop a mottled yellow and green pattern before dropping off. Bacterial canker can also cause yellowing and leaf drop, particularly following cool, wet spring weather.
Actionable Steps for Diagnosis and Treatment
A precise diagnosis requires a thorough inspection of the tree and its immediate environment before attempting treatment. Begin by assessing soil moisture by digging down a few inches near the tree to check if the soil is excessively dry or constantly waterlogged. If poor drainage is suspected, soak the area and observe how long the water takes to dissipate. For irrigation issues, adjust watering practices to provide deep, infrequent soaks during dry periods.
To confirm a nutritional issue, submit a soil sample to a professional lab for analysis of pH and nutrient levels. If the soil test confirms a high pH, which makes iron or manganese unavailable, treatment involves applying a chelated form of the nutrient or using soil acidifiers like sulfur.
When fungal diseases like Cherry Leaf Spot are identified, immediate action includes raking up and destroying all fallen leaves to reduce overwintering fungal spores. For active or recurring fungal issues, a targeted fungicide application, often beginning at bud break, may be recommended as a preventative measure. For pest infestations like aphids, a soft-bodied insecticide, such as horticultural oil or insecticidal soap, can be applied to the affected foliage.