2019 Phytophthora Project Results

Results are in from our 2019 study of Phytophthora on herbaceous ornamentals. See project details below for a full summary including detailed results. Summary results — Printable PDF 

Landscape bed with flowers at Mills River, NC

Project Summary: Several popular bedding plants are susceptible to Phytophthora root and crown rot, a soilborne plant pathogen that can cause devastating economic losses to the ornamental plant industry. Controlling the disease can be particularly difficult once the pathogen is established, making the use of resistant or tolerant plants ideal for infested landscapes.

In summer 2019, we evaluated 14 annuals and 16 herbaceous perennials (Table 1) to multiple species of Phytophthora. Some of the plant species and/or cultivars chosen for this study were re-evaluated from our 2018 study (https://content.ces.ncsu.edu/phytophthora-root-and-crown-rot-in-the-landscape). Three landscape beds were established in different 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.

Flowers in a raised landscape bed

Results: Fifteen cultivars of the evaluated annuals and twelve cultivars of the evaluated herbaceous perennials appeared excellent or good (Table 1) throughout the growing season (June 3 – September 20, 2019). Phytophthora was recovered from four cultivars of the evaluated annuals and four cultivars of the evaluated perennials and these plants were rated as Fair or Poor. Plants that in the ‘Other’ category had other problems and will need to be reevaluated to confirm resistance. This study provides evidence of 27 ornamental plant cultivars that can be used as acceptable alternatives in landscape beds infested with Phytophthora.

Table 1. List of annual and herbaceous perennial cultivars that were evaluated for tolerance or resistance to Phytophthora spp. from June 3 to September 20, 2019.

Ratinga Type Plant Species Cultivar Disease IDb
Excellent Annuals Angelonia (Angelonia angustifolia) ArchAngel Blue
Serenita White
Sweet Potato Vine (Ipomoea batatas) Ace of Spades
West Indian Lantana (Lantana camara) Miss Huff
Lantana (Lantana hybrida) Little Lucky Pot of Gold
Little Lucky Peach Glow
African Marigold (Tagetes erecta) Antigua Yellow
Zinnia (Zinnia angustifolia) Star White
Star Orange
Perennials Hybrid Yarrow (Achillea filipendulina) Moonshine
Ornamental Sedge (Carex flacca) Blue Zinger
Ornamental Sedge (Carex testacea) Prairie Fire
Tickseed (Coreopsis auriculata) Nana
Coneflower (Echinacea purpurea) PowWow Wildberry
Cheyenne Spirit
Verbena (Glandularia canadensis) Homestead Purple
Ornamental Grass (Miscanthus sinensis) Little Zebra
Ornamental Grass (Panicum virgatum) Shenandoah
Good Annuals Celosia (Celosia cristata) Dracula A, U
African Marigold (Tagetes erecta) Antigua Orange F
French Marigold (Tagetes patula) Janie Deep Orange A
Janie Spry A
Verbena hybrid (Verbena hybrida) Superbena Royal Chambray A
Perennials Rose mock vervain (Glandularia canadensis) Homestead Purple A
Peruvian mock vervain (Glandularia peruviana) EnduraScape Red A
Fair Annuals Verbena hybrid (Verbena hybrida) Lanai Upright Rose with Eye P
Annual vinca (Catharanthus roseus) Cora Cascade Lilac L, P
Cora Cascade Strawberry L
Poor Annuals Petunia (Petunia x hybrida) Night Sky P
Perennials Verbena hybrid (Verbena hybrida) Superbena Stormburst P
Alyssum (Alyssum wulfenianum) Golden Spring P
Yarrow (Achillea leisii) King Edward P
Other Annuals Lychnis (Lychnis arkwrightii) Orange Gnome A, U, P
Perennials Tickseed (Coreopsis verticillata) Starlight U
Sunfire U
Bee balm (Monarda didyma) Pardon My Cerise I, M
Pardon My Purple I, M
Balmy I, L, M, N
Black-eyed Susan (Rudbeckia hirta) Little Goldstar A, S

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) the plants died before end of growing season. Poor = severe disease symptoms, more than half (>6) the plants died before end of growing season. Other = more than half (>6) the plants had abiotic or unknown issues that prevented a fair trial of the cultivars resistance or tolerance to Phytophthora.

bDisease ID Letter Code: A: Abiotic, F: Fusarium, I: Insect, L: Leaf spot, M: Powdery Mildew, N: Nematodes (parasitic), P: Phytophthora, S: Southern Blight, U: Unknown

Future Implications: This project has been funded for an additional two years (2020-2021) by the Horticultural Research Institute. We plan to evaluate additional cultivars of annuals and herbaceous perennials in the upcoming growing season(s) and selecting plants based on input from NC landscapers and nursery growers.

Acknowledgements: We’d like to thank area specialized agents Amanda Taylor and Stacey Jones for their help with this project in its second year. We’d also like to thank the nurseries in North Carolina who generously provided plants that made this study possible: Hawksridge Farms, Metrolina Greenhouses, Hoffman Nursery, and King’s Nursery.

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