We've received a number of samples this spring of deciduous tree branches with similar problems. The symptoms include blackening of the bark (apple, magnolia) and shriveled flower buds and reduced flowering (magnolia). The black discoloration is usually a powdery substance that can be rubbed off of the branches. Most clients assume a fungal problem.
When we look under the microscope we find this:
Those definitely look like fungal spores! This is, in fact, a fungus … but it's only part of the issue at hand.
The fungus is responsible for the black discoloration of the bark. It's called sooty mold, and will show up on twigs, branches, and leaves. However, the sooty mold isn't what's causing the shriveled buds and reduced flowering. Sooty mold fungi grow on the sugar-y excrement (honeydew) of various insects, including aphids and scale insects (yes, the black stuff rubbing off on your fingers is actually poop fungus). These insects are sap-feeders, meaning they siphon away the photosynthate that the tree produces as an energy source. If the population of insects gets large enough, they can weaken the tree and cause reduced flowering, reduced fruit set, and overall reduced vigor.