Common Tree and Shrub Diseases in Michigan

Apple scab, caused by the fungus Venturia inaequalis, is an unsightly infection which affects leaves, fruit and twigs. Infections are most common on apple and crabapple trees but both pear and hawthorns are also frequently infected. Spring spores develop into brownish-green spots on the leaves or fruit. As the infection progresses, the spots on the fruit become darker and more prominent. Leaves will turn yellow and drop off prematurely in summer. Fruit will become deformed and/or drop off prior to ripening. Repeated annual infections can weaken the tree making it more susceptible to other insect or disease problems.

Anthracnose (leaf blight) is a fungus disease impacting a variety of tree species, including: Ash, Birch, Dogwood, Elm, Hickory, Linden, Maple, Oak, Sycamore and Walnut. Spores are transported to new buds and shoots in spring. This disease is enhanced by cool, wet conditions. Signs of anthracnose are tan to red-brown lesions that extend along the veins and edges of the leaf. Considerable defoliation, sometimes complete leaf loss, is expected.

Diplodia tip blight (Sphaeropsis sapinea) is a serious disease of exotic pines such as Austrian pine and Scots pine. Tip blight has been documented on 20 pine species throughout the Central and Eastern United States. Infection spreads from needles to twigs and can kill trees by clogging the vascular flow of water and nutrients. Stress from drought, compaction, mechanical damage and other abiotic factors make this disease worse.

Dothistroma needle blight is a common needle disease that can affect several species of pine trees. Dothistroma needle blight first appears as dark green, water-soaked spots on the needles. The spots become tan, yellow, or reddish-brown, and may encircle the needles to form bands. The tip of the needle beyond the band eventually dies leaving just the base of the needle.

Oak wilt is a vascular disease of oak (Quercus sp.) caused by the fungus Bretziella fagacearum. The fungus is spread below ground via root graft transfer and above ground by nitidulid picnic beetles and possibly oak bark beetles. Members of the red oak family can die rapidly (4-6 weeks) while members of the white oak family have more tolerance and may live longer. The disease is widespread throughout the eastern and central United States.

Powdery Mildew, Blumeria graminis, is a common and relatively recognizable disease affecting a wide range of host trees. This disease infects plants during times of high humidity, most often in the late spring through summer. Symptoms are most commonly found on the upper side of leaves, but can also infect stems, buds, flowers, and fruit. Proactive fungicide treatment is necessary to control this unsightly fungus. This disease occurs across the entire United States.

Iron or manganese chlorosis describes an intervenial condition in which a tree’s foliage loses its healthy green color and fades to a pale green or yellow. Over time, this condition will cause premature leaf drop, slow growth and eventually tree death. Chlorosis inhibits photosynthetic food production causing a tree to starve. Trees growing in compacted, nutrient deficient and poorly drained soils are more susceptible to chlorosis.