Ash Diseases- Ash anthracnose
|Don't confuse oak anthracnose with oak wilt
Our frequent rains this spring and summer caused many susceptible white oaks (lobed leaf edges) to become affected by a leaf, twig, and bud disease called oak anthracnose. Rain and wind spread anthrcanose spores from infected twigs, buds and dead leaves to green leaves and uninfected buds and twigs. Young leaves turned brown and shriveled or developed scattered necrotic blotches. Mature leaves developed small necrotic spots or blotches and veins of many leaves browned or turned black. Infected buds and twigs also died. The infection spread and caused heavy leaf and twig fall by August. There was no wilting of all the leaves on all larger branches, as occurs in oak wilt. Oak wilt is limited in its distribution to southeastern counties and twelve counties in the Metropolitan area. Oak wilt is also more common on red oaks (pointed leaf edges).
To help your oak trees recover from anthracnose, rake and destroy fallen leaves this fall and next spring, and fertilize next spring. You may choose to apply protective fungicides next spring, especially if rainfall and cool weather becomes prominent and your oak trees were heavily infected this year.
If you do suspect that your oak tree has oak wilt, next summer you can send several six inch long and one inch thick sections from a freshly wilted branch to the Lab Services Division, Minnesota Department of Agriculture, 90 W. Plate Blvd., St. Paul, MN 55l07-2094 for confirmation. There is no charge for this service and you can telephone the Lab for more information at 651-296-4749.
Ash anthracnose, a leaf blight, was especially common on black ash growing in both forest stands and urban sites this fall in Region 1. It's likely that most ash trees lost their leaves to blight before they normally would have fallen. I have never observed as much anthracnose in forest stands as I have this fall. After receiving anywhere from 16-30+ inches of rain this summer around Region 1, it's not hard to see why. It makes me what next spring will bring.
Anthracnose damage is characterized by irregular areas of dead leaf tissue which includes the veins. Occasionally, the fungus kills tissue down the petiole into the younger twigs. Often lower branches and twigs can be killed during severe infections. Anthracnose fungi overwinters on fallen affected leaves from which they reinfect new leaves the following year. Anthracnose occurs both in the spring and fall of the year and is usually not a serious problem but causes concern for shade tree owners. Trees are seldom killed but may be weakened or predisposed to damage by other agents. Infected leaves usually fall prematurely.
Rake up the fallen leaves and twigs and destroy by incineration if possible. Some added fertilizer with nitrogen may be helpful next spring when trees once again leaf out.