Phytophthora Resistant Plants
Phytophthora root rot, a soil borne disease caused by a fungus-like organism, is a widespread problem which can affect many woody and herbaceous landscape plants. Azalea, holly, rhododendron, and juniper are the most frequently affected plant groups in the eastern landscape. Submit a sample including roots to the Plant Diagnostic Clinic and it can be tested for Phytophthora. An infested site should not be replanted with susceptible hosts. Increase drainage and plant a more resistant shrub or tree. The best control for Phytophthora is to avoid getting it started in the first place. Phytophthora is favored by planting susceptible species and cultivars in poorly drained soils, or by overwatering in sites with adequate drainage. Once a Phytophthora problem is established in a landscape site, the best option is to remove the affected plants and replace with resistant species. Some good choices include Ilex glabra (inkberry holly), Clethra alnifolia (summersweet), Itea sp (sweetspire), Physocarpus opufoliius (Eastern ninebark), and Leucothoe fontanesiana. Examples suggested by extension agents in North Carolina include Nandina, Chinese holly (cultivars including ‘Rotunda’, ‘Dwarf Burford’ and ‘Carissa’), liriope, Indian hawthorn, and Camellia sasanqua cultivars (Camellia japonica is highly susceptible). The rhododendron hybrids: ‘Caroline’, ‘Martha Isaacson’, ‘Professor Hugo de Vries’ and ‘Red Head’ are considered resistant. In addition, the azalea cultivars ‘Formosa’, ‘Fred Cochran’, ‘Fakir’ and ‘Corrine Murrah’ are considered highly resistant. Other resistant azalea cultivars include Rhododendron poukhanese, ‘Formosa’, ‘Fakir’, ‘Corrine Murrah’, ‘Merlin’, ‘Hampton Beauty’, ‘Higasa’, ‘Pink Gumpo’ and ‘Delaware Valley’. Susceptible cultivars include ‘White Gumpo’, ‘Hinodegiri’, ‘Hershey Red’, ‘Coral Bells’, ‘Pink Pearl’ and ‘Hino Crimson’. Susceptible cultivars are the most widely planted, due to desirable horticultural characteristics. Resistant hybrids are not immune to disease, but more tolerant. Chemical control is generally not practical for the homeowner, and involves a drench or injection by a certified applicator.