Hypericum Rust infected St. John’s Wort - Ask Extension
Hi. My St. John’s wort is getting brown and yellow spots and dying back. I think (from online images I’ve seen) it may be hypericum rust infection...
Knowledgebase
Hypericum Rust infected St. John’s Wort #840824
Asked July 15, 2023, 2:40 PM EDT
Hi. My St. John’s wort is getting brown and yellow spots and dying back. I think (from online images I’ve seen) it may be hypericum rust infection (from fungus?). I have large areas of planting. Should I treat with fungicide? Should I mow back affected plants and clear out debris? I’m hoping to keep the infection from spreading to the , as of now, healthy looking plants. How urgent is this? Thank you.
Anne Arundel County Maryland
Expert Response
We agree this looks like a rust infection, which St. Johnswort (Hypericum) does have a vulnerability to. (To be fair, many different plant species can contract their own rust infections as rust fungi are diverse and widespread.) The rust particular to Hypericum does not need an alternate host plant, as some other rust fungi can. This means spores produced on the Hypericum foliage will re-infect the Hypericum and not other species.
The best recourse for now is to prune back infected growth (all of the above-ground growth) and let the plant rejuvenate; rake up and discard, do not compost, the debris. Improving airflow by avoiding a build-up of fallen tree leaves or other debris among the plant's stems as well as not irrigating in a way that wets the foliage (if at all avoidable) should help to reduce the severity of any future outbreaks. Although the Pacific Northwest Plant Disease Management Handbook mentions planting resistant cultivars as one prevention technique, we are not familiar with Hypericum selections that are notably rust-resistant.
While some rust infections can be treated preventatively (nothing will cure existing infection) with fungicides labeled for rust management, Washington State University Extension does not list any chemical treatments for this particular disease, suggesting they are either not effective enough to bother with or not recommended for use around plants attractive to pollinators. The source above does include a few chemicals, but these might not be easy or cheap to source, or might require application by a certified pesticide applicator. Even if successful, treatments would have to occur every year, and multiple fungicide applications made each treatment cycle, so this would likely be not very practical or cost-effective. If the infection returns in future years and is deemed too much of an eyesore, replacing some or all of the plants with a mix of different groundcover or perennial species would be a more suitable long-term option since mixed-species plantings tend to be more resilient overall.
Miri
The best recourse for now is to prune back infected growth (all of the above-ground growth) and let the plant rejuvenate; rake up and discard, do not compost, the debris. Improving airflow by avoiding a build-up of fallen tree leaves or other debris among the plant's stems as well as not irrigating in a way that wets the foliage (if at all avoidable) should help to reduce the severity of any future outbreaks. Although the Pacific Northwest Plant Disease Management Handbook mentions planting resistant cultivars as one prevention technique, we are not familiar with Hypericum selections that are notably rust-resistant.
While some rust infections can be treated preventatively (nothing will cure existing infection) with fungicides labeled for rust management, Washington State University Extension does not list any chemical treatments for this particular disease, suggesting they are either not effective enough to bother with or not recommended for use around plants attractive to pollinators. The source above does include a few chemicals, but these might not be easy or cheap to source, or might require application by a certified pesticide applicator. Even if successful, treatments would have to occur every year, and multiple fungicide applications made each treatment cycle, so this would likely be not very practical or cost-effective. If the infection returns in future years and is deemed too much of an eyesore, replacing some or all of the plants with a mix of different groundcover or perennial species would be a more suitable long-term option since mixed-species plantings tend to be more resilient overall.
Miri
