Common Oak Tree Diseases

Oaks (Querus spp.) are classic ornamental trees for a reason -- they grow tall and sturdy and provide generous shade. They're also relatively low-maintenance because they're less susceptible to pests than other trees. But that doesn't mean oaks are without problems. Oak trees suffer from several common diseases which can affect everything from the leaves to the roots. While some diseases are fatal, being able to identify them may help you treat the tree before it's too late.

Several fungal diseases can affect the foliage of oak trees and these diseases are likely to spread in cool, moist conditions. Oak leaf blister causes bulges and a velvety, brown-black fungus to develop on the leaf surfaces, while anthracnose results in infected leaves developing small, irregular dots along the margins or veins and eventually dropping from the tree. To control these diseases, remove affected leaves and rake those that have fallen and discard them to prevent the further spread of the disease. Powdery mildew causes a white, powdery substance to grow on the upper surfaces of oak leaves. Small, black fruiting bodies may also develop in late summer or fall. Control powdery mildew by watering oak trees early in the morning and improving air circulation around them. Remove infected leaves, as well those that have fallen off and remove them from the area. Oak trees infected with leaf spot disease develop small, reddish brown dots with yellow halos between leaf veins. Stressed trees, such as those experiencing drought conditions, are most susceptible. Treat infected oaks by removing affected leaves and raking up those that have fallen for disposal. It also helps to prune the tree to thin out the branches to improve air circulation.

Weakened oak trees can develop hypoxylon canker, which is usually first evident when at least one branch dies back. The leaves on affected limbs typically turn yellow and dry up, and affected branches may also show signs of sunken, decayed portions of bark that eventually expose a silver or gray fungus. Pruning infected trees is the most effective way to control the disease. Cut affected limbs approximately 24 to 26 inches below visible damage to ensure you get rid of the fungus. Apply a protective wound paint after pruning to prevent additional infection. Heart rot is a fungal disease that affects the trunks of oak trees. The fungi attack the heartwood of infected trees and usually do the most damage to older trees that have experienced trauma, such as damage from wind or insects. The rot typically begins at the tree's base, but can eventually spread to the trunk and branches. Prune away affected sections to prevent the spread of the disease, though if the oak isn't structurally sound, it's best to remove it. If you're pruning trees with any type of disease, disinfect the tools before making cuts and between cuts by dipping them in a solution of 1 part bleach and 9 parts water.

Oak wilt is a systemic disease that usually kills the tree. The leaves at the top of the tree turn brown and wilt, eventually falling off. The disease can spread down the tree, with branches and twigs dying and sucker sprouts developing on the trunk. Infected trees can die in as little as seven days, though some trees take up to two years to die. Controlling oak wilt is extremely difficult, and it's best to remove a tree that has this disease. Oak decline is a fungal disease that attacks the tree’s vascular system. The first signs of infection include dying twigs and thinning of foliage at the top of the tree. The dieback continues, with larger branches becoming affected. In most cases, trees with oak decline die within five to 10 years. Remove trees with an advanced case of the disease. Trees in the early stages may be treated by pruning infected limbs and deep watering in summer. Remember to disinfect pruning tools with a bleach solution so you don't spread the disease to other trees.

Armillaria root rot is a fungal disease that typically kills major roots and portions of the trunk of an oak near the ground. Infected trees show symptoms such as dieback of branches at the crown, leaf loss and slower growth. The base of the tree usually develops light brown mushrooms in fall. Armillaria root rot is difficult to control because the fungus that causes it is indigenous to many areas. Oaks that are growing vigorously can often keep the disease contained to specific areas so it doesn't spread, so good general care is important. Inonotus root rot also affects the root of oak trees, but the disease often spreads before symptoms show, so affected trees may topple before it is detected. Branch dieback may occur and the infecting fungus may form large, irregular, brown shelves at the base of the tree. Because oak trees are usually severely damaged before you notice this disease, remove the trees soon as you notice symptoms. Gandoderma root rot also causes a fruiting structure with a shelflike appearance to grow along the base of oak trees at the soil line. Affected trees show reduced growth, branch dieback and yellowed leaves, and typically die within 10 years. Once the fruiting structures are visible, remove the tree from your yard because it make topple over unexpectedly, causing damage or injury.