Species of fungus-like organisms belonging to the genus Phytophthora are soil-borne or plant-borne and all are plant pathogens. Some species of Phytophthora attack woody ornamentals including Arborvitae, Azalea, Dogwood, Forsythia, Fir, Hemlock, Holly, Juniper, Maple, Rhododendron, and White Pine. Other species attack herbaceous plants including Annual Vinca (Catharanthus), Begonia, Poinsettias, African Violets, and many other plants.


Symptoms:

 

Hostas with mild Root Rot:

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

Hostas 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

Hostas 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:

Soil moisture just below saturation allows sporangia to form in 4-8 hours, poorly drained or wet soil favors the pathogen. Zoospores infect feeder roots just behind the root cap.

Phytophthora overwinters primarily infected plant roots or stems and only to a small extent free of plant material in 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 this disease usually stays associated with infected plants, an important reason why it is isolated long distances is through the shipment of infected plants. Therefore when new plant are purchased, keep those plants together and observe them carefully for any symptoms.
  • Remove and destroy infected plants
  • Plant only in well-drained soil
  • If the area previously harbored Phytophthora, sterilize with a registered material before planting when soil temperatures are 50 F or warmer at 6" depth and when soil moisture levels are adequate for seed germination. Allow adequate aeration time.
  • Irrigate plants to prevent drought stress
  • Avoid overhead watering especially in late afternoon
  • Do not plant where infected plants were previously located
  • Do not plant too deep. Soil line should not be more than 1" over upper roots