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.