Q: Why would my perennial lobelia be only partially wilting? I keep an eye on watering so it doesn’t get too dry.

A: This can often be the work of an infection or, less commonly, stem-boring insects. Since stem-boring insects in perennials are rarely a cause for concern, I’ll focus on the pathogens.

Sometimes it’s hard to diagnose the cause of wilting since distinctive symptoms identifying the culprit aren’t always visible; there’s a lot of symptom overlap. During summer, heat-loving fungi like southern blight can cause wilting because the pathogen kills the stem tissue close to the soil line, cutting the top growth off from its water supply and causing it to collapse. A wide variety of fungi and fungus-like “water molds” can cause above- or below-ground infections throughout the year, with some prospering in cool conditions while others need heat. Wilting when the soil isn’t dry can be a telltale sign of infection or physical stem or root damage.

To reduce vulnerability to diseases, plant crowns need to get good airflow. Avoid over-mulching, which covers the crown and stem bases; nudge it a few inches away instead so it’s not piled against the stems. High humidity and dew are sometimes enough moisture for fungi to survive and begin infections, but over-watering plants by irrigating too often can make fungal outbreaks more likely.

Mycelium, which is the “body” of a fungus, is sometimes visible as white webbing or threads covering the infected part of the plant, but it’s not always prominent. Any wilting or collapsed stems need to be removed since they will not recover. Prune them off and dispose of the debris; don’t compost it.

One or two wilted stems doesn’t necessarily doom the entire plant patch or garden bed, but keep an eye out for worsening symptoms like more rampant dieback. No fungicides can reverse this damage or eliminate spores from the soil permanently. Nor would you want to attempt chemical soil sterilization, as plenty of beneficial fungi, bacteria, and other organisms inhabit the soil.

If southern blight is responsible, you can dig out and dispose of the affected plant, plus remove a few inches of the surface soil with it. This is not because the plant’s roots are going to also be infected, but because plant regrowth could be infected again by the spores remaining on the soil surface, and you don’t want to give the fungus more fuel for expansion. If you dig out soil but don’t want to throw it out (soil is a precious resource, after all), just bury the layers with potential spore contamination to a depth of 6 inches or so, where the spores should not survive.

Q: I see what looks like sawdust or fine wood shavings on a couple of my shrub and perennial leaves. I don’t have any deck rails or fencing nearby, so I’m assuming it’s not carpenter bees. Where is it coming from? The plants are not underneath a tree that has borers.

A: This sounds like the work of stem-boring insects, specifically small stem-nesting bees using stems that are already dead. As it happens, a few of those species are carpenter bees, though a different species from the large ones we notice mainly in spring. Others belong to different bee groups, like plasterer bees. Our Maryland Grows blog post, stem-nesting bees in Maryland, showcases a few species.

Some stem-nesting bees chew their own nest chambers in the stems, while others use hollow stems already move-in ready. A variety of plant species can be used, including hydrangea, perennial hibiscus, and brambles.

If you leave stems uncut during the winter, or only cut them partially down instead of fully removing them, it provides valued habitat for these native pollinators.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.