AMES, Iowa — Why is the ash tree in the front yard dropping leaves? What are those brownish, orange things in the cedar tree out back? Horticulturists with Iowa State University Extension and Outreach answer questions about trees; to have additional questions answered contact Hortline at firstname.lastname@example.org or 515-294-3108.
The leaf drop is probably due to anthracnose. Anthracnose is a common fungal disease of trees in Iowa. Anthracnose may occur on ash, sycamore, maple, oak, walnut and other deciduous trees. Cool, rainy weather in spring favors anthracnose development. Symptoms of anthracnose vary with the tree species. On ash, brown or black blotches typically appear on the leaflets. Affected leaflets often become distorted (they tend to curl toward the blighted areas) and fall from trees.
Fortunately, anthracnose does not cause serious harm to healthy, well established trees. The affected trees continue to leaf out. Leaves that develop later in spring usually are not affected, as weather conditions are less favorable for anthracnose development. There is no need to apply a fungicide to affected trees.
Cedar-apple rust is likely responsible for the brownish orange structures. Cedar-apple rust is a fungal disease. It requires both an apple and cedar or juniper to complete its life cycle. On the cedar, the fungus produces reddish brown galls that are up to golf-ball size on young twigs. During wet weather, these galls swell and begin to push out bright orange gelatinous tubular structures. Wind carries fungal spores from these gelatinous structures to susceptible apple or crabapple cultivars. Infection occurs when these spores land on a susceptible apple cultivar and moist conditions exist. Small, yellow spots begin to appear on the upper leaf surface shortly after the infection. Spots gradually enlarge and become a bright yellow-orange color. These brightly colored spots make the disease easy to identify on apple leaves. Heavily infected leaves may drop prematurely. In late summer, small tube-like structures develop on the underside of the apple leaves. Spores are released from these structures and are blown by wind back to susceptible cedars or junipers, completing the disease cycle.
In most cases, cedar-apple rust does not cause serious harm to cedars (junipers), apples and crabapples. Apple cultivars, such as Haralson, Red Delicious, Redfree, Liberty and Jonamac, are resistant to cedar-apple rust as are many crabapple cultivars.
The gray-green patches are probably lichens. Lichens are unusual organisms. They consist of two unrelated organisms, an alga and a fungus. These two components exist together and behave as a single organism. The alga provides food via photosynthesis. The fungus obtains water and minerals for itself and the alga.
Lichens are common on trees because the bark provides a suitable place to gather sunlight and grow. They grow especially well on dead branches because they receive more sunlight. In addition to growing on the trunks and branches of trees, lichens can be found on exposed soil surfaces, rocks, wooden fence posts, shingles, gravestones, stone walls and other sunny surfaces. Lichens may be flat, leafy or branched and hair-like. The lichens on trees are often gray-green. Other species may be orange, yellow, slate blue or black.
Lichens are fascinating, unique organisms. They do not harm trees.