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

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).

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.

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

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

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).

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.

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.

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 introduced 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 separate 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.

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.

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.
   

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

4. You may include a root sample so the diagnostic laboratory can check for soil problems or root diseases caused by other pathogens. For a sufficient sample, provide a large handful of the fibrous roots in at least one pint of potting media or soil. Place the 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 lab shortly after sampling. Do not place the sample in the refrigerator before shipping or allow it to heat up(for example, in a vehicle). Try to ship the sample on the day of collection and maintain it at room temperature until shipping. Ensure that samples are sealed in a plastic bag or bags, and package the sample to avoid damage during shipment. Mail samples early in the week to arrive before Friday.Ideally, ship by two-day delivery.

References

Diagnosing This and Other Plant Diseases

The Virginia Tech Plant Disease Clinic can test for this and other plant diseases. Refer to the Plant Disease Clinic website (https://bit.ly/VTplantclinic) for the current diagnostic form, fees, and instructions on collecting an appropriate diagnostic sample and submitting samples to the Plant Disease Clinic.

https://bit.ly/VTplantclinic