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Vascular Streak Dieback: An Emerging Problem on Woody Ornamentals in the U.S.

ID

SPES-483P

Authors as Published

Authored by 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

Current Situation and Symptoms

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 (fig. 1).

 A cultivar of flowering dogwood (three rows in the foreground) with a high incidence of stunted and dying plants.
Figure 1. A cultivar of flowering dogwood (three rows in the foreground) with a high incidence of stunted and dying plants. (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)

Early symptoms include leaf chlorosis, scorched leaf margins, and stunting and/or wilting of current year’s growth (figs. 2 A, B, and C), eventually leading to death of individual branches and progression into the main stem.

Redbud showing leaf scorch and yellowing in the upper portion of the tree.
Figure 2 A. Redbud showing leaf scorch and yellowing in the upper portion of the tree. (Photo by Nicole Kopas, Virginia Department of Agriculture and Consumer Services.)
 Redbud with leaf scorch and scattered branches wilting and dying back.
Figure 2 B. Redbud with leaf scorch and scattered branches wilting and dying back. (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)
Flowering dogwood with severely stunted new growth.
Figure 2 C. Flowering dogwood with severely stunted new growth. (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)

Wilting typically starts on the top of the plant and progresses downwards into and along the main stem (figs. 3 A and B).

Branch on flowering dogwood showing wilt that is progressing from the branch tip downwards toward the main stem.
Branch on flowering dogwood showing wilt that is progressing from the branch tip downwards toward the main stem.
Figures 3 A and 3 B. Branch on flowering dogwood showing wilt that is progressing from the branch tip downwards toward the main stem. (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)

Streaking or discoloration within the vascular, or waterconducting, tissue occurs when symptomatic branches and/or main stems are cut (fig. 4). However, vascular symptoms may be subtle or absent on dogwood or other hosts, adding a challenge for diagnosis

Vascular discoloration in redbud (A and B) and red maple (C and D).
Figure 4. Vascular discoloration in redbud (A and B) and red maple (C and D). (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)

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. Branches and main stems that are cankered with secondary fungi may have sunken, split, and/or callused bark; necrosis typically shows on one side of the branch or main stem (fig. 5).

A. Botryosphaeria canker on maple. B. Canker progressing downwards on flowering dogwood; the bark associated with the canker is darker compared to non-cankered branch tissue. C. Cracking of a cankered location on the mainstem of redbud. D. The bark associated with the cankered area on the main stem of this redbud is darker than non-cankered stem tissue; however, cankers do not always cause apparent symptoms on the bark and cutting into the vascular tissue may be necessary to reveal the discoloration.
Figure 5. A. Botryosphaeria canker on maple. B. Canker progressing downwards on flowering dogwood; the bark associated with the canker is darker compared to non-cankered branch tissue. C. Cracking of a cankered location on the mainstem of redbud. D. The bark associated with the cankered area on the main stem of this redbud is darker than non-cankered stem tissue; however, cankers do not always cause apparent symptoms on the bark and cutting into the vascular tissue may be necessary to reveal the discoloration. (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)

Vascular Streak Dieback: What We Know and Don’t Know

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. The distribution and host range of C. theobromae in the United States is not known; to date, it has been detectedfrom a variety of woody ornamentals exhibiting VSD symptoms in six states (table 1). In Virginia, redbud, maple, and dogwood appear to be most commonly affected.

Table 1. List of woody ornamental plants diagnosed with vas- cular streak dieback in six U.S. states as of March 2023.

Genus

Species

Common Name

Acer

griseum

paperbark maple

Acer

rubrum

red maple

Acer

x freemanii

Freeman’s maple

Amelanchier

canadensis

serviceberry

Calycanthus

florida

sweetshrub

Catalpa

bignonioides

Southern catalpa

Catalpa

speciosa

Northern catalpa

Cercis

canadensis

redbud

Cornus

florida

flowering dogwood

Cornus

kousa

Kousa dogwood

Crataegus

viridis

green hawthorn

Fothergilla

spp.

witch alder

Hamamelis

virginiana

witch hazel

Lindera

benzoin

spicebush

Liriodendron

tulipifera

tulip poplar

Magnolia

tripetala

umbrella magnolia

Myrica

cerifera

wax myrtle

Nyssa

sylvatica

black gum

Prunus

salicina

Chinese plum

Rhus

aromatica

fragrant sumac

Syringa

reticulata

Japanese tree lilac

The problem has been observed on field stock (fig. 6) and container stock (fig. 7), and in the landscape (fig. 8). Symptoms can be confused with those of other common nursery problems, including Verticillium wilt, Phytophthora root rot, and boring insects, as well as environmental stress such as winter injury or drought.

