Phytophthora root rot and stem blight affects more than 100 of the most popular and commonly grown landscape perennials and annuals, including annual vinca, petunia, and daylily, throughout the U.S. and worldwide. The best strategy is to avoid introducing Phytophthoras into the landscape because once Phytophthora species are established, they are impossible to eradicate without extraordinary measures that are impractical or expensive because they can survive for many years. Therefore, resistant plants are the next best option, but there is limited information on which plant species or cultivars make for suitable replacements for infested landscape beds.
How We Determined Phythophthora Resistance
In 2018, we evaluated one to three cultivars each of 15 annual and 12 herbaceous perennial plant species in each of three landscape beds in North Carolina. After plants were established, each bed was inoculated with three species of Phytophthora commonly found affecting ornamentals in North Carolina: P. dreschleri, P. nicotianae, and P. tropicalis.
Plants were evaluated for appearance and disease presence every two weeks. Known susceptible plants such as gerbera daisy, petunia, dusty miller, and annual vinca were planted in each bed to ensure inoculum was active. All susceptible plants died from Phytophthora by the end of the season.
At the end of the season, we identified 14 cultivars among nine annual plant species and eight cultivars among six herbaceous perennial plant species that were rated as excellent or good throughout the growing season.
Other plants rated as fair or poor were affected by other diseases either identified or not identified including Verticillium wilt, Phytophthora, Fusarium, powdery mildew, or abiotic disease of unknown origin. More results are posted on the Meadows Plant Pathology Lab website.
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.