NC State Extension Publications

 

Many ornamental crop species (including trees, shrubs, and bedding plants) are susceptible to diseases caused by Phytophthora, a genus of plant-pathogenic oomycetes (also known as water molds) that can persist in soil for several years. Phytophthora root rot or crown rot symptoms often include loss of older foliage, foliar blight, chlorosis, decline in vigor, branch dieback, and wilting (Figure 1, Figure 2, and Figure 3). Phytophthora species in a landscape setting may be managed by improving soil drainage, avoiding overwatering or overfertilizing, and preventing the spread of soil, water, and plants from infested areas (see Phytophthora Blight and Root Rot on Annuals and Herbaceous Perennials). Chemical means of managing these diseases are limited and generally impractical in landscape settings.

Because there are few effective chemical management options, using resistant plant species is a better option. However, there have been limited studies of the plant species and cultivars that can thrive and grow in soils in which Phytophthora crown rot and root rot have occurred. Table 1 includes some suggested plant species for landscapes with a history of Phytophthora spp. infestations, some of which we evaluated in a summer 2018 study (Table 2). In our study, we evaluated multiple species and cultivars of summer annuals and herbaceous perennials for tolerance or resistance to Phytophthora.

Color photo of red-flowering plant with browned, wilted foliage

Figure 1. Annual vinca with severe aerial blight symptoms.

Color photo of stems with defoliated leaves

Figure 2. Annual petunia showing severe defoliation symptoms.

Color photo of plant with wilted foliage.

Figure 3. Herbaceous perennial dusty miller showing wilt symptoms.

 


Table 1. Ornamental plants tolerant or resistant to Phytophthora species.

Annuals

Herbaceous Perennials

Woody

ageratum (Ageratum sp.)

alyssum (Alyssum sp.)

angelonia (Angelonia angustifolia)

ornamental cabbage, kale (Brassica sp.)

impatiens (Impatiens spp.)

sweetpotato vine (Ipomoea batatas)

morning glory (Ipomoea sp.)

West Indian lantana (Lantana camara)

lantana (Lantana sp.)

moss-rose (Portulaca sp.)

marigold (Tagetes spp.)

tree marigold (Tithonia diversifolia)

Mexican sunflower (Tithonia rotundifolia)

purpletop vervain (Verbena bonariensis)

narrow-leaf zinnia (Zinnia angustifolia)

zinnia (Zinnia elegans)

yarrow (Achillea sp.)

narcissus/daffodil (Narcissus spp.)

common bugleweed (Ajuga reptans)

amaryllis (Amaryllis/Hippeastrum spp.)

bluestar (Amsonia spp.)

creeping phlox (Phlox subulata)

anemone (Anemone spp.)

aster (Aster spp.)

canna lily (Canna spp.)

ornamental sedges (Carex spp.)

tickseed (Coreopsis auriculata)

dahlia (Dahlia spp.)

purple coneflower (Echinacea purpurea)

gazania (Gazania sp.)

cranesbill (Geranium sanguineum)

verbena (Glandularia canadensis)

candytuft (Iberis sempervirens)

lychnis (Lychnis spp.)

mints (Mentha spp.)

bee balm (Monarda spp.)

pachysandra (Pachysandra spp.)

ornamental grass (Panicum virgatum)

Black-eyed Susan (Rudbeckia hirta)

ferns (several species)

ornamental grasses (several species)

trillium (Trillium sp.)

rain lily (Zephyranthes spp.)

abelia (Linnaea x grandiflora)

river birch (Betula nigra)

spice bush (Calycanthus spp.)

wintersweet (Chimonanthus praecox)

honeylocust (Gleditsia triacanthos)

ginkgo (Ginkgo biloba)

Burford holly* (Ilex x burfordii)

Chinese holly* (Ilex cornuta)

winterberry (Ilex verticillata)

yaupon (Ilex vomitoria)

sweetgum (Liquidamber styraciflua)

magnolia (Magnolia spp.)

dawn redwood (Metasequoia glyptostroboides)

black gum (Nyssa sylvatica)

currants, gooseberry (Ribes spp.)

rose** (Rosa spp.)

willows (Salix spp.)

bald cypress (Taxodium districhum)

Cultivars of plant species listed in bold were tested in our 2018 study.

*Other hollies with I. cornuta parentage may also be good alternatives. Note that many Ilex species are susceptible to species of Phytophthora—for example, blue hollies (Ilex x meserveae), Japanese holly (I. crenata), possumhaw (I. decidua), and inkberry (I. glabra).

**Rose is occasionally infected by Phytophthora spp. in the nursery, but we do not consider it a primary pathogen in the landscape.


