Garden Detective

Did neem oil treatment cause cherry leaf damage?

Powdery mildew, shown here on a peony, is a fungal disease that attacks many plants including cherry trees.
Powdery mildew, shown here on a peony, is a fungal disease that attacks many plants including cherry trees. The Associated Press

Experts tackle readers’ garden questions.

Q: I have Bing and Royal Ann cherry trees, both about 5 years old. In the spring, the trees had powdery mildew, which I treated with neem oil. In late summer, the Bing started dropping leaves. They turn yellow and then drop. They have small holes on them as if someone scraped the outside of them. The trees get watered regularly and are sprayed when dormant. What do I need to do?

Bill Conner, Folsom

Master gardener Rachel Tooker: Northern California’s sunny, temperate weather offers ideal conditions for growing Bing and Royal Ann cherry trees. Unfortunately, powdery mildew also tends to thrive in Sacramento’s Mediterranean climate with its mild winters and hot dry summers, particularly in spring when there is rain followed by warm temperatures. Treating for powdery mildew with a fungicide often can lead to leaf damage and drop.

Planting trees in a sunny area, pruning to allow good air circulation and using slow-release fertilizers can help discourage powdery mildew. However, in cases where powdery mildew already exists, many home gardeners turn to fungicides to assist with control.

Neem oil is a plant-based insecticide that also works as a fungicide and has the advantage of being less toxic for backyard use. However, it can injure leaves when applied in temperatures over 90 degrees or when plants are drought-stressed, as is often the case in the Sacramento area. Oil sprays deposited on foliage and stems can block the air openings of plants. This may result in leaf burn, darkened bark and dead twigs and branches.

Oil burn can be minimized or avoided if trees are not stressed when sprayed. Lack of moisture, extreme or sudden change in temperature, prolonged winds, disease or a heavy pest infestation can predispose plants to damage. Also, more significant damage can occur if neem oil is applied within a few weeks of a sulfur spray.

If the trees are located in a lawn area, they also could be receiving too much water. Overwatering, especially in soil that does not drain well (such as clay found in much of the Sacramento region), can damage roots and create an ideal environment for tree-root diseases. Cherries are very susceptible to root and crown rots, which can cause leaves to discolor, wilt and/or drop prematurely.

For established trees, irrigation should be every 10 to 14 days and sufficient to encourage the water to penetrate down at least 3 feet a few days after watering. This will encourage deeper root growth. In addition, trees should be watered at the drip line (the area directly under the outer circumference of the tree branches) and away from the trunk. Adding a 3-inch layer of mulch under the tree (a few inches away from the trunk) will help keep the roots happy, encourage earthworms to aerate the soil and allow less frequent watering.

The leaf problem also may be due to cherry leaf spot, a fungal disease (caused by the pathogen Blumeriella jaapii) that tends to thrive in more humid conditions. Although this disease is less common in California, the same damp conditions that helped promote the powdery mildew also may be setting the stage for this fungus to develop.

Cherry leaf spot causes spots to appear on the upper surface of leaves. The spots may range in color from light gray or brown to dark purple and an overall yellowing of the leaves may occur. As the affected tissue dries, the internal tissue may fall out, giving the leaves a tattered appearance. The undersides of leaves may be covered with spores. Leaves may dry and fall off the tree. Fruit may be spotted and soft.

To manage leaf spot, remove and destroy diseased leaves as soon as they appear. Removing all older leaves at the end of each season also may be helpful in reducing carryover of the pathogen. Some fungicides, such as flowable sulfur, are available to the home gardener, but are rarely needed.

Another possible problem might be spider mites, which feed on leaves, resulting in a stippling of light dots followed by the leaves turning yellow and dropping off. Typically, leaves, twigs and fruit will be covered with large amounts of webbing. Damage is usually worse when compounded by water stress. Spider mites reproduce rapidly in hot weather and become numerous in June through September. They thrive in hot, dusty conditions and often become a problem after application of insecticides, which also can kill the natural enemies of the mites. Regular, forceful spraying of the plants with water will often reduce mite populations sufficiently to keep them under control.

For more information on managing powdery mildew, check out the University of California, Integrated Pest Management (UCIPM) Pest Note 7494, Powdery Mildew on Fruits and Berries, available free online at http://ipm.ucanr.edu/PMG/PESTNOTES/pn7494.html. For more information on watering fruit and nut trees, visit the UCIPM website at www.ipm.ucdavis.edu/PMG/GARDEN/FRUIT/CULTURAL/fruitwatering.html. For more information on web-spinning spider mites, go to the UCIPM website at http://ipm.ucanr.edu/PMG/r105400211.html.

Rachel Tooker is a UC Cooperative Extension master gardener for Sacramento County.

Garden questions?

Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to h&g@sacbee.com. Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:

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