People have been asking me about spots appearing on the leaves of some of their trees. Since I’m no expert I pulled an article written by Katharine Widin in the May-June issue of Northern Gardener magazine. She is a plant pathologist who owns Plant Health Associates located in Stillwater.
Katharine’s article states that anthracnose disease affects many types of deciduous trees in the United States and in our area those trees are oaks, maples and ash trees. The anthracnose fungi mainly infect leaves on the lower and inner canopy of the trees first. Rainy weather and moderate temperatures create ideal conditions for the spread of this disease.
Oak anthracnose will attack most oak species in Minnesota but is most commonly seen on bur oak, bicolor swamp oak, and white oak with the fungus attacking new growth in the spring. Brown to blackish lesions appear on the leaves mainly along the veins and leaf edges.
Affected leaves curl and may fall off the tree early in the season. In rainy years the first infected leaves may fall off and the second crop of leaves may also fall. This crop of leaves may deplete the tree’s energy each year. Some confuse oak anthracnose with Oak Wilt but they are not the same. Oak Wilt is usually first noticed at the top of the tree’s canopy in mid to late summer and mostly affects red oak and pin oak.
Several species of anthracnose fungi affect maple trees. Leaf symptoms include individual irregular shaped, light to dark brown lesions along the veins and midrib of the leaf. Lesions may cause blotching and-or wilting of the entire leaves and leaf dropping may occur in spring or early summer. Sugar maple and Norway maple are the most common maples found in Minnesota.
Ash anthracnose can cause leaves to shed in the spring when the infection starts. Early infections produce lesions the size of pinheads on smaller leaves. As this disease develops the lesions enlarge and cause the leaves to curl along the edges. Green Ash is the most susceptible to this disease among the species of ash trees.
The best way to manage this diseases to remove overwintering twigs, fallen leaves, bud scales and other fallen plant parts by raking and removing all the debris and disposing of it. Improve air circulation by pruning trees between October and March which will help deter the disease.
Watering and fertilizing during the growing season will improve the trees vigor but avoid watering with a sprinkler system near those trees that are most susceptible. Fungicides are usually unnecessary because most years the damage to the tree is minor.