Three Ash Diseases You Should Know
By Wendy White-McDougall (MDA)
Green ash is a widely planted shade tree throughout Minnesota. Several diseases of ash can render them very unsightly, weakening or even killing them. Trees can suffer from a many diseases but a few relatively common diseases on green ash in Minnesota include ash anthracnose, Verticillium wilt and ash yellows. Insect damage, salt damage and other stresses such as drought sometimes resemble disease
symptoms, and vice versa. To avoid confusion and provide accurate treatment options, knowledge of the organism is necessary. Confirmation of the causal agent by a lab or plant pathology consultant is suggested.
Ash anthracnose is probably the most common disease of green ash. It is caused by a fungus and occurs during cool, rainy weather. It can be confused with ash plant bug and herbicide damage. Individual trees vary in their susceptibility to anthracnose to the extent that some trees may be infected every year and others only rarely or never at all. Initial symptoms on green ash include very small (grain of sand-sized) purple or brown spots, sometimes with a cream-colored center, on young leaves. These spots may enlarge and coalesce forming brown blotches. Leaves often become distorted and drop and young shoots may be killed. Defoliation and leaf browning cause homeowners a great deal of concern, but treatment is generally not warranted. Affected trees usually put out a second flush of leaves and make up for lost time later in the growing season. However, trees that have been defoliated three to five years in a row might benefit from chlorothalonil, mancozeb or thiophanate treatment at budbreak to prevent infection. The fungus can overwinter on the ground in infected leaves and twigs, so raking and disposing infected material may help reduce infection the following spring.
Another disease of ash, as well as many other plant species, is Verticillium wilt. Verticillium wilt is caused by a soil-borne fungus that enters through the roots. It has a very wide host range and can infect any time during the growing season. Verticillium wilt can be mistaken for drought stress, salt damage, girdling roots and nutrient deficiency.
Symptoms may be chronic or acute and can vary widely from tree to tree. Symptoms most often appear in July and August and sometimes include a noticeable yellowing of leaves, wilting and scorched-looking leaf margins. In green ash, the leaves may also simply drop off the tree in the absence of other obvious symptoms. Sometimes only a portion or branch of the tree will appear affected or the whole tree may die in a very short period of time. The greenish discoloration seen in the sapwood of infected maples is usually absent in green ash. The lifespan of trees suffering from chronic symptoms may be extended by keeping them as healthy as possible through ample watering and fertilization. Because this fungus exists in the soil and has a very wide host range, proper identification of the pathogen as the cause of death is critical if considering another plant for the same location. The Verticillium fungus can survive in the soil for many years in a special survival structure that can germinate and infect susceptible roots that come into contact with it. There are no effective chemical controls available.
Ash yellows is much more difficult to diagnose than ash anthracnose and Verticillium wilt. It is not caused by a fungus but rather a plant inhabiting, bacteria-like organism called a phytoplasma. Phytoplasmas cannot be grown in culture like fungi, so testing is more complicated and costly. Trees infected with ash yellows may exhibit reduced growth, dieback, small, chlorotic leaves growing in tufts at the end of branches, and witches' brooms. These symptoms resemble salt spray damage, which can cause bushy growth at branch tips, and nutrient deficiency. Leaves on ash yellows witches' brooms tend to be simple, small and chlorotic. Green ash is thought to be more resistant to ash yellows than white ash and may exhibit only a witches' broom or no other obvious symptoms (although growth may be reduced). Infected trees may look fine and live for many years with good care or become unsightly and decline. There are no effective chemical controls or treatments and the biology of this organism is still being researched. The long-term effects of ash yellows infection on a green ash dominated urban forest are not yet known.
The predominance of green ash in our urban, suburban and rural landscape makes disease identification very important. Insect damage, site stress, and pathogen damage can be easily confused. Correctly identifying the problem is important because the treatment methods for each are different. Knowledge of each condition, and/or relying on a plant pathologist or lab for diagnosis, will ensure that any treatment is based on accurate information. Many tree diseases can be managed when you are armed with knowledge about the organism. Controlling diseases like ash anthracnose, Verticillium wilt and ash yellows will help keep the green ash in our community forests green.