Diagnosing and Treating Black Sooty Mold on Weeping Pussy Willow

A concerned gardener from Hickman, Kentucky reached out about a weeping pussy willow tree covered in a black, oily substance dripping onto the ground below. The sticky residue coating the leaves and soil is a classic sign of an insect infestation—most likely aphids, though scale insects can produce similar symptoms.

Understanding the Problem

When aphids feed on plant sap, they excrete a sugary substance called honeydew. This sticky coating attracts a fungal growth known as sooty mold, which turns leaves and surrounding surfaces black. While the mold itself doesn't directly damage the tree, it blocks sunlight and indicates an underlying pest issue that needs addressing.

Treatment Options

Foliar Sprays for Immediate Control

Several contact insecticides effectively control aphids when sprayed directly on foliage. Recommended options include acephate (Orthene), bifenthrin (Talstar), malathion, and pyrethrum. Since aphids can develop resistance, rotating between different chemical classes may be necessary if initial treatments prove ineffective. Orthene typically provides reliable results and makes a good first choice.

Systemic Soil Treatment for Long-Term Protection

For larger trees that are difficult to spray thoroughly, a systemic insecticide applied to the soil offers season-long control. The tree absorbs the product through its roots and distributes it to leaves and twigs where feeding insects ingest it. Bayer Advanced Tree and Shrub Insect Control (containing imidacloprid) is a widely available option for homeowner use.

Recommended Action Plan

Given the late-season timing, apply a foliar spray of Orthene now to knock down the current population. Next spring, treat the soil with the Bayer systemic product for preventative, season-long protection. Both products should be available at local nurseries or garden centers.

Controlling the aphids will stop honeydew production, allowing the sooty mold to gradually weather away naturally.