Cause There are several organisms that have been associated with root rots of lilac including Phytophthora, Rhizoctonia and Armillaria. These diseases are generally found where plants are in cold wet soils and/or overwatered. Young, nursery-grown plants can get root rot as well as mature bushes grown in landscapes.
Symptoms Plants infected by Phytophthora may show decline and dieback of one to many shoots and branches. Leaves may turn chlorotic before becoming necrotic and may not fall from dead plants. A reddish-brown discoloration can be found in the root crown of plants. There may be a line between discolored and healthy tissue.
Armillaria mycelial fans can be observed beneath the bark of the root crown of infected plants. Mycelial fans are thick, white layers of fungus that adhere to the inner root bark and/or the wood beneath the bark. Armillaria also makes black, shoestring-like structures called rhizomorphs, which are occasionally found within the bark and/or extending into surrounding soil.
- Do not overwater plants.
- Provide good drainage for plants in beds, fields, or containers. Place containers on gravel beds (4 inches or more deep) to allow drainage. Do not place containers on poly sheets; they can prevent containers from draining into soil and allow contaminated drainage water to spread from the base of one container to another.
- Avoid reusing pots from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
Chemical control The following have been found to be safe on this crop and may be helpful for disease management.
- Aliette WDG at 2.5 to 5 lb/100 gal water may be useful for Phytophthora management. Group P7 fungicide.12-hr reentry.
- Empress at 1 to 3 fl oz/100 gal water can be used for cuttings or seedlings. May be useful for Phytophthora management. Group 11 fungicide. 12-hr reentry.