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Applied Research

Wael Elwakil
Extension Agent II, Fruit & Vegetable Production
UF/IFAS Extension, Hillsborough County


Central Florida nursery growers have reported disease management challenges since 2004 impacting the production of ornamental Viburnum spp. Reported symptoms included blighting and rapid defoliation that were indicative of downy mildew. Growers indicated that common labeled fungicides failed to provide acceptable levels of disease management. Beginning in the spring of 2020, symptomatic plant samples were collected from local nurseries. Identification of isolated fungi, revealed the presence of multiple pathogens throughout the growing seasons (spring, summer, and fall), including Plasmopara sp., Cercospora sp., Corynespora sp., Colletotrichum sp., Phoma sp., Phylosticta sp., and Pestalotiopsis sp. Several isolates were collected and preserved for subsequent pathogenicity testing. Subsequently, two trials were conducted at a commercial nursery, to evaluate the performance of a range of fungicides. Both trials utilized natural pathogen populations present on diseased plant materials, with overhead watering and fertilization per grower standards. The first trial, conducted in July thru August, evaluated thirteen fungicides available to nursery growers. The second trial, conducted in September thru October, focused on seven fungicides. Both trials included a non-treated control, with all treatments replicated (n=6) and arranged in randomized complete blocks.  Disease severity, based on percent symptomatic foliage, was rated weekly and used to calculate Area Under Disease Progression Curve (AUDPC). Initial plant samples collected in May, identified downy mildew (Plasmopara sp.), Cercospora sp. and Colletotrichum sp. as the primary pathogens.  However, additional later sampling failed to find any sign of downy mildew. Rather, isolations recovered Colletotrichum sp., Corynespora sp., Phylosticta sp., Phoma sp., and a Pestalotiopsis sp.  Leaf symptoms were similar to those commonly associated with downy mildew. Not surprisingly, fungicides that target oomycetes (i.e., Plasmopara sp.), containing ametoctradin, cyazofamid, dimethomorph, fluopicolide, mandipropamid, mefenoxam, and oxathiapiprolin, failed to statistically reduce disease severity relative to the non-treated control based on AUDPC. Benzovindiflupyr, difenoconazole, fluxapyroxad, and pyraclostrobin fungicides that typically target true fungi, statistically reduced disease severity. Copper sulfate and mancozeb, or a systemic fungicide, flutriafol, failed to reduce disease severity, while a generic phosphite gave an intermediate level of control. Results stress the importance of an appropriate disease diagnosis to avoid making an ineffective fungicide application.

Poster has NOT been presented at any previous NACAA AM/PIC

This poster is being submitted for judging. It will be displayed at the AM/PIC if not selected as a State winner. The abstract will be published in the proceedings.

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Authors: S. Steed, W. Elwakil, L. Vallad, G. Vallad
  1. Steed, S. Multi-county Environmental Horticulture Production Agent III, UF/IFAS Extension, Florida, 34221
  2. Elwakil, W. Extension Agent II, Commercial Horticulture, UF/IFAS Extension, Florida, 34221
  3. Vallad, L. Research Assistant, UF/IFAS, Florida, 33598
  4. Vallad, G. Professor of Plant Pathology, UF/IFAS, Florida, 33598