Managing Foliar Leaf Spots in Viburnum Production: What Florida Nursery Trials Reveal

Viburnums are a cornerstone crop for many Florida nurseries, but over the past two decades growers have faced increasingly severe foliar disease problems—especially during propagation. What was once a nearly trouble‑free crop has, in some cases, dropped from near‑100% propagation success to total loss. Extension and research by UF/IFAS and funding help from the Florida Nursery Grower and Landscape Association help explain why—and what growers can do to protect their plants.

It’s Not Just Downy Mildew Anymore

For years, downy mildew (DM) (Plasmopara viburni) was considered the primary culprit behind leaf spots, blighting, and defoliation in viburnum. But UF/IFAS researchers found that today’s disease pressure is far more complex. Across multiple nursery trials, they consistently isolated a mixture of true fungal pathogens—including Cercospora, Colletotrichum, Corynespora, Phyllosticta, Phoma, and Pestalotiopsis species.
These pathogens produce symptoms nearly identical to downy mildew, making field diagnosis extremely difficult. In fact, in several seasons researchers found no downy mildew at all, even when symptoms looked like classic DM.

Viburnum cutting propagation with heavy disease pressure (photo credit: UF/IFAS Extension Hillsborough County)

Why Many Fungicides Fail

Because many growers were targeting downy mildew, they relied heavily on oomycete‑specific fungicides. The trials showed that these products—those containing active ingredients ametoctradin, cyazofamid, dimethomorph, fluopicolide, mandipropamid, mefenoxam, or oxathiapiprolin—did not reduce disease severity when true fungi were the real problem.

In contrast, systemic fungicides effective against true fungi performed significantly better. Products containing active ingredients benzovindiflupyr, difenoconazole, fluxapyroxad, and pyraclostrobin consistently reduced leaf spot severity in both container and propagation trials.

Propagation Trials: Dip Treatments Shine

During propagation, both fungicide drench and dip applications helped reduce disease, but dip treatments had two major advantages:

• They reduced disease severity without the negative impact on root development seen in some drench treatments.
• They possibly avoided diluting the rooting hormone, which can contribute to weaker, less vigorous liners.

Postiva® (benzovindiflupyr + difenoconazole) and Orkestra® (pyraclostrobin + fluxapyroxad) were the most consistently effective products across trials.

Best Practices for Growers

The research highlights three key takeaways:

1. Identify the pathogen—or at least recognize that multiple pathogens may be involved. Misidentification leads to ineffective control.
2. Use the right fungicide for the right pathogen group. When true fungi dominate, oomycete‑targeting products won’t help.
3. Reevaluate production practices. Overhead irrigation, high humidity, and dense canopies create ideal conditions for foliar pathogens. Reducing leaf wetness—especially in propagation—can dramatically improve outcomes. Creating less stress (possibly with some shade) can also help plants fight diseases.

While fungicides remain an important tool, cultural practices and clean stock are equally critical. With better pathogen awareness and targeted management, growers can regain control over viburnum production and reduce costly losses.

For more complete information, publications of the multi-year research findings were published in the Florida State Horticulture Society Proceedings and summarized for nursery growers and propagators in the publication Foliar Leaf Spots of Viburnum during Nursery Production in Florida: Research and Fungicide Efficacies

Disclaimer

UF/IFAS recommends following all pesticide labels. The label is the law. The selection and use of these products over any other products do not indicate an endorsement. During the preparation of this work, the author used Copilot to summarize research findings. After using this tool, the author reviewed and edited the content as needed and takes full responsibility for the content of the publication.