If you notice children playing in fallen leaves around your river birch tree and it's not fall, your tree is likely suffering from some type of fungi. River birch trees (Betula nigra) grow rapidly and eventually become 40 to 70 feet tall with a pyramidal shape. They make excellent ornamental and shade trees for U.S. Department of Agriculture plant hardiness zones 4 though 9. While they are fairly disease-resistant, the trees are susceptible to certain fungal diseases.
1. Sooty Mold
Sooty mold is a fungal condition that results from insect pests, such as scale and aphids, that secrete honeydew. Once leaves are covered with sooty mold, a secondary, non-parasitic fungus develops that is gray-green to black. Sooty mold affects the leaves' ability to take in sunlight and prevent river birch from completing photosynthesis, which inhibits growth. Premature leaf drop can also occur from sooty mold infection. Controlling sooty mold consists of controlling insects that secrete honeydew and cutting off limbs with heavily covered leaves.
Rusts include a broad range of fungal diseases caused by related fungi. Symptoms of rust on river birch include rusty brown, red, orange or yellow pustules on leaves. Infected leaves turn brown or yellow, and often drop prematurely. In severe cases, trees can die from rusts. Control consists of cutting off and destroying affected leaves, pruning trees to provide adequate air circulation and avoid overcrowding trees.
3. Armillaria Root Rot
Armillaria root rot, also known as oak root fungus, often goes undetected until the plant dies. The fungus can infect a variety of trees, shrubs and other plants, attacking their cambial tissue, which is a layer of tissues in the stems and roots of plants. Fungi cause roots and trunks near the ground to die. Above-ground symptoms include stem die back and discolored, withering leaves. Trees may die suddenly. Control consists of digging up and disposing of dead roots, and allowing the soil to air dry before planting new trees.
Anthracnose is a fungal disease that attacks a variety of deciduous and evergreen trees and shrubs, including river birch. Anthracnose fungi require wet conditions and is prevalent during wet springs. Affected leaves develop small brown spots and can become distorted. Heavily infected leaves can drop prematurely. Control of anthracnose includes collecting and destroying fallen leaves. Trees may require pruning to increase air circulation.
- The New Sunset Western Garden Book; Kathleen Norris Brenzel, Editor
- UC IPM Online: Birch (Betula spp.)
- UC IPM Online: Sooty Mold
- The Practical Encyclopedia of Garden Pests and Diseases; Andrew Mikolajski
- UC IPM Online: Rusts
- UC IPM Online: Armilliaria Root Rot (Oak Root Fungus)