QUESTION: I had a big, fragrant patch of sweet woodruff in a shady raised bed that suddenly began turning gray and dying off in the center. Within a few days almost all of it had turned gray or black, and now there are just a few sprigs left around the edges. What happened?
Rapid die-off is often an indication of some kind of blight, and a little research into this symptom in sweet woodruff (Galium odoratum) turned up the disease called Rhizoctonia web blight. It’s caused by the fungus Rhizoctonia solani, which is the source of an array of cankers, rots and diseases. Web blight develops in hot, humid weather.
Look closely at the dead and dying plants, and you may be able to see fine webbing that sticks to the leaves and stems and across the surface of the soil. It’s common on sweet woodruff, and also attacks Dianthus, Coreopsis, ferns, hibiscus, goldenrod and yarrow.
I found this information at the web site of the University of Maryland Extension, where they explain that the blight usually doesn’t kill the roots of the plants, but it’s best to remove dead plants and debris right away.
To reduce the chance of development of the disease, thin the plants out a bit to improve air circulation. Remove mulches and other debris from the area. Cut plant debris at ground level and remove it in the fall.