Should leaf spot on hydrangeas cause worry?
Q: My hydrangeas seem to develop spots on their leaves each summer, but they still bloom fine. Do I need to be worried?
A: Generally not, especially if the plant is otherwise growing normally. Fortunately, few leaf spot diseases impact the plant’s overall growth because areas of unaffected leaf tissue can still photosynthesize, and since damage tends to be lighter earlier in the season, plants have had most of the spring and summer to sufficiently feed themselves.
Collectively called
“leaf spots,” a wide range of pathogens can cause prominent leaf damage by late summer. Sometimes, plant reactions are more visible than the dead tissue itself — the reddish blush that often encircles such infections is akin to a plant’s immune response. Most plants are vulnerable, including hydrangeas, roses, dogwoods, photinia, rhododendrons, black-eyed susans, peonies, and tomatoes.
Many of these diseases seem to appear all at once, even if they’re unrelated pathogens, because they thrive in similar weather conditions — think warm and wet. Differences in weather patterns is one reason outbreaks can be severe one year and all-but-gone another. One approach to control is to avoid wetting the leaves when you water, or irrigate in the morning so foliage has time to dry by nightfall. You can also pick off the worst-looking leaves if the plant has enough left to sustain itself. At the end of the growing season, collect and dispose of infected leaf debris, unless you can hot-compost well enough to sterilize them for reuse.
Crowded plants and those wedged up against walls have limited airflow which can promote outbreaks, so periodic thinning can be beneficial. Fungicide use is only preventative and will not cure existing infection. Consider its use only as a last resort, since unintended exposure to insects, beneficial microbes, and other organisms may be detrimental; always follow label directions. Have the infection identified first (which isn’t always possible visually, but some symptoms are more distinctive than others), since you may learn that intervention is not necessary. Search “leaf spot” on the HGIC website for more information.
Q: I’ve had problems with individual stems on my Rhododendron wilting and dying, or an entire plant will die suddenly. What’s going on?
A: These somewhat-fickle shrubs have two strong suspects for this kind of problem: canker or root rot. Each is a disease and neither can be directly cured after it appears, but understanding what causes them can be useful for preventing recurrence. For now, prune out wilted stems about six inches below any stem discoloration, and keep an eye on the plant’s watering needs, being sure not to keep it saturated. A completely collapsed plant will have to be replaced.
Drought and heat stress can make plants more vulnerable to infection. Botryosphaeria canker fungi can infect stem tissues and disrupt water
flow to the branch tips, causing leaves to curl and wilt, and then the entire stem past that lesion will die. It’s very
common on Rhododendron, especially if not sited
in their preferred conditions of moist, well-drained woodland soil. Make sure they get checked for water regularly when we’re lacking rain, especially if the plant receives a lot of direct sun.
“Root rot” is a collective term for several pathogens, including fungus-like “water molds,” that infect roots stressed by over-watering or poor drainage. Essentially, soil that is too saturated does not retain enough oxygen for the roots to “breathe,” and this stress makes them vulnerable to disease. Once root loss begins, it is difficult to halt, and trying to improve drainage in problematic soils can be difficult. These organisms are ubiquitous and trying to eliminate them from soil is impractical. Instead, use plants well-adapted to damp conditions so they won’t be stressed, or monitor soil moisture carefully before automatically irrigating. Search “rhododendron problems” on the HGIC website for more information.
University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension. umd.edu/hgic. Click “Ask Maryland’s Gardening Experts” to send questions and photos.