Verticillium wilt was discussed in issue no. 8 of this newsletter, so I won’t go into all the details of diagnosing the disease. The Plant Clinic has received many ash samples with dieback symptoms, marginal necrosis of leaves, and general lack of vitality. Several of these have been confirmed to contain the Verticillium fungus, the cause of Verticillium wilt.
This disease causes a staining of the vascular system in most species, but it rarely causes a discoloration of the wood of ash. Because you do not have that symptom to guide you in diagnosis, look for poor stem growth (compare to healthy ash trees in your area), scorching of leaf margins (though not always present), and a thin canopy. If you would like to have the disease confirmed, you will need to send branch and leaf tissue to a diagnostic lab. The ideal sample is a terminal foot of stem growth with leaves still attached and those leaves exhibiting scorching. The lab personnel will remove the leaves on their own because the leaves stay fresh that way. Cultures will be prepared from cross sections of the base of the leaf petioles. Dry samples are not of any use in the lab. Because the Plant Clinic is now closed, follow the management recommendations listed below, and sample next spring if symptoms recur.
There will not be any miracle cure once Verticillium wilt is confirmed, but this knowledge will help somewhat in management. The worse scenario is that the tree will die; and that is possible. Because Verticillium is soilborne, you will have to replace the tree with a nonhost species; and your list will be limited. On a more cheerful note, ash trees grow quickly and may be able to “wall off” the fungus in the wood and continue to grow and develop for years. Water the tree in periods of drought lasting 2 weeks. This fall, apply a general tree fertilizer and remove any dead wood. More information about Verticillium wilt is in Report on Plant Diseases No. 1010.