Table 2. 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

Annual

narrow-leaf zinnia (Zinnia angustifolia)

Star Orange

-

Star White

-

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

-

Serenita White

-

lantana (Lantana hybrida)

New Gold

-

West Indian lantana (Lantana camara)

Miss Huff

-

Perennial

purple coneflower (Echinacea purpurea)

PowWow Wildberry

-

Cheyenne Spirit

-

ornamental grass (Panicum virgatum)

Shenandoah

-

Shasta daisy (Leucanthemum x superbum)

Becky

-

tickseed (Coreopsis auriculata)

Nana

-

Jethro Tull

-

verbena (Glandularia canadensis)

Homestead Purple

-

Good

Annual

zinnia (Zinnia elegans)

Magellan Orange

L

annual vinca (Catharanthus roseus)

Cora Apricot1

U

Cora Strawberry1

P, U

New Guinea impatiens (Impatiens hawkeri)

Harmony

L

Perennial

ornamental grass (Panicum virgatum)

Rotstrahlbusch

L, P

Fair

Annual

geranium (Pelargonium hortorum)

Calliope Dark Red

A, L, U

Bullseye Cherry

A, P

celosia (Celosia argentea)

New Look

P

Perennial

bee balm (Monarda didyma)

Petite Delight

M

Jacob Cline

M

mealy blue sage (Salvia farinacea)

Victoria Blue

P

fernleaf yarrow (Achillea filipendulina)

Moonshine

P, U

Poor

Annual

African marigold (Tagetes erecta)

Proud Yellow

A, U, F

Inca Yellow

A, U, P

petunia (Petunia hybrida)

Violet Picotee

P

Yellow Madness

P

calibrachoa (Petunia x calibrachoa)

SuperCal

P

Perennial

Shasta daisy (Leucanthemum x superbum)

Snow Lady

L, U

blanket flower (Gaillardia grandiflora)

Goblin

P, L, F

Mesa Bi-Color

L, P

Other

Annual

French marigold (Tagetes patula)

Disco Mix

A, U

Disco Yellow

A, U

annual phlox (Phlox drummondii)

Phlox Star

A, U

Intensia Red Hot

A, U

Perennial

garden phlox (Phlox paniculata)

Amethyst True Gal

A, U

yarrow (Achillea millefolium)

Desert Eve Red

A

black-eyed Susan (Rudbeckia hirta)2

Indian Summer

A, U, V

Prairie Sun

A, U, V

aDisease Rating Scale:

Excellent = no disease symptoms, excellent floral quality, survived entire growing season.
Good = minor disease symptoms (< 25% leaf area affected), good floral quality, most survived entire growing season.
Fair = moderate disease symptoms (~ 50% leaf area affected), less than half (< 6) the plants died before end of growing season.
Poor = severe disease symptoms (> 50% leaf area affected), 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 cultivar’s resistance to or tolerance of Phytophthora.

bDisease ID Letter Code:

A: Abiotic disorder, F: Fusarium wilt, L: Leaf spot, M: Powdery mildew, P: Phytophthora root rot or stem rot, or aerial blight, U: Unknown, V: Verticillium wilt.

1Other research has shown the Cora® series to be susceptible to some strains of P. nicotianae.
2Verticillium wilt is not generally a concern in warmer climates (> 81°F).


Methods

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We evaluated one to three cultivars each of 15 annuals (26 cultivars total) and 12 herbaceous perennials (20 cultivars total) in three landscape beds for tolerance or resistance to three species of Phytophthora. Plant species and cultivars were selected based on anecdotal evidence of resistance or tolerance to Phytophthora spp., resistance to other common pathogens (such as powdery mildew and downy mildew), desirability in landscape beds, and consumer availability.

Three landscape beds were established in western North Carolina (Mills River, Waynesville, and Salisbury) (Figure 4, Figure 5, and Figure 6) and were infested in June 2018 (Figure 7 and Figure 8) with three species of Phytophthora known to cause disease of nursery crops: P. nicotianae, P. tropicalis, and P. dreschleri. Each landscape bed was divided into four quadrants, and one plant of each cultivar was planted in each of the quadrants. Plants known to be susceptible to Phytophthora spp. (gerbera daisy, annual vinca, petunia, and dusty miller) were included as positive controls to confirm inoculum was active. The landscape beds were irrigated daily using soaker hoses to excessively water the raised beds in order to create environmental conditions conducive for the disease.

