Disease in the Spotlight

The Knowns, Unknowns, and Updates about Vascular Streak Dieback (VSD) of Redbud

By Dr. Fulya Baysal-Gurel 1, Dr. Prabha Liyanapathiranage 2 , Dr. Janna Beckerman 3, Dr. Tom Creswell 4 , and Dr. John Bonkowski 5

1 Interim Associate Dean for Research and Research Associate Professor, Otis L. Floyd Nursery Center, Tennessee State University

2 Post-doctoral Researcher, Otis L. Floyd Nursery Center, Tennessee State University

3 Professor of Plant Pathology, Department of Botany and Plant Pathology, Purdue University

4 Director, the Purdue University Plant and Diagnostic Laboratory

5 Plant Disease Diagnostician, the Purdue University Plant and Diagnostic Laboratory

What is VSD? How long has this problem been happening?

Vascular streak dieback (VSD) describes plants that exhibit a constellation of symptoms including stunted growth, declining trees with water sprouts/epicormic shoots below the dead branches, chlorosis, necrosis, scorching on leaves, tip dieback that may continue into the main stem or stems of the tree and ultimately cause tree death, poor root development, and brown- to- gray vascular streaking of infected branches (Figures 1 – 4). In recent years, multiple laboratories throughout the country have received numerous plant samples with symptoms similar to VSD on multiple hosts. This issue has been detected in seedlings, grafted plants, and older nursery stock produced in container and field nursery production settings, along with established landscape plants. It is possible that this issue has been in the United States for some time but has been overlooked and/or misdiagnosed.

FIGURE 1. VSD associated vascular streaking on redbud.

Photo: Tennessee State University.

FIGURE 2. Leaf scorch associated with VSD symptomatic redbud plants.

Photo: Tennessee State University.

FIGURE 3. Dieback symptom observed in VSD-symptomatic redbud plants.

Photo: Tennessee State University.

FIGURE 4. Ct growing from the incubated VSD symptomatic redbud plants.

What’s causing (or not causing) VSD?

Positively identifying the causal agent is the first step in diagnosing what is causing VSD in woody ornamentals. Ceratobasidium theobromae (Ct) [synonym, Rhizoctonia theobromae], identified by both molecular tests and isolations, has been consistently found in association with redbud exhibiting vascular discoloration and other symptoms of VSD, in Virginia, North Carolina and Tennessee. However, it has not yet been determined if this fungus is causing the observed symptoms or if it is simply present within the affected plants as wood-inhabiting endophytes or mycorrhizal associates.

To determine if Ct is causing dieback, we must first re-create the disease in the laboratory after inoculating plants with Ct, something that has so far been unsuccessful. The fastidious nature of this fungus (hard to isolate, grow and maintain in culture) presents challenges to obtaining sufficient material for DNA analysis and for inoculation work. Future work needs to be conducted to identify if VSD is caused by Ct alone or if is it caused by multiple organisms, including cankercausing pathogens that may make host plants more susceptible to Ct.

Many redbuds with VSD diagnoses also are infected with common canker pathogens, like Botryosphaeria dothidea, Didymella spp., and Diaporthe spp. Finally, multiple diagnostic tests were used to check for other diseases that cause similar symptoms, including Verticillium wilt, bacterial leaf scorch, and other vascular diseases - all of which were ruled out as causing the VSD symptoms.

What are the other host plants affected by VSD?

Recently, additional VSD-symptomatic tree and shrub species have been found (Table 1). Detections have occurred primarily in nurseries in Virginia, North Carolina, and Tennessee, but individual detections have occurred in nurseries in Indiana, Florida, and Oklahoma. VSD has also been found in newly planted landscape plants, a botanic garden, and in a natural setting in North Carolina.

TABLE 1. Trees and shrubs with VSD diagnoses.

Are there any redbud cultivars that are tolerant to VSD?

According to research conducted by the Tennessee State University (TSU) team in 2022, redbud cultivars with yellow-colored foliage and papery leaf texture were more susceptible to VSD than other tested cultivars. The more VSD-susceptible cultivars began to present leaf-scorching symptoms in late May. VSD-tolerant cultivars that possess dark green and purple foliage and thick leathery leaves exhibited VSD-associated leaf-scorching symptoms in late August. None of the tested cultivars in this trial exhibited 100% resistance to VSD-associated symptoms during the growing season. The TSU team will be continuing with the cultivar screening this year as well, including more redbud species, cultivars, and hybrids received from different regions to identify VSD-resistance. Susceptibility screening information will be beneficial in initiating breeding programs to obtain hybrids with promising horticultural characteristics and increased resistance to VSD.

What is the impact of VSD on the redbud nursery industry?

Since the first detection, grower concern for VSD has increased, leading to rejection of plant shipments, cancellation of redbud orders, and destruction of symptomatic plant material prior to sale by nurseries. As Tennessee is the #1 redbud producer in the United States, the economic impact has been severe, with redbud producers in other states in the southern region, including North Carolina and Virginia, also experiencing economical losses. To identify the extent of economic losses that occurred due to VSD in the southern U.S., Tennessee State University is currently conducting a multi-state survey and the findings of this survey will be crucial in understanding the dimensions of the issue. If you are a redbud producer in the southern U.S., please contact Dr. Fulya Baysal-Gurel (fbaysalg@tnstate.edu) or follow the QR code below to participate in this survey.

If growers and landscape managers have only incomplete information, what can be done to prevent VSD or limit the spread and severity of VSD?

