The most common form of root rot is caused by the soil-inhabiting water mold fungus Phytophthora. Many species are susceptible to this disease and in the home landscape, the most commonly bothered plants include azalea, rhododendron, Pieris, yews, mountain laurel, heather, and high-bush blueberries.

This disease is promoted by high soil moisture content and warm soils so the real problem is often poor drainage. Root rot is very unusual in well-drained soils such as sand and is very common in those high in clay content. Over-watering, unusually heavy rain seasons and hard pan soils may all contribute to the development of root rot.

When the roots rot away, it inhibits the plant's ability to take in water so, the result is wilt on the leaves and twigs above ground. Infected roots turn reddish-brown, become brittle and die.


 
  1. Proper Drainage - Plants that are especially susceptible to this root rot must be planted in sites that have good drainage. Sometimes amending the soil with porous material or planting in raised beds may avoid the problem.

  2. Planting Depth - Be sure to set plants in the soil at the same depth at which they were growing in the container or at their previous site. Planting too deep and burying the base of stem may stress plants and encourage the development of the rot in poorly drained sites.

  3. Resistant Plants - If a site has been plagued by root rot in the past, plant only species not susceptible to Phytophthora. Even within a genus or species, there may be cultivars that are more or less susceptible to the problem. The fungus stays around in the soil so new, susceptible plants are very likely to acquire the disease too.

Generally the most effective control of this root rot disease is to follow the preventative measures listed above. As a last resort, there are some soil drenching treatments that may be attempted. This is usually more effective in the nursery where the plants are all in separate containers with a small volume of soilless media. In the home landscape, these treatments are usually not very effective.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

Some plants susceptible and resistant to Pythophthera:
 
SUSCEPTIBLE TREES AND SHRUBS
Abelia
Abies - Fir
Acacia - Thorntree
Calluna - Heather
Castanea - Chestnut
Ceanothus - Wild Lilac
Cedrus - Cedar
Chamaecyparis - False Cypress
Cornus - Dogwood
Cupressus - Cypress
Daphne
Eleagnus - Olive
Erica - Heath
Eucalyptus
Fatsia - Aralia
Hibiscus
Hypericum - St. John'swort
Juglans - Walnut
Juniperus - Juniper
Larix - Larch
Myrtus - Myrtle
Picea - Spruce
Pieris - Andromeda
Pinus - Pine
Platanus - Planetree, Sycamore
Pseudotsuga - Douglas Fir
Quercus - Oak
Rhododendron - Rhododendron, Azalea
Salix - Willow
Sequoia sempervirens - Coast Redwood
Taxodium - Bald Cypress
Taxus - Yew
Thuja - Arborvitae
Viburnum

RESISTANT TREES AND SHRUBS
Chamaecyparis nootkotensis - Alaska cedar
C. pisifera - Sawara Cypress
C. thyoides - White Cedar
Daphne eneorum - Rock Daphne
Juniperus chinensis - Pfitzers
J. sabina - Savin
J. squamata - Meyer
Pinus mugo var. mugo - Dwarf mugho pine
Rhododendron obtusum - Hiryu Azalea
Thuja occidentalis - American Arborvitae

 

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