Ajuga crown rot is one of the few major disease problems on this host. The fungus, Sclerotium delphinii, thrives in warm, wet weather, so it has done well in 1998. Sites that are particularly wet or poorly drained will have this problem more than well-drained areas.
The fungus causing this crown rot enters the plant through the roots and crown. The fungus cuts off the water supply from the roots, so the owner usually complains of a sudden wilt and death of plants. The crown and roots just below the crown are totally rotted in severe cases. Overwintering structures called sclerotia form in this tissue and in the soil around the plant. Look for hard yellow-brown to buff-colored bodies that are about 1/4 inch in diameter. These sclerotia have more of a reddish brown color with age, making them more difficult to see in the soil.
The fungus can be spread by tools, water, or in soil. Homeowners cannot eradicate this fungus from the soil, so it may be necessary to reestablish the planting in a new site with clean plants. The sclerotia can remain in the soil for several years. The only chemical we can recommend for homeowner use is thiophanate methyl. This chemical will protect healthy plants but will not eradicate the fungus.
Disease management includes removing and discarding infected plants and the soil immediately surrounding the rootball. The idea is to remove the fungus, including sclerotia. It would be wise to plant only disease-free plants in an entirely new site, but if the infected site is the only choice, try drenching plants and soil with a registered thiophanate-methyl fungicide at planting. Follow label directions carefully.