WSU Extension


Annosus root rot 
Phytophthora root rot 
Hemlock scale 
Hemlock woolly adelgid 

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Caption: Dieback on creeping hemlock due to Phytophthora root rot
Photo by: R.S. Byther
Hemlock : Phytophthora root rot
(revision date: 4/28/2014)

Phytophthora root rot is usually a problem only in areas with poor drainage or where flooding occurs. The fungus attacks the roots, which rot and die. Phytophthora root rot decreases the number of lateral roots, especially on seedling trees, and causes the existing roots to turn red-brown inside. The infection moves into the crown, where the cambium (soft inner bark) turns reddish-brown instead of the normal greenish-white. Lower branches may wilt, turn brown, or die back. Young trees are often killed outright, while infected mature trees may show brown or yellow needles, wilting, branch dieback, or other signs of inhibited water and nutrient uptake. Western hemlock (Tsuga heterophylla) and mountain hemlock (T. mertensiana) are particularly susceptible.
Management Options

Non-Chemical Management
  • Plant only healthy, disease-free materials.
  • Do not plant in waterlogged, poorly drained, or frequently flooded areas.
  • Improve soil drainage by incorporating organic material or use raised beds.
  • Avoid soil compaction, which reduces drainage. Construction, heavy foot traffic, or machinery can compact soil.
  • Remove and destroy all infected plants and plant debris.
  • Plant resistant or tolerant species in affected areas. Your county extension agent or WSU Master Gardeners can make recommendations.
Select non-chemical management options as your first choice!

Chemical Management

None recommended


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Caption: Dieback on creeping hemlock due to Phytophthora root rot
Photo by: R.S. Byther
Caption: Phytophthora root rot infecting trunk
Photo by: R.S. Byther