Phytophthora Root Rot

Several species of fungi belonging to the genus Phytophthora cause root rot of herbaceous and woody ornamentals. Phytophthora cinnamomi attacks woody ornamentals including arborvitae, azalea, Chamaecyparis, dogwood, forsythia, Fraser fir, hemlock, Japanese holly, juniper, Pieris, Rhododendron, Taxus, and white pine. Phytophthora spp. are soilborne fungi, all known species of which are plant pathogens.

Symptoms

• Hosts with mild root rot
  • smaller than normal foliage
  • dead feeder roots
  • dark streaks up stem wood
• Hosts with severe root rot
  • stunting of entire plant
  • wilting
  • smaller than normal leaves or 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
  • death of plant

Conditions Favoring Development

Infection can occur from 15 degrees C to 28 degrees C (59-82 degrees F). 22 degrees C (71degrees F) is optimum.

Soil moisture just below saturation allows sporangia to form in 4 to 8 hours and motile zoospores to be released in 10 to 60 minutes. Therefore, poorly drained soil or wet sites favor the disease. Zoospores infect feeder roots just behind the root cap. Soil pH plays little role in this disease.

Phytophthora overwinters in the soil. The fungus can be splash-dispersed during heavy rains or overhead irrigation. Also, the fungus can be carried in runoff from plant to plant in the field or from an infected plant to the drain holes of containers of nearby healthy plants.

Management

Purchase Phytophthora-free plants. Grow resistant cultivars. See the list below. Disease prevention must be the primary goal since no chemicals cure this disease. Remove and destroy infected plants.

• Nursery--Field
  • Plant only in well-drained locations.
  • If the area previously harbored Phytophthora, fumigate with methyl bromide-chloropicrin before planting when soil temperatures are 10 degrees C (50 degrees F) or warmer at 15 cm (6 in) depth and when soil moisture levels are adequate for seed germination. Allow adequate aeration time.
  • Avoid overhead watering especially in late afternoon.
  • Avoid using runoff water for irrigation.
  • See chemicals below which can protect healthy plants.
• Nursery--Container:
  • Use a well drained sterile mix. Sand should not be considered sterile. Composted hardwood bark not only drains well, it inhibits Phytophthora development.
  • Use clean containers
  • Place containers on an area that has been graded to insure drainage away from the growing area. Or, place containers on a 7- to 10-cm (3-4 in) thick bed of gravel or other well drained material. Black plastic under this bed will prevent weed growth.
  • Group different types of plants by water requirement so that plants are not over or under watered.
  • See chemicals below that are used to protect healthy plants.
• Final Planting Site--Home or Commercial Planting:
  • Plant only in well-drained areas or grade or tile the site to insure good drainage.
  • Do not plant in locations of previously Phytophthora-infected plants.
  • Do not plant too deeply. Soil line should not be more than 2.5 cm (1 in) over upper roots.

Chemicals Used to Protect Healthy Plants

Read the product label to be certain the plant to be treated is listed. If nearby plants were infected with Phytophthora, remove those and make more than one soil drench application of a chemical listed below as instructed by the label to protect the healthy plants.

COMMON NAME	TRADE NAME
etridiazole	Truban, Terrazole
etridiazole + thiophanate methyl	Banrot
fosetyl-Al	Aliette
metalaxyl	Subdue
propamocarb	Banol

References

Benson, D. M. 1982. "Phytophthora root rot: Phytophthora cinnamomi" (pp. 14-17); and "Rhododendron diseases" (pp. 69-71). In Diseases of woody ornamental plants and their control in nurseries. Ed. by R. K. Jones and R. C. Lambe. North Carolina Agricultural Extension Service, NCSU, Raleigh.

Lambe, R. C. and R. E. Baldwin. 1975. "Phytophthora root rots and wilt of rhododentron, azalea, and related ornamentals." Plant Disease Control Notes. Extension Division, Virginia Polytechnic Institute and State University, Blacksburg. 6 pp.

The following list is from: R. K. Jones and D. M. Benson. 1982. "Phytophthora root rot and its control in nurseries." Plant Pathology Info. Note #202. Dept. of Plant Pathology, North Carolina State University, Raleigh.

Resistance of azalea cultivars to root rot caused by Phytophthora cinnamomi:

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