Fungi feed on dead matter and sometimes feed on living matter as well. Trees are usually resistant to fungi and produce natural fungicides. However, some fungi are too powerful for trees to handle and some trees become weakened by a variety of conditions. In these cases, a fungus infection can occur that might need human intervention.
Twig cankers are sores found on twigs, as if a part of the twig had been sliced off the tree. These cankers are a kind of infection that will usually develop a fruiting body. Trees can also develop spots that appear soaked with water, according to the University of Rhode Island. These spots grow larger and turn brownish-green.
A part of the tree might start dying off, which often causes that part to turn brown or black. When twigs die, new shoots start to grow, according to the University of Rhode Island. But the fungi can cause these shoots to wilt and die.
Fruiting bodies sometimes form on trees when the fungus is going through reproduction. The fruiting body is what most people think of when they envision fungi such as mushrooms, according to the University of Rhode Island. The fruiting bodies come in different colors and appear as growths on the tree.
Trees with fungi can have a difficult time developing leaves and many of the leaves might fall off the tree. The leaves can change color and develop lesions. Leaves can also become twisted or wrinkled, according to the University of Rhode Island. The lack of sugar that the tree is receiving due to the fungi attack can cause some areas of the leaves to turn reddish-brown. Leaves can also develop a white and powdery mildew and tiny black dots, according to North Dakota State University. Some leaves become dull light green and then suddenly drop.
Trees near cool and moist areas likely have fungi diseases if they are suffering from any of the other conditions described previously, according to the University of Rhode Island. Hot areas often slow down or stop the growth of fungi. Very large wounds caused by pruning or heavy storms can cause fungi to infect the wound and cause heart rot, which can gradually advance and destroy the entire tree if the tree is not healthy, according to North Dakota State University.
Some fungi live on trees but do not harm the trees. Other fungi are actually beneficial to trees by eating dead tree matter, allowing new shoots to grow, and by increasing nutrient availability in the tree's roots. The sooty mold that creates black growth on the surface of leaves is harmless. Many leafspots are caused by harmless fungi.