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 with certainty 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 like Southern Blight, plant crowns need to get good airflow. (The crown is where stems emerge from the root system.) Avoid over-mulching, which covers up or crowds the crown and stem bases; nudge it a few inches away 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. (This includes lawn grasses, if sprinklers are run frequently.)
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. (It is usually noticeable with Southern Blight.) 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. Many of them can out-compete, directly kill or consume, or otherwise stifle the development of pathogens like these. Fungicide impacts are not limited to the pathogens, and some could even have harmful consequences for pollinators.
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.
Southern Blight, as with many similar blight and rot diseases, have an extensive range of plants they are capable of infecting. Why, then, don’t we see rampant die-offs in cultivated and wild plants for such common and destructive fungi? Interactions between organisms are complex, and as with insect diversity, there are microbe species which are beneficial, especially in healthy soil that is not too compacted, oxygen-deprived, wet or poorly drained, or low in organic matter (all conditions that favor common pathogens).
By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Miri writes the Garden Q&A for The Baltimore Sun and Washington Gardener Magazine. Read more by Miri.
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