Fungus Among Us: High rainfall worsens tree diseases

by | Nov 4, 2015

Forestry-Files-740x420Rain, rain, go away! Usually a children’s song, it’s also been a plea from many North Carolinians this fall. While this season’s excessive rain is eliminating drought statewide, it may also be the source of problems for trees, particularly diseases.

It’s no secret that fungi are moisture-loving organisms. The extended wetness from rain creates an environment that is home sweet home to many disease pathogens. Two common diseases homeowners may encounter that are especially moisture-loving affect opposite ends of the tree: anthracnose disease of leaves and root rot.

Lesions on leaves caused by anthracnose generally begin along the major veins.  Image: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org.

Lesions on leaves caused by anthracnose generally begin along the major veins. Image: Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org.

Anthracnose is a common fungal disease that causes lesions on tree leaves, twigs, and fruit. It is often noticed on landscape or ornamental trees because of its aesthetically displeasing look. During a wet season, the disease may be more pronounced and has greater ability to spread. Many different fungi cause anthracnose, but each one typically only attacks one type of host. Therefore, the disease is named for the tree on which it is found. Some of the most common ones in North Carolina are sycamore anthracnose, oak anthracnose and maple anthracnose. Generally, anthracnose diseases are a cosmetic concern and do not cause long-term damage to the tree.

Prevention is key! As in football, the best defense is a good offense. Perhaps the best recommendation to combat anthracnose is sanitation: rake and destroy tree leaves in the fall to reduce occurrence the following year. Not a fan of raking? Like any fungus, give it time … it will grow on you! Throughout the year, remove and bag/dispose or remove and destroy diseased leaves and/or fruit from the tree itself and the surrounding ground. Pruning can improve canopy air circulation, reducing moisture and making a less hospitable environment for disease pathogens. However, be sure to prune when leaves are dry, as wet leaves more easily facilitate the spread of disease pathogens. Disease can spread through contaminated equipment as well, so remember to sanitize shears and pruners with a 50 percent alcohol solution or 25 percent bleach solution before moving on to another tree (rinse with water or allow to air dry before using tools again). Similarly, wash your hands with soap and running water.

Conks are often the external signs that disease is occurring.  Ganoderma sp. root rot conk at the base of an an American beech tree.  Image: Edward L. Barnard, Florida Department of Agriculture and Consumer Services, Bugwood.org.

Conks are often the external signs that disease is occurring. Ganoderma sp. root rot conk at the base of an an American beech tree. Image: Edward L. Barnard, Florida Department of Agriculture and Consumer Services, Bugwood.org.

Root rot is another common disease that is increasing in frequency after this fall’s rain. It is more common on older trees or trees with injury on the roots or at the base of the trunk. An infected tree may exhibit dieback in the canopy, discolored foliage, and/or leaf drop. Diseased trees generally decline over many years and eventually die, but some may live years without exhibiting symptoms. The presence of conks, or fruiting bodies of the fungus, is a sign that a tree has root rot.

To prevent root rot, avoid injuring roots and tree trunks, as trees with such injuries more commonly have the disease. If a tree has succumbed to root rot, before replanting in the vicinity, consider removing the old root system and stump to reduce future infection. Finally, if possible, encourage and improve soil drainage in the area.

Above all, remember your umbrella!