Phytophthora Root Rot On Woody Ornamentals

Some species of Phytophthora attack woody ornamentals, including arborvitae, azalea, beech, Chamaecyparis, dogwood, forsythia, Fraser fir, hemlock, Japanese holly, juniper, maple, Pieris, Rhododendron, Taxus, and white pine. Other species attack herbaceous plants, including annual vinca (Catharanthus), begonia, poinsettias, African violets, and many other plants.

Symptoms

Hosts with Mild Root Rot

• smaller than normal foliage
• foliage with nutrient deficiency-like symptoms
• dead feeder roots
• dark streaks up stem

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

Hosts with Cankers or Stem Rot

• slightly sunken dead area just under the bark (in the cambium)
• wood under the outer edge of the canker may be pink or reddish-brown in color
• bleeding from various points in the cankered area but the ooze does not smell badly
• stems and branches of herbaceous plants collapse

Conditions Favoring Development

Infection can occur over a wide range of temperatures: 15°C (59°F) to 28°C (82°F). 22°C (71°F) is optimum.

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

Phytophthora overwinters primarily in infected plant roots or stems, and only to a small extent, free of plant material in the soil. The fungus can be splash-dispersed during heavy rains or overhead irrigation. It can be carried in run-off from plant to plant in the field or from an infected plant to the drain holes of containers of nearby healthy plants, particularly if containers share puddles of water for an extended period.

Management

• Because Phytophthora usually stays associated with infected plants, a very important reason why it is dispersed long distances is through the shipment of infected plants. Therefore, when new plants are purchased, keep those plants together and observe them carefully for any symptoms. Do not co-mingle plants from different sources, so that if Phytophthora is detected, you will be able to determine its source. Do not recycle the irrigation water for these plants until you are certain they do not harbor Phytophthora. (If Phytophthora ramorum is detected in plants, ALL plants in that shipment and all plants sharing recycling water will be ordered destroyed.)
• Grow resistant cultivars in any areas known to harbor Phytophthora. See the list below.
• Disease prevention must be the primary goal since no chemicals cure a Phytophthora-infected plant.
• Remove and destroy infected plants.

Nursery - Field

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

Nursery - Container

• Use a well-drained, pasteurized potting mix. Sand should not be considered sterile. Composted hardwood bark not only drains well, but it also usually contains other microbes that inhibit Phytophthora.
• Use clean containers
• Place containers on an area that has been graded to ensure drainage away from the growing area. Or place containers on a 7–10 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 underwatered.
• Irrigate plants to prevent drought stress. Drought-stressed, Phytophthora-resistant plants lose their resistance and can be attacked.
• Certain chemicals can protect healthy plants.

Final Planting Site - Home or Commercial Planting

• Plant only in well-drained areas or grade or tile the site to ensure good drainage.
• Do not plant where Phytophthora-infected plants were previously located.
• Do not plant too deeply. Soil line should not be more than 2.5 cm (1 in.) over the upper roots.

References

1. Benson, D. M. 1982. Phytophthora Root Rot - Phytophthora cinnamomi (p. 14–17); and Rhododendron Diseases (p. 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.
2. 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.

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

Prepared by Gary W. Moorman, Professor Emeritus of Plant Pathology