Splotchy leaves on our redbud. Photo: CATHY CALDWELL
Like most folks, I’m particularly fond of redbuds, and I treasure several in our yard. Two are volunteers — the most welcome volunteers ever! — but one of those volunteers suddenly started looking unwell last summer. Its leaves were spotty, but not the familiar brown spots of fungi, such as Cercospora, which I’ve learned to take in stride. See Illinois Ext. Then the leaves began to turn brown way too early, and eventually, some branches appeared to be dead or dying. Research seemed to be in order; after some time on the computer, I began to worry that our redbud might have a serious disease that had only recently appeared on trees in the U.S. But there were a few other diseases that presented in a similar fashion, but none of them eased my mind at all.
Brown leaves, branches wilting and dying. Photo: CATHY CALDWELL
The most likely suspect was Botryosphaeria canker and dieback — a fungal disease to which redbuds are quite susceptible, especially if they’ve been stressed by drought. Symptoms of Botryosphaeria canker and dieback — which affects many species of trees and shrubs — are wilted or dead branches on a tree or shrub that looks healthy in other respects. My redbud does have a few branches that appear to be dead. Here’s what the experts at the Virginia Tech Plant Disease Clinic have to say about Botryosphaeria canker:
This is a common disease on redbud. Cankers are often centered at branch stubs or other wounds where the fungus gains entry. The fungus can also infect trees that have been weakened by drought, winter injury, or other environmental factors. No effective chemical control is available. Prune out affected branches. Make pruning cuts several inches below the cankered area, through healthy (white) wood. Pruning tools should be disinfested between cuts by dipping in alcohol or in a 10% solution of household bleach to prevent spread to the new wounds. Refer to the VCE factsheet on this disease for further information.
Botryosphaeria canker on redbud. Photo: Mary Ann Hansen, VA Tech Plant Disease Clinic.
How to protect your redbud from Botryosphaeria fungal infection? First of all, protect it from stress, and remember that any wound could be an entry point for the fungus. Do not prune when it’s wet; only when it’s dry. And if you’re pruning out diseased branches, follow the directions in this helpful video from Va Tech: www.youtube.VCE/Botryosphaeria Dieback – Common Plant Diseases in the Landscape and Garden. Also, if you’re planting a new redbud, be sure to provide its favored conditions.
If you think one of your trees or shrubs has Botryosphaeria — and I was beginning to suspect that’s what mine had — you’ll want to send a sample to the Plant Disease Clinic at Virginia Tech. It’s critical to provide them with a sample branch that contains both healthy and diseased tissue; i.e., a branch that contains a “junction of healthy (white) and diseased (brown to reddish-brown) tissue.” This type of sample is essential for diagnosis.
The next suspect on my list was Verticillium Wilt, another fungal disease that affects many trees and ornamental shrubs, and for which there is no cure. It can be caused by two different fungi, both of which reside in the soil, and sadly, can persist in the soil for a long time, infecting new plantings. The verticillium pathogen infects trees via roots and basically blocks the tree’s vascular system, preventing the transport of water and nutrients, leading to dieback and death. Just like Botryosphaeria, this disease is less likely to infect trees that have optimum growing conditions.
My redbud had the usual symptoms of verticillium wilt: “wilted, shriveled, scorched or browning leaves; off-color foliage; stunting; defoliation; dieback and death.” Verticillium Wilt of Shade Trees and Woody Ornamentals/VA Tech. Reading about verticillium wilt was a rather scary experience because my redbud is surrounded by much-loved perennials that could be infected. Even if the disease progression could be slowed by careful watering during summer’s dry periods, transplanting those perennials would simply spread the pathogen to a new area of the garden.
Verticillium wilt on cherry tree. Photo: H.J. Larsen, Bugwood.org
On top of the spreading-through-the-soil issue, there’s even a problem with removing an infected tree — which is recommended if most of the tree appears to be sick. The remains cannot be composted or chopped into mulch (also contagious) and must be landfilled — but not in an area where free mulch is being prepared! All of these contagion problems dictate that only resistant species be planted in areas where verticillium has been diagnosed. For a list of resistant or immune species, as well as a list of susceptible species, see Verticillium Wilt of Shade Trees and Woody Ornamentals. Verticillium wilts also affects a number of vegetable crops, though mostly in areas north of us. See Verticillium Wilt of Vegetables/Univ.Md.Ext.
