Because biorational pesticides have a relatively short residual effect, it is necessary to monitor the aphid population regularly and retreat when necessary. Orthene TM is translocated to leaf surfaces and green stems. Once Orthene TM has dried, it has minimal impact on beneficial insects. Diseases Most diseases of yarrows are prevented or controlled using cultural methods, such as removal of infected plant parts, improved aeration, crop rotation, and proper plant nutrition and spacing. Rust is reported on the West Coast and in the Mountain States, but not on the East Coast.
Powdery mildew is identified by white powdery growth on upper and lower leaf surfaces.
This disease does not kill the plant, but it may reduce flower quality. If powdery mildew appears, remove the infected plant parts. Horticultural oil sprays and insecticidal soap will suppress powdery mildew about as well as standard systemic fungicides. Rhizoctonia stem rot can kill yarrow plants during hot humid weather. Hybrids with A. taygetea in their parentage, such as A. x 'Moonshine' and the Galaxy hybrids, are more susceptible to stem rot and other foliar diseases during hot, humid growing conditions.