Black mold or fungal spore masses developing on the trunk of a pine tree can indicate a variety of disease problems. Typically, several specific fungi species can cause these symptoms on pine trees. Unfortunately, many of these fungal diseases are difficult or impossible to cure, meaning that you may need to cut down and remove the infected pine tree before the disease spreads to your other trees, shrubs and landscape plants.
The most common types of fungal diseases that can cause symptoms of black mold on the pine tree's trunk include Annosus root and butt rot, fusiform rust, pine-oak gall rust, blister rust, pitch canker and Armillaria root rot. Annosus root and butt rot is caused by the fungus Heterobasidion annosum, while Armillaria root rot or "oak root fungus" is caused by Armillaria mellea. Pine pitch canker is caused by different Fusarium species fungi, while pine-oak gall and fusiform rusts are caused by the fungus Cronartium quercuum. Blister rust in pine trees is related to the fungus Cronartium ribicola.
You can diagnose what fungal disease is causing the black mold on the trunk of your pine tree by the other symptoms that the tree is showing. Annosus root and butt rot causes stringy fungal bodies around the roots and on the trunk near the soil line, along with thinning foliage. Fusiform rust causes elongated galls or swellings on the pine tree's trunk and branches, typically with orange powdery spores emerging on the gall surfaces during spring. Pine-oak gall rust produces similar symptoms as fusiform rust, only the galls are rounded instead of elongated. Blister rust causes swollen areas on the pine tree's branches and trunk, developing cankers that become slightly sunken and ooze pitch. If your pine tree has blister rust, you might also see whitish blisters on the woody parts of the tree with yellowish-orange spore masses during early summer. The symptoms of pitch canker include slightly sunken cankers on the pine tree's trunk that ooze heavy pitch flow and branch cankers as well. Armillaria root rot causes stunted, discolored and prematurely-dropping needles, branch dieback in the lower canopy, mushroom clusters growing on the trunk, and blackish rootlike fungal structures on the lower trunk or upper roots.
Annosus root and butt rot can eventually kill a pine tree, affecting all pine species, but especially white, loblolly and slash pines. Fusiform rust affects loblolly and slash pines, and pine-oak gall rust affects shortleaf and Virginia pines, often killing young trees with galls on their trunks or stunting the growth of mature pines. Blister rust affects only white pines, sometimes killing the infected branches or the whole tree when the trunk is affected. Pitch, longleaf, shortleaf, slash and Virginia pine trees are all susceptible to pitch canker, killing the branches or the entire tree if cankers appear on the trunk. Armillaria root rot affects most species of pine trees, as well as various other conifers and broadleaf trees.
Treat fusiform rust and pine-oak gall rust by pruning off any branches with galls to help prevent the spread of the fungal disease to the trunk. You'll typically need to cut down and remove pine trees with fusiform or pine-oak rust galls on their trunks or on their branches within 8 to 12 inches of the trunk. Also prune away any pine tree branches that are infected with pitch canker to help control the fungal disease's spread to the trunk.
If your pine tree has Annosus root and butt rot, you'll likely need to remove the infected tree and treat the cut stump with dry granular borax. Avoid planting pine trees within 20 feet of diseased tree and plant hardwoods instead. Blister rust has no effective cure, so you may need to cut down the infected pine trees and plant disease-free species other than white pines. Also, avoid planting white pines near currant shrubs of the Ribes species, which can promote blister rust disease. No proven treatment works for Armillaria root rot either, so you may need to cut down the pine trees and replant the area with tree species that are resistant to this fungal disease. Pitch canker fungi are believed to spread via different insects, such as engraver, twig, cone and deathwatch beetles, as well as spittlebugs.