Yarrow Diseases

Yarrow, Achillea millefolium, typically is a pest- and disease-free perennial for the garden.  Along with the native Yarrow species, many hybrid cultivars are available to add either bright or subtle color to a cottage garden, pollinator garden, or open meadow setting. The most common diseases in Yarrow are fungal rust, powdery mildew, crown gall, and bacterial basal root rot. All of these diseases are easily controlled without the use of synthetic or organic chemical sprays when diagnosed early. 

Basal Root Rot

Yarrow prefers soil that is well draining with low fertility. Planting sites with highly fertile soil or poor drainage increase the risk of basal root rot, which can quickly lead to the decline of the plant. Basal root rot can occur in either well-draining or wet soils, making the diagnosis sometimes difficult. The disease is caused by Pythium fungus, found mainly in poor-draining and heavy soils, or by Rhizoctonia fungus, which prefers well-draining and rich soils. 

Identifying Basal Root Rot

The disease is characterized by brown or black roots that become mushy. The lower portions of stems may have dark marks or streaks, eventually causing the plant to collapse.  

Treating Basal Root Rot

Fungicides are often not effective in wet situations. Removing plants from the area is the most effective method to clear root rot pathogens. Monitor the drainage of the area closely to determine if the soil needs amending. Avoid overwatering at any point in the growing season. Digging up and removing affected roots and stems may help to save plants only lightly infected. You can also drench the roots with 3% hydrogen peroxide solution (dilute 1 part hydrogen peroxide in 2 parts water) to kill the pathogen. Make sure to replant the Yarrow in a location with better drainage or less rich soil. Avoid over-fertilizing this light feeder with chemical fertilizers or heavy layers of organic compost. 

Powdery Mildew

Powdery mildew is a fungus that is most prevalent when the conditions in the garden are hot and dry during the day and cooler during the night. The spores travel on the wind rather than being transported through water or high humidity like other fungi. The powdery white mildew grows on the upper side of the leaves, absorbing nutrients from the host plant. Heavy infections will cause plants to decline in health but do not often kill the whole plant. Powdery mildew can travel quickly between susceptible plants and in some areas become an annual occurrence. 

Identifying Powdery Mildew

Patches of dusty white mildew appear on the upper sides of the leaves. Eventually, the mildew will spread to cover the foliage. As the foliage is being fed on, it will begin to brown, dry, and curl, eventually dying. 

Treating Powdery Mildew

Because powdery mildew thrives in drier conditions, proper watering of the plants can help to stop the spread. Plant Yarrow in full sun and well-draining soil. Avoid over-fertilizing plants, which encourages dense foliage growth that may be more susceptible to attack. Fungicides can be used to protect plants that are not visibly infected. Ensure that the spray you choose is safe for Yarrow (Achillea). 

Rust

Rust, Puccinia spp, is the easiest fungus to identify. The characteristic rusty or orange-colored spots attack both the top and bottom of leaves and can spread quickly on a plant. The spores eventually cause yellowing of the leaf and can affect the flowers as well. Plants may decline in vigor and eventually die. Perennials such as Yarrow that have been affected by rust will continue to develop it annually. Fungal rusts are host specific and do not normally spread to other plant species.

Identifying Rust

Rusty, reddish or orange spots are the first sign that rust has invaded your plant. Eventually, the small dots of fungal spores will grow larger and cause the leaves to yellow. Flower buds can also be affected, resulting in distorted and smaller than normal growth. 

Treating Rust

Early diagnosis is crucial for protecting Yarrow plants from the spread of fungal rust. Remove any plant material that has signs of spores or that is turning yellow. Whole plants may need to be removed if the disease has spread between plants. Choose varieties and cultivars of Yarrow that are resistant to diseases.

Crown Gall

Crown Gall is a bacterial infection that usually invades the roots and lower stems of Yarrow after the plant has been damaged either mechanically or by another pest. Crown Gall causes large (or small) knots to grow on the roots or stems and can spread quickly when established. The galls prevent the plant from taking up nutrients and water efficiently, eventually causing a serious decline in the growth and overall health of the plant. The bacteria, Agrobacterium tumefaciens, can remain in infected soils for many years. 

Identifying Crown Gall

Brown, rough-textured knots on the roots or at soil level will appear on the stems of the plant. The size can range from unnoticeable to very large. The top growth of the plant may appear to decline with no explanation if the galls are only on the roots. Plants may have to be dug up to make an accurate diagnosis. 

Treating Crown Gall

Ensure that any new plants in the garden are completely disease-free before planting. Sterilize pruners and other tools regularly when taking cuttings or pruning back plants. There is no chemical control for crown gall, and affected plants should be removed and discarded or burned. Do not replant Yarrow in the same spot for 3-4 years. The bacteria will die off if no host plant is present. 

Yarrow Disease Chart

Sources:

"Common Yarrow." The United States Department of Agriculture, Natural Resources Conservation Service. nrcs.usda.gov

"Yarrow (Achillea)." The Connecticut Agricultural Experiment Station. portal.ct.gov

Author Robbin Small - Published 8-12-2023