Scouting and early diagnosis

Regular scouting of susceptible plants (Table 1) for VSD-related symptoms is highly recommended. Remove symptomatic plants and remove all associated debris associated with symptomatic plants from crop production areas. Have plant material tested by reaching out to a plant and pest diagnostic laboratory regarding their submission forms and shipping recommendations.

Recently, Dr. Baysal-Gurel’s team at TSU was able to develop a novel molecular tool for the accurate detection of Ct from potentially infected host plant material. These real-time polymerase chain reaction primers are currently being tested further by several other institutions. With demonstrated success, these primers may soon be released for use as a diagnostic tool enabling efficient and accurate diagnosis of Ctinfected plant materials.

Use of healthy source plant material for propagation

The use of healthy rootstock and asymptomatic plant chips or buds for propagation is recommended to prevent the spread of the disease. If a rootstock is already infected, it is unlikely that the subsequent grafted plant will be marketable. Infected plant rootstocks will also pose a threat to healthy plants located nearby. Frequently sanitizing tools, blades, and pruners during propagation and pruning practices can also reduce the risk of plant-to-plant disease transmission.

Avoid unnecessary pruning

Pruning provides injuries that may serve as avenues of infection for the Ct fungus. Until we properly understand the epidemiology of VSD, we advise growers to:

1. Minimize pruning to reduce cut surfaces that are receptive to infection.

2. Sanitize pruners with disinfectants.

3. Remove pruned material and debris from the fields and destroy.

4. If pruning is required, the growers can apply protectant fungicide within 24 h of pruning to protect pruning wounds from pathogens.

Practice appropriate crop rotation

If you experienced losses due to VSD in a particular area of your nursery, it is advisable to avoid replanting redbuds or other known susceptible host plants (Table 1) into that area for the next few growing seasons.

Apply proper amounts of fertilizer

Excess nitrogen fertilization has been implicated in increasing plant susceptibility to several plant pathogens. Nitrogen fertility should be on the lean side, 50–100 ppm nitrate when plants are actively growing. The TSU team observed that based on foliar nutrient analyses, some of the yellow foliage cultivars that were exhibiting higher susceptibility to VSD also had higher foliar levels of nitrogen, sulfur, and phosphorus, and low levels of micronutrients such as boron, iron, and aluminum. Elemental accumulation conditions in foliage may be a trait associated with the cultivars, yet the results we observed could also be associated with the high degree of susceptibility observed in these cultivars. Future experiments will help determine what is occurring during seasonal growth and how fertilization may be affecting disease development. In the meantime, however, we advise that growers make wise use of fertilizers especially if they are growing susceptible cultivars. It is also helpful to understand that redbud is tolerant of a wide pH range, yet trees grow best in soils in which the pH is above 7.5.

Use appropriate depth for planting

Plants should be planted at the appropriate site, correct depth and be properly spaced in production rows. Trees that are planted too deeply will have less uniform and effective root development, and these trees will have increased exposure to crown and root pathogens (Figure 5).

FIGURE 5. Deep planted redbud.

Photo: Gary Clendenon, Tennessee Department of Agriculture.

Apply proper irrigation

Redbuds are intolerant of flooding and/or poorly aerated soils. Flooding results in oxygen deficiency and increases the risk of root rot and attacks by ambrosia beetles. Drought stress should also be avoided, as it is associated with increased canker disease severity.

Avoid accidental herbicide injury

Redbuds are highly susceptible to phenoxy/2,4-D herbicides and other post-emergent herbicides and care should be taken whenever these are being used around young trees to avoid injury. Avoid the use of aminopyralid, triclopyr, and clopyralid herbicides that are known to damage redbud in nursery plantings.

Practice appropriate crop management with chemical fungicides

At this time, there are no chemical treatment recommendations as a specific pathogen has not been identified. The TSU team was able to conduct fungicide efficacy trials during 2022 using redbud plants that were naturally exhibiting VSD-related symptoms. Results indicate that foliar application of Postiva (FRAC 3 + 7) at 20 fl oz/100 gal and Mural (FRAC 7 + 11) at 7 oz/100 gal in 14-day application interval were the most effective treatments in reducing leaf scorching associated with VSD on eastern redbud seedling, in 3-year-old plants, and in several 2-year-old budded cultivars that were growing in field soil and container settings. These treatments were also effective in reducing the population levels of canker-causing pathogens such as Botryosphaeria spp. and Didymella spp., yet did not protect plants from new infections nor do treatments cure already-infected plants. Until more information regarding the causal agents(s) becomes available, we can advise that growers focus on management practices that improve the plant’s health while avoiding the practices that create plant stress.

Proper management of soilborne pathogens such as Phytophthora spp., Pythium spp., Rhizoctonia spp., and Fusarium spp. might be beneficial in reducing the damage caused by vascular streak dieback. Management for these pathogens would provide protection against other opportunistic root rots while improving plant health. If using preventative fungicide drenches to protect roots against Rhizoctonia and other soilborne pathogens, fungicides with different modes of action should be applied in rotation. Some recommended rotations include:

• Empress or Heritage (FRAC 11), rotated with Prostar (FRAC 7) or Medallion (FRAC 12) or Terraguard (FRAC 3). OR

• Mural or Orkestra (both FRAC 7+11) rotated with Terraguard (FRAC 3) or Medallion (FRAC 12).

If you encounter redbud plants in nurseries or managed landscapes with VSD symptoms, contact Dr. Fulya Baysal-Gurel at fbaysalg@tnstate.edu.