Many ornamental crop species (including trees, shrubs, and bedding plants) are susceptible to disease caused by members of the genus Phytophthora, a group of fungal-like, plant-pathogenic oomycetes (also known as water molds) that can persist in soil for several years. Phytophthora species typically cause root rot or crown rot, and 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 root and crown rot may be managed in a landscape setting 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 costly and generally impractical in landscape settings.

The most sustainable and environmentally friendly management tool is the cultivation of tolerant or resistant plants. However, there has been a limited number of studies investigating species and cultivars of ornamental landscape plants that can thrive in beds with a history of Phytophthora infestation. During the summers of 2018, 2019, and 2020, we evaluated multiple species and cultivars of ornamental plants (including summer annuals and herbaceous perennials) for tolerance or resistance to Phytophthora root and crown rot in a landscape setting.

Figure 1. Annual vinca with severe symptoms of aerial blight.

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Figure 1. Annual vinca with severe symptoms of aerial blight.

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Figure 2. Annual petunia showing severe symptoms of defoliation.

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Figure 2. Annual petunia showing severe symptoms of defoliation.

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Figure 3. Herbaceous perennial dusty miller showing symptoms of wilt caused by Phytophthora root rot.

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Figure 3. Herbaceous perennial dusty miller showing symptoms of wilt caused by Phytophthora root rot.

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We evaluated one to five cultivars each of 21 annuals (53 cultivars total) and 27 herbaceous perennials (42 cultivars total) in three landscape beds in North Carolina for tolerance or resistance to four species of Phytophthora responsible for causing root and crown rot in the landscape setting. Plant species and cultivars were selected based on anecdotal evidence of resistance or tolerance to Phytophthora spp. as well as 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 twice each year with species of Phytophthora known to cause disease in ornamental landscape plants in North Carolina: P. nicotianae, P. tropicalis, P. dreschleri and, in 2019 and 2020, P. cryptogea. 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 that the inoculum was active. The landscape beds were irrigated daily using soaker hoses to create environmental conditions conducive for the disease.

Plants were rated every two weeks throughout each growing season (June through October) for disease incidence and severity based on visible symptoms of root rot, crown rot, or aerial blight. The disease rating scale was Excellent, Good, Fair, Poor, and Other (Table 1). As disease progressed, symptomatic plants were harvested and processed to check for the presence of Phytophthora and other common pathogens. The North Carolina State Extension Plant Disease and Insect Clinic provided disease diagnoses for some plants.

^a Disease 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 other diseases or had abiotic or unknown issues that prevented a fair trial of the cultivar’s resistance to or tolerance of Phytophthora.

^b Disease ID Letter Code: A: Abiotic disorder, F: Fusarium wilt, L: Leaf spot, M: Powdery mildew, S: Southern blight, Py: Pythium root rot, N: Nematodes, P: Phytophthora root rot or stem rot, or aerial blight, U: Unknown, V: Verticillium wilt. ¹ Other research has shown the Cora^® series to be susceptible to some strains of P. nicotianae.
² Verticillium wilt is not generally a concern in warmer climates (> 81°F).

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

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

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

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

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

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

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Figure 7. Suzette Sharpe (research assistant) creating shallow trenches in the landscape bed at the Mountain Research Station before applying inoculum.

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Figure 7. Suzette Sharpe (research assistant) creating shallow trenches in the landscape bed at the Mountain Research Station before applying inoculum.

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Figure 8. Applying vermiculite-based inoculum infested with Phytophthora spp. to the landscape bed at the Piedmont Research Station.

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Figure 8. Applying vermiculite-based inoculum infested with Phytophthora spp. to the landscape bed at the Piedmont Research Station.

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

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

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We identified 18 cultivars among 10 annual plant species, as well as 13 cultivars among 10 herbaceous perennial plant species, whose performance was rated as Excellent (Figure 10, Figure 11, and Figure 12). Phytophthora spp. were not isolated from the roots of any of these plants, suggesting that they are not hosts of this pathogen. Nine cultivars among 7 annual plant species and 11 cultivars among 9 herbaceous perennial plant species were rated as Good, although some of these cultivars were diagnosed with diseases such as Fusarium wilt, leaf spot, and abiotic disorder. Phytophthora spp. were isolated from the roots of three cultivars rated as Good. In many cases, the cause of disease for plants rated as Fair was difficult to identify, although Phytophthora spp. were isolated from the roots of some of these plants. Phytophthora spp. were frequently isolated from the roots and crowns of plants rated as Poor (Table 1). Based on the results of this study, we recommend that the 27 cultivars of annuals and 24 cultivars of herbaceous perennial landscape plants whose performance was rated as Excellent or Good in this study be grown in landscape beds with a history of Phytophthora infestation.

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

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

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

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

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

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

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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.

Phytophthora Blight and Root Rot on Annuals and Herbaceous Perennials Disease Factsheet

NC State Extension Plant Disease and Insect Clinic

Southeastern US Pest Control Guide for Nursery Crops and Landscape Plantings

North Carolina Agricultural Chemicals Manual

Inga Meadows Plant Pathology Lab for Vegetables and Herbaceous Ornamentals

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, North Carolina) for their input into this publication. We also want 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 Michael Munster, Fulya Baysal-Gurel, and Margery Daughtrey for providing valuable feedback on this publication.

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