The mugo pine (Pinus mugo), cultivated throughout U.S. Department of Agriculture plant hardiness zones 2 through 7, is enjoyed for its relatively dense crown, symmetrical form and slow growth rate. This tree, which grows as a large shrub or small tree, is potentially bothered by a handful of different pests or diseases, including the sooty mold fungus. Although sooty mold does not directly injure a mugo pine it is growing on, this fungus is a good indicator of an underlying pest problem and is sometimes considered unattractive enough to warrant treatment.
Sooty Mold Description
Sooty mold, a term used to describe various species of fungi, generally appears as a soot-like layer of fungal growth on needles or leaves and bark. Some species of sooty mold instead grow as a thick, spongy mass. Sooty mold can grow on honeydew anywhere that it drips, including on walkways, cars, other vegetation and furniture under the pine. Sooty mold does not directly injure the mugo pines or any other plants, but a large quantity of this fungus can have a negative impact on the plant by interfering with photosynthesis.
Sooty mold grows on honeydew, a sugary, shiny-looking substance produced by certain insects as a by-product of feeding on the mugo pine. Honeydew-secreting insects that are potential pests of pine include various species of aphids, mealybugs and soft scales. Sooty mold can persist on the mugo pine even after the responsible insects are no longer present in damaging numbers, so careful inspection of the plant is necessary before making any treatment decisions.
In order to prevent any further sooty mold growth, the honeydew-producing pests have to be addressed. If the insect activity, honeydew and sooty mold are concentrated on just a few parts of the mugo pine, pruning off and disposing of these sections can offer control. Often, natural predators and parasites control pest insects sufficiently unless they are disrupted by broad-spectrum, persistent pesticides, dusty conditions or ants. Occasionally spraying the pine with a forceful stream of water can knock certain pests off the pine and wash off honeydew. Once pests are controlled, the sooty mold will gradually weather away. To speed up sooty mold removal and address many types of pests, prepare a mixture of 2 to 5 tablespoons of dish-washing soap per gallon of water and spray it over affected areas of the mugo pine. Before spraying the entire pine, test the soap spray on a small, hidden part of the tree, and monitor it for damage for about a week before treating the entire specimen.
Not only does honeydew host sooty mold growth, it also attracts ants that harvest the honeydew and protect honeydew-producing insects from their natural enemies. In order to curb pest activity, ants must first be addressed. Cutting back branches that are touching the ground or other plants or structures and banding the trunk of the mugo pine with a sticky material such as a commercially available ant trapping goo will force the ants to travel up the trunk where they will be trapped in the material. Various forms of baits, including stakes and disposable or refillable stations sometimes acceptable for use around small children and pets, can effectively control ants when placed near nest openings or in active ant trails.