Yellow Fungus on an Oak Tree

Sulfur Fungus

• Sulfur fungus (Laetiporus sulphureus) causes a brown rot disease that decays living heartwood. This pathogen can damage oaks as well as chestnut, locust and several fruit trees. Sulfur fungus invades the internal living wood of a healthy tree through a wound, crack or puncture in its bark, according to the University of California. The fungus decays wood inside the trunk and branches of the infected tree, causing structural problems and disrupting the circulation of moisture and minerals to the outer leaves and stems of the plant. The reproductive bodies, called conks, appear once the fungus has decayed a significant portion of its host's heartwood. The conks can be up to 1 foot wide and are often yellow or orange.

Oyster Mushroom

• Oyster mushroom (Pleurotus ostreatus) is also a heart rot that can invade its host's internal tissues through openings or punctures in the bark. Unlike sulfur fungus, an infection of oyster mushroom produces a white rot disease, which causes infected wood to turn white and lose its rigid consistency. The internal decay on a living tree can extend up to 10 feet beyond the growth of its yellow and cream-colored conks, according to the University of California.

Rust

• A rust disease of oaks and pines caused by the Cronartium strobilinum fungi can also produce yellowish colonies on the surface of oak foliage. This disease has a severe impact on pines by destroying their cones and preventing reproduction. This ailment is less severe on oaks, which simply serve as secondary hosts that allow the pathogen to complete its developmental cycle. Symptoms of a rust infection on an oak tree include yellow-orange colonies on the plant's leaves. Some leaf damage may occur as a result of infection, but it is rarely severe. The rust fungus distributes spores with the assistance of air currents, allowing it to spread to suitable hosts across significant distances, according to the Neighborhood Association of Southwestern Williamson County.

Armillaria

• Armillaria root rot, also called oak root rot, is caused by several fungal species in the Armillaria genus. Many infections are attributed to the A. mellea species. This fungus is one of the most dangerous diseases of oak trees growing in North America. This soil-borne pathogen invades the subterranean tissues of its host, decaying its root system. Symptoms of an infection include sections of discolored foliage, sudden wilting and death of new shoots and stems. The reproductive bodies of armillaria are small, yellow or tan mushrooms that spring up near the base of infected trees.