Black disease is not an actual disease. It is a byproduct of infection unique to magnolias. Magnolias have very thin bark and delicate vascular systems, which swell with fluid and create what appears to be a blackened bruise in the wood when damaged. This and other forms of infection are to blame for black disease.
Magnolia scale are insects similar to aphids in appearance, but significantly larger. They prey solely on magnolia trees, using their needle-like mouthparts to pierce the thin bark and drink its sap. Magnolia scale, alone or in colonies, excrete a waxy substance called honeydew. Most pesticides are unable to penetrate this wax layer, making infestations difficult to destroy. What's worse, honeydew becomes the home of an opportunistic fungus called sooty mold, which covers the bark of the magnolia in powdery black spores.
The magnolia borer feeds primarily on the sap of magnolia trees, but lacking the capacity to simply puncture the wood, it chews grooves across the surface of the tree beneath the bark. These grooves weep sap, resulting in the characteristic darkening along the tree's trunk. In addition to this, magnolia borer excrement can provide a toehold for sooty mold infection.
Asian Long-horned Beetle
The Asian long-horned beetle, Anoplophora glabripennis, is an invasive species to North America that infests many deciduous hardwood and softwood species, magnolias included. Adults are black, 1 inch long, have white spots on the thorax and antennae ranging from 1 to 2 inches long. They bore individual depressions in the tree's bark, each 3/8 inch in diameter and lay eggs in the host tree. These eggs appear as black bundles beneath a magnolia's thin bark, and when they hatch, the larvae tunnel into the tree's heartwood, weakening if not killing it in the process. The only way to control the Asian long-horned beetle is to cut down infected trees and burn them.