Project Results – Phytophthora on Herbaceous Ornamentals

We had a successful 2018 field season and were able to evaluate plant cultivars that could  be used as acceptable alternatives in Phytophthora-infested soils. We will be publishing the results of this study in the coming year, as well as an extension publication that will be made accessible to the public. Additionally, you can find a printable PDF file of the summary results here:  Summary Results – Printable PDF

Landscape Bed at Piedmont Research Station

Landscape Bed at Piedmont Research Station

Project Summary: Many ornamental crops are susceptible to diseases caused by Phytophthora, a genus of fungal-like organisms that can persist in the soil for many years. Since chemical applications are not economical or practical for managing Phytophthora-infested landscape beds, knowledge of Phytophthora-resistant or tolerant plant species would make disease avoidance possible in landscape settings.

In summer 2018, we evaluated 15 annuals and 12 herbaceous perennials based on desirability and anecdotal evidence of resistant or tolerant cultivars to multiple species of Phytophthora. Three landscape beds were established in different climatic regions of western North Carolina (Mills River, NC; Waynesville, NC; Salisbury, NC) and were infested with three species of Phytophthora known to cause disease of nursery crops. As disease symptoms progressed, symptomatic plants were harvested and processed to determine the presence of Phytophthora and/or other common diseases that may have been suspected.

Landscape Bed at the Mountain Research Station

Landscape Bed at the Mountain Research Station

Results: Ten cultivars of the evaluated annuals and seven cultivars of the evaluated herbaceous perennials appeared excellent throughout the growing season (June 2 – October 3, 2018). Phytophthora was recovered from six cultivars of the evaluated annuals and five cultivars of the evaluated perennials. This study provides evidence of 17 ornamental plant cultivars that can be used as acceptable alternatives in landscape beds infested with Phytophthora.

Future Implications: The landscape beds will remain intact and will be used to evaluate additional annuals and perennials in the upcoming 2019 growing season. In addition to evaluating new plant species and cultivars, we are re-evaluating cultivars that had a 2018 performance rating or ‘excellent’, ‘fair’, or ‘good’. Cultivars rated as excellent are assumed to be at least tolerant to Phytophthora, but additional evaluations are needed to confirm tolerance or resistance.

Acknowledgements: Thank you to the nurseries who generously provided plants that made this study possible: Hawksridge Farms, Metrolina Greenhouses, Hoffman Nursery, King’s Nursery, Plantworks Nursery, Cold Mountain Nursery, Rountree Plantation, Homewood Nursery and Garden Center, and Fairview Garden Center.

Table 1. List of annual and herbaceous perennial cultivars that were evaluated for tolerance or resistance to Phytophthora spp. from June 2 to October 3, 2018.

Ratinga Type Plant Species Cultivar Disease IDb
Excellent Annuals Zinnia (Zinnia angustifolia) Star Orange
Star White P, F
New Guinea Impatiens (Impatiens hawkeri) Sunpatiens Compact Orchid
Sunpatiens Lilac
Sweet Potato Vine (Ipomoea batatas) Bright Idea Tri-color
Ace of Spades
Angelonia (Angelonia angustifolia) ArchAngel Pink
Lantana (Lantana hybrida) New Gold
Lantana (Lantana camara) Miss Huff
Perennials Coneflower (Echinacea purpurea) PowWow Wildberry
Cheyenne Spirit
Ornamental Grass (Panicum virgatum) Shenandoah
Shasta Daisy (Leucanthemum superbum) Becky
Tickseed (Coreopsis auriculata) Nana
Jethro Tull
Verbena (Glandularia canadensis) Homestead Purple
Good Annuals Zinnia (Zinnia elegans) Magellan Orange L
Vinca (Catharanthus roseus) Cora Apricot U
Cora Strawberry P
New Guinea Impatiens (Impatiens hawkeri) Harmony L
Perennials Ornamental Grass (Panicum virgatum) Rotstrahlbusch P
Fair Annuals Geranium (Pelargonium hortorum) Calliope Dark Red A, U
Bullseye Cherry A, U
Celosia (Celosia argentea) New Look P
Perennials Bee balm (Monarda didyma) Petite Delight M
Jacob Cline M
Salvia (Salvia farinacea) Victoria Blue P
Hybrid Yarrow (Achillea filipendulina) Moonshine P, U
Poor Annuals French Marigold (Tagetes patula) Disco Mix A, U
Disco Yellow A, U
African Marigold (Tagetes erecta) Proud Yellow A, U, F
Inca Yellow A, U
Phlox (Phlox drummondii) Phlox Star A, U
Intensia Red Hot A, U
Petunia (Petunia hybrida) Violet Picotee P
Yellow Madness P
Calibrachoa hybrid (Petunia x calibrachoa) SuperCal P
Perennials Shasta Daisy (Leucanthemum superbum) Snow Lady L, U
Phlox (Phlox paniculata) Amethyst True Gal A, U
Yarrow (Achillea millefolium) Desert Eve Red A
Blanket Flower (Gaillardia grandiflora) Goblin P, L, F
Mesa Bi-Color L, P
Black-eyed Susan (Rudbeckia hirta) Indian Summer U, V
Prairie Sun U, V

aDisease Rating Scale: Excellent = no disease symptoms, excellent floral quality, survived entire growing season. Good = minor disease symptoms, good floral quality, most survived entire growing season. Fair = moderate disease symptoms, less than half (<6 plants) died before end of growing season. Poor = severe disease symptoms, more than half (>6 plants) died before end of growing season.

bDisease ID Letter Code: A: Abiotic, F: Fusarium, L: Leaf spot, M: Powdery Mildew, P: Phytophthora, U: Unknown, V: Verticillium.

Excellent Cultivar PowWow Wildberry

Excellent Cultivar: Echnicacea ‘PowWow Wildberry’

Excellent Cultivar: Sweet Potato Vine 'Ace of Spades'

Excellent Cultivar: Sweet Potato Vine ‘Ace of Spades’

Excellent Cultivar: Tickseed 'Jethro Tull'

Excellent Cultivar: Tickseed ‘Jethro Tull’

Excellent cultivar Angelonia 'ArchAngel Pink'

Excellent cultivar: Angelonia ‘ArchAngel Pink’



This research was supported by a grant from the Horticultural Research Institute (HRI). Its contents are solely the responsibility of the authors and do not necessarily represent the views of HRI.

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