Plants were rated every two weeks throughout the 2018 growing season (June 2 to October 3) for disease incidence and severity based on visible symptoms of root rot, crown rot, or aerial blight (Figure 9). The disease rating scale was Excellent, Good, Fair, Poor, and Other (Table 2). As disease progressed, symptomatic plants were harvested and processed to check for the presence of Phytophthora and other common pathogens. If other diseases were diagnosed, these cultivars were placed in the “Other” category, as these circumstances prevented a fair trial of the plants’ resistance to Phytophthora. The North Carolina State University Plant Disease and Insect Clinic provided disease diagnoses for some plants.

Photo of framed raised bed of multicolored blooming plants

Figure 4. Landscape bed at the Mountain Horticultural Crops Research and Extension Center in June 2018.

Photo of framed raised bed of multicolored blooming plants

Figure 5. Landscape bed at the Piedmont Research Station in June 2018.

Photo of framed raised bed of multicolored blooming plants

Figure 6. Landscape bed at the Mountain Research Station in July 2018.

Photo of woman digging with trowel among red-bloomed plants

Figure 7. Suzette Sharpe (research assistant) digging trenches into the landscape beds before applying inoculum to the Mountain Research Station landscape bed.

Close-up of hand sprinkling particles in flower bed trench

Figure 8. Applying Phytophthora spp. inoculum to the Piedmont Research Station landscape bed.

Color photo of woman with clipboard next to flower bed

Figure 9. Michelle Henson (Research Assistant) rating plants for disease incidence at the Mountain Research Station in July 2018.

Results and Conclusion

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At the end of the season, we identified 14 cultivars among 9 annual plant species and 8 cultivars among 6 herbaceous perennial plant species that rated Excellent or Good throughout the 2018 growing season (June 2 to October 3) (Figure 10, Figure 11, and Figure 12). Phytophthora was recovered from seven cultivars of the evaluated annuals and five cultivars of the evaluated perennials, which ranged in appearance from Excellent (asymptomatic) to Poor (symptomatic). P. nicotianae or P. drechsleri was recovered from each known susceptible host, which confirmed that inoculum was active in the beds. Interestingly, P. cryptogea was recovered from three cultivars, even though the species was not intentionally introduced into the landscape beds. Other diseases such as Verticillium wilt, Fusarium wilt, and powdery mildew, as well as abiotic disorders, were diagnosed on plants that performed Fair, Poor, and Other. These susceptibilities are noted in Table 2 and are the cultivars not listed in our recommendations. P. nicotianae and Fusarium spp. were isolated from root tissue of one Zinnia ‘Star White’ plant, but this variety retained its vigor and the population was not considered diseased. This study provides evidence of 22 ornamental plant cultivars within 15 herbaceous plant species that can be used as acceptable alternatives in landscape beds infested with several species of Phytophthora.

We plan to continue using our landscape beds to evaluate other annuals and perennials. In addition to evaluating new plant species and cultivars, we will re-evaluate selected cultivars that had a 2018 performance rating of Excellent, Good, Fair, or Other to ensure consistent results. Cultivars rated as Excellent or Good are assumed to be at least somewhat tolerant of Phytophthora, but additional evaluations are needed to confirm that they have a practical level of tolerance or resistance.

Color photo of plant with green leaves and orange flowers

Figure 10. Plant species and cultivar with excellent disease resistance: West Indian lantana ‘Miss Huff.’

Color photo of plant with green leaves and magenta flowers

Figure 11. Plant species and cultivar with excellent disease resistance: Purple coneflower ‘PowWow Wild Berry.’

Color photo of plant with green leaves and white flowers

Figure 12. Plant species and cultivar with excellent disease resistance: Angelonia ‘Serenita White.’

Disclaimer

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Some of the plants listed in this publication may be susceptible to other plant pathogens. Good cultural management practices are recommended to keep pests and pathogens out of the landscape.

Acknowledgments

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The authors would like to thank Area Specialized Extension Agents Amanda Taylor and Stacey Jones and the participants at the 17th Ornamentals Workshop in Kanuga (Hendersonville, NC) for their input into this publication. We’d also like to thank the nurseries in North Carolina who generously provided plants that made this study possible: Hawks Ridge Farms, Metrolina Greenhouses, Hoffman Nursery, King’s Nursery, Plantworks Nursery, Cold Mountain Nursery, Rountree Plantation, Homewood Nursery and Garden Center, and Fairview Garden Center. Thanks also to M. Munster, F. Baysal-Gurel, and M. Daughtrey for providing valuable feedback on this publication.

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.

Authors

Research Assistant
Entomology & Plant Pathology
Research Assistant
Entomology & Plant Pathology
Extension Associate, Vegetable and Herbaceous Ornamental Pathology
Entomology & Plant Pathology

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Publication date: Feb. 5, 2020
AG-747

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