Several diseases are caused by fungi that destroy/cause decay of the interior wood of an elm tree. They are known as heart rot or sap rot fungi. The rot fungi can take years to decay portions of a tree or they can move quickly and destroy large portions of the interior of the tree in months. Heart rot destroys the conductive tissues and sapwood storage of the tree.
What Heart Rot Does
Heart rot can destroy a plant's internal structure, which includes: cellulose, hemicellulose and lignin. The decay can go undetected unless the rot produces fruiting bodies or "conks," an open cavity is seen, or if part of the bark is removed and the rotting interior is revealed. The breakdown of the plant's structure makes the rotting tree dangerous due to its instability. Decaying trees cannot support the weight of their limbs and branches. They are easily blown over in a storm.
Identification of Wood Rot Fungi
There are three groups of wood rot fungi: white rots, brown rots, and soft rots. You can identify wood rot fungi by their shape, color, and the formation of their fruiting bodies that appear on the tree. Conks or brackets can be found at wound sites on the bark, by branch scars and by the root crown.
How They Spread
Decaying fungi are spread by airborne spores. When these airborne spores reach areas of a tree that are injured (wounds, branch stubs, exposed internal wood), they enter the tree and begin their destruction. The fungi can also enter through holes made by insects, such as borers.
Heart rots or sap rots are diseases that are generally found in older trees. Management is difficult in older trees--try to refrain from removing large limbs so you do not create an entryway for the fungus. Also monitor them for stability; if they become unstable due to decay, they should be removed. With younger trees, practice correct pruning, sufficient watering and by following a fertilization program.
This fungus attacks elm trees as well as a vast number of ornamental trees. It is known as Ganoderma applanatum. Wounds provide the entryway for the fungus, causing destruction of the sapwood. Conks generally appear at the ground level, and they are usually a semicircle that can be 2 to 30 inches wide, and up to 8 inches thick. Conks are also called shelf mushrooms due to their size--they are rough, horny and brown. It is called artist's conk because you can draw or carve on the underside of the conk and the artwork will remain.
Sulfur fungus is also known as Laetiporus sulphurens. This fungus will attack living trees as well as dead trees. Conks range from 2 to 12 inches, they are bright orange/yellow on the top and red/yellow on their underside. The conks will turn white as they age. Conks only appear after the disease has been present for several years, which means there is severe damage to the interior wood.
Other Heart Rotting Fungi
Artist's conk and sulfur fungus are two of the fungi that attack the elm tree. Elm trees can also be damaged by oyster mushroom, common split gill, parchment fungus, hairy turkey tail and turkey tail.