Phytophthora Root Rot on Woody Ornamentals: Host Range, Symptoms, and Integrated Management

Phytophthora on African violet

Pathogen and Hosts

Fungus-like organisms in genus Phytophthora are soil- or plant-borne plant pathogens. Some species attack woody ornamentals: arborvitae, azalea, beech, Chamaecyparis, dogwood, forsythia, Fraser fir, hemlock, Japanese holly, juniper, maple, Pieris, Rhododendron, Taxus, white pine. Others attack herbaceous plants: annual vinca (Catharanthus), begonia, poinsettias, African violets, many others.

Symptoms

Mild Root Rot

• Smaller-than-normal foliage
• Foliage with nutrient deficiency-like symptoms
• Dead feeder roots
• Dark streaks up stem

Severe Root Rot

• Stunting of entire plant
• Wilting
• Smaller-than-normal leaves/needles
• Reddish-brown discoloration of wood at soil line
• Greatly reduced root systems
• Roots with reddish-brown discoloration
• Dead feeder roots
• New shoots do not develop
• Plant death

Cankers or Stem Rot

• Slightly sunken dead area under bark (in cambium)
• Wood under outer canker edge pink/reddish-brown
• Bleeding from various points in cankered area (ooze not foul-smelling)
• Stems/branches of herbaceous plants collapse

Conditions Favoring Development

Infection range: 15°C (59°F) to 28°C (82°F); optimum 22°C (71°F). Soil moisture just below saturation allows sporangia formation in 4–8 hr and motile zoospore release in 10–60 min. Poorly drained/wet soils favor pathogen. Zoospores infect feeder roots just behind root cap. Soil pH plays little role.

Phytophthora overwinters primarily in infected plant roots/stems, minimally free in soil. Splash-dispersed during heavy rains/overhead irrigation. Carried in runoff plant-to-plant in field or from infected plant to container drain holes of nearby healthy plants, especially if containers share puddles.

Management

• Because Phytophthora stays associated with infected plants, long-distance spread often via shipment of infected plants. When purchasing new plants, keep together and observe carefully for symptoms. Do not co-mingle plants from different sources so source can be traced if Phytophthora detected. DO NOT recycle irrigation water until certain they don't harbor Phytophthora. (If P. ramorum detected, ALL plants in shipment and sharing recycled water will be ordered destroyed.)
• Grow resistant cultivars in areas known to harbor Phytophthora (see list below).
• Disease prevention primary goal—no chemicals cure infected plants.
• Remove and destroy infected plants.

Nursery - Field

• Plant only in well-drained locations.
• If area previously harbored Phytophthora, fumigate with registered material before planting when soil temp ≥10°C (50°F) at 15 cm (6 in.) depth and soil moisture adequate for seed germination. Allow adequate aeration. (Requires specific pesticide license.)
• Irrigate to prevent drought stress. Drought-stressed, resistant plants lose resistance and can be attacked.
• Avoid overhead watering, especially late afternoon.
• Avoid using runoff water for irrigation.
• Certain chemicals protect healthy plants.

Nursery - Container

• Use well-drained, pasteurized potting mix. Sand not sterile. Composted hardwood bark drains well and usually contains microbes inhibiting Phytophthora.
• Use clean containers.
• Place containers on graded area ensuring drainage away from growing area, or on 7–10 cm (3–4 in.) thick gravel/well-drained material. Black plastic under gravel prevents weeds.
• Group plants by water requirement to avoid over/under watering.
• Irrigate to prevent drought stress (drought-stressed resistant plants lose resistance).
• See protective chemicals below.

Final Planting Site - Home/Commercial

• Plant only in well-drained areas or grade/tile for good drainage.
• Do not plant where Phytophthora-infected plants were previously located.
• Do not plant too deeply—soil line ≤2.5 cm (1 in.) over upper roots.

References

1. Benson, D.M. 1982. Phytophthora Root Rot - Phytophthora cinnamomi (pp. 14-17); Rhododendron Diseases (pp. 69-71). In Diseases of Woody Ornamental Plants and Their Control in Nurseries. R.K. Jones & R.C. Lambe, eds. NCSU.
2. Lambe, R.C. & R.E. Baldwin. 1975. Phytophthora root rots and wilt of rhododendron, azalea, related ornamentals. Plant Disease Control Notes. Virginia Tech. 6 pp.

Resistance of azalea cultivars to root rot caused by Phytophthora cinnamomi (from Jones & Benson 1982, NCSU):

Resistant	Susceptible	Highly Susceptible
Formosa (I), Fakir (GD), Corrine Murrah (BA), Merlin (GD), Hampton Beauty (P), Higasa (S), Glacier (GD), Rose Greeley (G), Polar Seas (GD), Redwing (I), Chimes (I), Alaska (R), New White (I), Shin-Ki-gen (S), Rachel Cunningham (BA), Pink Gumpo (S), Eikan (S), Sweetheart Supreme (I), Morning Glow (K), Barbara Gail (P), White Gumpo (S), Rentschler's Rose (W), Dorothy Gish (R), White Gish (R), Pink Hiawatha (P)	Margret Douglas (BA), Gaiety (GD), Gloria (R), Kingfisher (W), White Christmas (W), Sensation (P), Prince of Orange (I), White Jade (BA), Copperman (GD), Hexe (K), Massasoit (K), Martha Hitchcock (GD), China Seas (G), Warbler (W), California Sunset (I), Amaghasa (S), Pride of Summerville (I), Hinodegiri (K), Flanders Field (P)	Robinhood (GD), Hershey Red (K), Herbert (K), Fortune (P), Catawba (GD), Marion Lee (BA), Snow (K), Royalty (G), Kow-Ko-Ku (S), Rosebud (G), Mrs. G.G. Gerbing (I), Coral Bells (K), Treasure (GD), Pat Kraft (BA), Saint James (BA), Carror (N), Purple Splendour (G), Pinocchio (GD), General MacArthur (K), Pink Pearl (K), Johga (S), Sunglow (N), Hino Crimson (K), Elaine (N), Emily (N), Pink Cloud (N), Adelaide Pope (N), Jane Spaulding (N)

• BA = Back Acres
• N = NCSU
• G = Gale
• P = Pericat
• GD = Glenn Dale
• R = Rutherford
• I = Indian
• S = Satsuki
• K = Kurume
• W = Whitewater

Phytophthora bleeding canker on beech; Apple bait for Phytophthora detection; Discoloration of base of rhododendron from Phytophthora; Dieback of Ilex (Shamrock holly) from Phytophthora root rot; Phytophthora on begonia. Prepared by Gary W. Moorman, Professor of Plant Pathology.