Vascular streak dieback in a nursery field of Freeman’s maple.
Figure 6. Vascular streak dieback in a nursery field of Freeman’s maple. (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)
Vascular streak dieback on container redbuds.
Fig. 7. Vascular streak dieback on container redbuds. (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)
VSD-symptomatic southern wax myrtles (Myrica cerifera) recently planted in a landscape with leaf scorch and branch dieback.
Fig. 8. VSD-symptomatic southern wax myrtles (Myrica cerifera) recently planted in a landscape with leaf scorch and branch dieback. (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)

Proof that C. theobromae is the causal agent of the VSD observed in U.S. nurseries has not been established, since pathogenicity tests have not yet been possible. This is because this fungus is very difficult to maintain in culture and a pure culture is necessary to perform pathogenicity testing. However, the consistent detection of this fungus on symptomatic woody ornamentals is concerning. On cacao, C. theobromae produces wind-dispersed spores that develop in leaf scars and cracked mid-veins of infected leaves during periods of rain and high humidity. Such spores provide fungal inoculum to infect young, succulent leaves. After a spore infects a leaf, the fungus travels into the branch and main stem via the vascular tissue, causing dieback. These spores are thought to be short-lived during favorable conditions and are not disseminated long distances.

Avoidance and Management of Vascular Streak Dieback

Currently, robust research data to inform best management practices of VSD in the United States are lacking. For now, we can only recommend using best cultural practices to maintain plant health. This may minimize the chance of infection, since cultural and environmental stress factors often play a role in predisposing plants to attack by pathogens and opportunistic organisms.

To minimize plant stress, provide conditions that are appropriate for each plant species:

  • Ensure an appropriate planting depth, proper soil pH, adequate drainage, plant spacing, and sufficient irrigation.
  • Avoid nontarget herbicide injury to plants.
  • Transplant containerized stock periodically to prevent roots from becoming pot-bound.

To avoid spreading VSD:

  • Use only healthy plants for bud grafting or clonal propagation.
  • Purchase clean plant stock, since VSD may be intro- duced to a nursery via infected material.
  • Disinfest grafting tools and pruners with a 10% bleach solution or a commercial sanitizing product between plants.
  • After pruning woody ornamentals, remove any pruned wood left lying beneath the plants.
  • Regularly monitor woody plants for signs of disease and boring insects.
  • Isolate any symptomatic stock that may be suspect for VSD from healthy stock.
  • Keep woody plants obtained from different vendors separated. This is particularly important with plants that have been reported as hosts for VSD.
  • If possible, place newly purchased plants in a sepa- rate holding area for 45 days and scout for symptom development before co-mingling with other stock.

For plants exhibiting VSD symptoms, there are currently no recommended pesticide or cultural treatments. It is unlikely that a vascular disease such as VSD can be pruned-out or eradicated from the plant.

To prevent healthy plants from infection, systemic fungicide soil drenches that are labeled for management of Rhizoctonia (table 2) may provide protection.

Table 2. Some professional-use fungicides registered in Virginia as a soil drench for protection against Rhizoctonia. Follow the product label directions and minimize pesticide resistance development by limiting the number of applications as specified on the product label and by rotating products from different Fungicide Resistance Action Committee (FRAC) code groups.

