vinca patches dying

Question: bed of shaded perennial vinca 15 years old has suddenly developed dying strings.  photos atch.  stems seem to blacken at the bottom, and the blackness proceeds up the stem and the leaves die.  The same root cluster may also have unaffected stems.  Photos show whole bed, whole plant, and stem sections from the root (very black).  Stem transitions to 'green and clean' higher up in infected but not dead plants.  Has been proceeding for about a month.

note: nursery shrubs (mountain mint and others) were recently planted 10 feet away, and road crews reseeded nearby grass after a curb repair.

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied June 14, 2023, 1:03 PM EDT
response: This appears to be a commonplace fungal blight called Phoma or Phomopsis. Even though such diseases are more prevalent in wet weather or in conditions of poor drainage, they can also more easily affect plants that are stressed for other reasons, such as drought. (We are currently in a drought and many landscape plants likely would benefit from occasional irrigation if you are not already doing so.)  Ironically, irrigation that wets foliage instead of only soil may exacerbate the situation because wet leaf surfaces are more vulnerable to infection, though these plants lie too low to the ground to make most drip or soaker-hose irrigation a practical alternative. Watering early enough in the day so foliage can dry by nightfall may help, but nothing is going to cure existing infections. Similarly, we do not recommend fungicide as a preventative treatment as such measures do not always work well (if they work at all) and because pesticides can cause harm to other organisms, even if only intended for certain issues (a fungus in this case). Plus, Vinca is an invasive species in our region and we don't encourage gardeners to keep it, if you're willing to replace it with a different mix of groundcovers. A mix of species is also less likely to succumb to the same issues because they will have different vulnerabilities and tolerances to stress.  For now, the best way to remedy the situation is to severely cut back all stems (withered or otherwise) close to the ground (a weed-whacker is probably the simplest tool for this), rake or leaf-blow out any debris (like rotted-off stems, shed Vinca leaves, and other fallen tree leaves), and let the Vinca regrow. This serves to remove most of the spore production while also improving air circulation in an overgrown patch to reduce the incidence of recurring disease. If the Vinca root systems are otherwise healthy, regrowth should not unduly stress the plants, unless they are simultaneously drought-stressed. Don't fertilize as the roots won't need it and the lush growth that can result from nutrient supplementation may also be more vulnerable to infection than growth produced at a plant's normal pace.  It's quite possible that a low-level presence of these fungi were in the Vinca bed for some time prior to this season but not noticeable, and only now flared-up due to accumulated stresses, like from drought.  Miri

responder: The Question Asker Replied June 15, 2023, 9:19 AM EDT
response: Also, disposal of the cut back material?

responder: The Question Asker Replied June 15, 2023, 9:19 AM EDT
response: Thank you for the clear explanation.  After the cut back should I do a single deep watering?

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied June 15, 2023, 10:50 AM EDT
response: You're welcome. Yes, a thorough watering after a cut-back is recommended to help the plants recover. (It would probably also be appreciated by, and partially used-up by, any tree or shrub roots growing into that area as well.) You can dispose of the removed material in a home compost pile if you hot-compost (cold or passive composting doesn't heat-up enough to kill pathogen spores), add it to regular household landfill-bound trash, or, better yet, send it out with yard waste pickup instead as municipal facilities almost undoubtedly use hot-composting to kill pathogens and weed seeds in the material before distributing it as finished compost.  Miri

responder: The Question Asker Replied June 18, 2023, 3:51 PM EDT
response: I have cut back the vinca with a weedwacker and deep watered.  Could the same problem be appearing in mountain mint (photos).  Black starting at the root cluster and appearing up the stem.  Has just appeared in multiple plants over the last few days.  The mountain mint was planted about a month ago and seemed to be doing well - standing straight and starting new growth.Hoping for the best in my first 'native plants' attempt.  Alan  On Thursday, June 15, 2023 at 10:50:49 AM EDT, Ask Extension <<personal data hidden>> wrote:

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied June 19, 2023, 5:28 PM EDT
response: Hello Alan,  We don't think the Mountain-mint has a Phomopsis infection, and the darkened streak on the stem close-up pictured could have an array of causes. The base of the plant in the third photo looks normal; is the plant showing any signs of wilting or stem dieback? If not, darkening on maturing older stem tissues can be normal for many different perennials.  Continue to monitor it for watering needs as the drought continues, but otherwise no action would need to be taken if the plant appears to be growing normally.  Miri