Issue 4, May 15, 2009

Ash Leaf Drop

Leaf drop of ash trees at this time of year (mid-late May) is usually caused by a fungal disease called ash anthracnose. The problem occurs on susceptible ash species (especially green ash) in extended periods of cool, moist weather. If these conditions persist, secondary infections occur, and lesions become more visible. Hot, dry weather shuts down the disease.

The causal fungus, Discula fraxini, infects expanding leaves, resulting in water-soaked to brown areas. This early infection may kill young leaves and shoots. When infection occurs at the base of the leaf rachis, the leaf may drop from the tree. This symptom looks much like a late frost. Often callers complain that their ash trees look like they are in the midst of autumn leaf drop.

Young leaves may have pin-point-sized, purple lesions. Older leaves may appear somewhat distorted as they continue to grow around these early lesions as seen in the first image. Later, but when leaves are still succulent, anthracnose lesions appear as black blotches. As leaves mature, the lesions become tan. When leaves reach full size and stems become woody, they resist infection. We do not see ash anthracnose on mature foliage in warm weather.

How do you know anthracnose is the cause? Look at the pattern on the tree. Anthracnose generally occurs in the lower portion of the tree where moisture is high and air movement and light low. Look for lesions on the foliage. The fruiting bodies are easiest to see on the underside of the leaf, on veins or within lesions. The fruiting bodies are acervuli, appearing as masses of spores in tiny piles as seen in the image of the underside of an infected ash leaf. You may need the help of a plant pathologist to confirm these fruiting bodies.

This disease causes concern to the grower when leaves fall in great numbers, but the long term effects are usually minor. Sprays are not warranted in the landscape. They are sometimes used as preventives in seedling beds in production. The greatest advantage to a positive diagnosis is to know the problem is not life threatening to your tree. Promote the production of new foliage by watering the tree in periods of drought, fertilizing with balanced fertilizer in spring or fall, and removing dead wood in dry weather.--Nancy Pataky

Nancy Pataky

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