FRAC

Code

Brand Name

Active Ingredient

FRAC 3

Terraguard

triflumizole

FRAC 3

Avelyo

mefentrifluconazole

FRAC 7

Prostar

flutolanil

FRAC 7
+ 11

Pageant

boscalid and pyraclostrobin

FRAC 7
+11

Broadform

fluopyram and trifloxystrobin

FRAC 7
+11

Mural

azoxystrobin and benzovindiflupyr

FRAC 7
+11

Orkestra

fluxapyroxad and pyraclostrobin

FRAC 11

Empress

pyraclostrobin

FRAC 11

Heritage

azoxystrobin

FRAC 12

Medallion

fludioxonil

Soil drenches will not benefit or cure plants already infected. Plants diagnosed with VSD should be removed and incinerated to prevent possible spread to landscape plants or other nursery stock. Trees may be lopped off at the collar and incinerated, and the root ball can be culled or buried but should not be composted.

Laboratory Diagnosis of Vascular Streak Dieback

Since symptoms of VSD may be confused with other abiotic or biotic problems, a laboratory diagnosis is necessary. For VSD to be diagnosed, the putative causal agent, C. theobromae, must be detected.

Instructions for collecting a sample for diagnosis:

1. Take samples from a plant in an early stage of symptom development (fig. 9) rather than a late stage of decline. Colonization from secondary organisms may make accurate diagnosis from late-stage samples difficult or impossible.

An ideal sample for laboratory diagnosis of VSD could be taken from this southern wax myrtle (M. cerifera), because it is in the early stage of symptom development. Plants that are far-gone in decline, have cankers caused by opportunistic fungi, or are dead are not typically useful for VSD diagnosis.
Figure 9. An ideal sample for laboratory diagnosis of VSD could be taken from this southern wax myrtle (M. cerifera), because it is in the early stage of symptom development. Plants that are far-gone in decline, have cankers caused by opportunistic fungi, or are dead are not typically useful for VSD diagnosis. (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)

2. Include branches or main stems with visible vascular discoloration (figs. 4, 10, and 11), if possible. If no discoloration is found, sample living branches with wilted, stunted, or scorched leaves that are still attached to the branch. Include living tissue from different parts of the symptomatic plant, including the main stem and branches. These may be cut into 6- to 8-inch pieces, then sealed in a plastic bag so the sample does not dry out.

A red maple shows vascular discoloration.
Figure 10. A red maple shows vascular discoloration. (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)
A redbud branch with the bark removed shows vascular streaking.
Figure 11. A redbud branch with the bark removed shows vascular streaking. (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)

3. If parts of the plant are dead, include the transitionzone between healthy (white) and dead (tan tobrown) wood (fig. 12). You may need to removebark or cut through the branch or main stem to finda transition zone. If bark is removed to locate thetransition zone, do not remove all the bark from thesample. Note: Completely dead tissue is typically notuseful for diagnosis.

The transition zone between healthy and dead wood on a cankered redbud.
Figure 12. The transition zone between healthy and dead wood on a cankered redbud. (Photo by Devin Bily, Virginia Department of Agriculture and Consumer Services.)

4. You may include a root sample so the diagnosticlaboratory can check for soil problems or rootdisease caused by other pathogens. For a sufficientsample, provide a large handful of the fibrous rootsin at least one pint of potting media or soil. Placethe roots and media or soil in a sealed plastic bag,separate from the branch and main stem pieces.

5. It is important that the sample is sent to the labshortly after sampling. Do not place the sample inthe refrigerator before shipping or allow it to heat up(for example, in a vehicle). Try to ship the sampleon the day of collection and maintain it at roomtemperature until shipping. Ensure that samplesare sealed in a plastic bag or bags, and packagethe sample to avoid damage during shipment. Mail samples early in the week to arrive before Friday.Ideally, ship by two-day delivery.

References

Beckerman, J., T. Creswell, J. Bonkowski, and F. Baysal-Gurel. 2022. Vascular Streak Dieback of Redbud: What Plant Pathologists Know So Far. https://www.hriresearch.org/vascular-streak-dieback-update.

Samuels, G. J., A. Ismaiel, A. Rosmana, M. Junaid, D.Guest, D., P. Mcmahon, P. Keane, A. Purwantara, S. Lambert, M. Rodriguez-Carres, and M.A.Cubeta. 2012. “Vascular Streak Dieback of Cacaoin Southeast Asia and Melanesia: In Planta Detection of the Pathogen and a New Taxonomy. FungalBiology 116: 11-23. https://doi.org/10.1016/j.funbio.2011.07.009.


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Publication Date

May 17, 2023