Without a description or photos of the fungus it is difficult to determine exactly what pathogen is affecting your tree. I highly encourage you to contact your County Extension Agent to assist you in getting the correct diagnosis and appropriate treatment for what is ailing your cottonwood tree.

There are two fungal pathogens common to cottonwoods, Marssonina leaf spot and Cytospora canker.

Marssonina leaf spot, cause leaves to develop brown spots that later turn black. This induces early leaf drop, gives an unsightly appearance and stunts growth. The disease survives on fallen leaves and can re-infect the tree year after year, therefore it is important each year to remove and destroy infected leaves. This disease can be treated with fungicides as either a foliar spray or a soil drench, but can be expensive depending on the size of the tree.

Cytospora canker starts by infecting young twigs developing brownish, sunken, circular areas in the bark and my spread down to larger branches and the trunk. Spores released from the reproductive structures allow the fungus to spread throughout the infected tree as well as surrounding trees. No chemical treatments for this disease are available. Keeping your trees healthy, only pruning when necessary and removing infected branches will reduce the chances of the fungus spreading.

If the fungus at the base of the tree is attached to the trunk or roots, you could be seeing Wood Decay Fungi. Decay fungi called conks or brackets cause the wood in the center of trunks and limbs to decay and are often located around root crowns, tree wounds or branch scars.

We discourage building raised beds around trees. Tree roots need oxygen to survive which is readily available in the top 18 inches of soil, piling additional soil over a root system puts the lower roots out of range of their oxygen source. This additional stress puts trees at a disadvantage in defending against insects and disease.