Next on the list of suspects was Vascular Streak Dieback, a disease that appeared in parts of the United States in the past two years, and which had formerly been known only as a disease of cacao plants in Southeast Asia. In Virginia, it has been redbud, maple, and dogwood that have been apparent victims of vascular streak dieback (“VSD”). Scientists are scrambling to learn more about VSD, and so far have not been able to make a positive identification of the cause. Here’s a summary recently prepared by a group of Virginia Tech researchers:
In the past two years, nurseries in Virginia and some other states have observed wilt and severe dieback on redbud, maple, and dogwood stock (Beckerman et al. 2022). In some cases, 90-100% of stock was unsellable due to the extent of damage. Early symptoms include leaf chlorosis, scorched leaf margins, and stunting and/or wilting of current year’s growth, eventually leading to death of individual branches and progression into the main stem. Wilting typically starts on the top of the plant and progresses downwards into and along the main stem. Streaking or discoloration within the vascular, or water conducting, tissue occurs when symptomatic branches and/or main stems are cut. However, vascular symptoms may be subtle or absent on dogwood or other hosts, adding a challenge for diagnosis. Commonly, opportunistic fungi such as Botryosphaeria and Phomopsis colonize the weakened branches and cause cankers, adding another challenge for detection of the primary causal agent.
The fungus Ceratobasidium theobromae (synonym: Rhizoctonia theobromae) has been consistently associated with vascular tissue of nursery stock showing the symptoms described above. This fungus has previously been reported as the cause of vascular streak dieback (VSD) on cacao in Southeast Asia (Samuels et al. 2012). Therefore, plant pathologists in the United States are calling the putative disease VSD.
Scientists are advising that trees diagnosed with C. theobromae be removed and incinerated. They are also suggesting that healthy plants might be protected from infection through the use of systemic fungicide soil drenches that are labeled for management of Rhizoctonia. To learn more about VSD, you’ll want to read Vascular Streak Dieback: An Emerging Problem on Woody Ornamentals in the U.S., which also provides detailed guidance on how to take a sample for diagnosis by the Virginia Tech Plant Problem Clinic.
Vascular Streak Dieback on container redbuds. Photo: Devin Bily, VA Dept. of Agriculture & Consumer Services
After studying the three suspects and numerous photos of each disease, it became clear that professional help was needed. So, I followed the directions for taking samples of branches and soil and took them all to the Extension Office, where staffers mailed them to the Plant Disease Clinic at Virginia Tech. When an email from the Clinic landed in my inbox, I took some deep breaths and opened it. Which of the suspects was it? None of them! That’s right, dear reader. Our redbud was a victim of a problem I had not even considered: decay. The report very helpfully detailed the testing that had been done and concluded as follows:
The trunk of our ailing redbud. Photo: Cathy Caldwell
Based on the damage and appearance of the trunk in the photo submitted, I think there is very likely decay in the main trunk of this tree. Many different fungi cause wood decay. Most decay fungi enter through wounds (e.g. planting wounds, lawn mower, weed whacker, etc.) on stressed trees.
I was quite surprised by the report, but also relieved. Although our redbud will eventually succumb to the decay, the end will not be immediate. And I was relieved that nearby plants were not in danger of catching a case of verticillium wilt. Frankly, I learned a great deal from this experience. There are a lot of fungal diseases out there, but if you protect your trees and shrubs from stress, they will likely be able to avoid them. If you do spot a problem, take advantage of the diagnostic expertise available from the university serving your extension office. You’ll be glad you did!
SOURCES:
Featured Photo: Redbud believed to have vascular streak dieback by Nicole Kopas, Va. Department of Agriculture and Consumer Services
“Vascular Streak Dieback: An Emerging Problem on Woody Ornamentals in the U.S.,” VA Tech (Devin Bily, Plant Pathologist, Virginia Department of Agriculture and Consumer Services, and Elizabeth Bush, Extension Plant Pathologist, School of Plant and Environmental Sciences, Virginia Tech, 2024)
