Achillea sp. (Yarrow) is often seriously inflicted with the disease Botrytis. Botrytis (gray mold) appears as brown dead areas and under proper moisture may have a gray fuzzy appearance. It attacks buds, flowers, leaves, and stems. It may eventually kill the perennial. In addition, Botrytis attacks a wide range of plants. Fungicides and sanitation are helpful.
Powdery mildew is another fungal disease that is often described as a white powder on the leaves. Several cool humid nights and warm dry days in a row help promote the outbreak of this disease. Powdery mildew tends to be worse on plants growing in shade. There are many fungi that can cause powdery mildew. Fungicides and improving sunlight and air circulation can help reduce infection.
Spittlebug will attack yarrow. The insect produces a protective foam that looks like "spit." The foam helps protect the insect from ultra violet light and predators and insecticides. Use a strong spray of water to wash the foam the insect. The resulting exposure to UV light will kill the insect and expose it to predators. A stronger spray of water may also remove the insect from the plant in addition to the "spit."
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Bruce Paulsrud, Extension Specialist, Pesticide Applicator Training and Plant Pathology, University of Illinois at Urbana-Champaign.