A small patch of Physostegia virginiana in my front, sunny bed is suffering. First leaves shrivel and turn brown despite watering. When I pull, it comes up easily and the crown and roots look like they have a white fungus. What is going on and what can I do about it? I have another patch in a different bed that is unaffected. Catherine
Anne Arundel County Maryland
The symptoms you picture and describe seem to match Southern Blight, a fungus of hot summer weather and which can infect a wide array of perennials. Check out the images and information on the page below and see if you find fungal growth or spore sclerotia on the soil surface under the Physostegia. If so, the plant will need removal and discarding, as will a few inches of the surface soil in that spot. There is no fungicide to use, but you can replant there once the fungal growth has been eliminated.
If Southern Blight does not seem to be a match, then another root- or crown-rotting fungus is at work, probably due to excessive moisture or poor drainage. Although the wild species can be found in areas such as stream banks, cultivars may be less wet-tolerant, especially if in compacted soils.
I don’t see any schlerota. This morning I noticed another plant affected and pulled it. I split open the stem and found some frass and a small (2-3 mm) white larva. The stem is completely hollowed out. Attached is a photo: stem on right of affected plant, on left healthy one from another area of the garden.
Your photo still suggests Southern blight, because the plant has very little root left since the blight has rotted off the roots. You also mentioned a white fungus on the base, which is a Southern blight symptom.
Unless you can find a borer in every stem, we think that Southern blight is the primary culprit here. Read through the above link carefully. Be sure not to spread the infected soil to other parts of your gardens.