There are several species of Phytophthora fungi that are known to attack landscape trees and shrubs. The Latin translation of the name Phytophthora as “plant destroyer” is appropriate. Tree and shrub diseases caused by Phytophthora are referred to as blights, cankers, dieback, root rots, and wilts.
Phytophthora diseases usually infect plants that are growing in poorly drained soils or receiving excessive irrigation. After the plant roots are infected the disease develops and grows in the cambium and sapwood of the plant. Plant decline may be rapid or slow, depending on the Phytophthora species involved, the vigor of the infected plant, and the particular planting location. A plant may not show symptoms of Phytophthora infection until months or years after the initial infection.
HOW TO CONTROL PHYTOPHTHORA DISEASE
Prevention is the best course of action - plant the right tree or shrub in the right place. Consult an arborist and consider site and soil conditions before tree and shrub planting or replacing trees or shrubs infected with Phytophthora disease. If the site is prone to excess soil moisture then plant a tree or shrub species that will tolerate these soil conditions. Make sure the soil pH is appropriate for the tree or shrub you want to plant. Reduce plant stress with periodic fertilization.
Cultural Control of Phytophthora
If possible, improve soil drainage. If soil drainage cannot be improved consider replacing plants with site-appropriate species. Do not replace with plant species susceptible to Phytophthora.
Avoid excessive mulching which will retain excessive soil moisture. Loose pine bark nugget mulch will allow the soil to breathe more than tight, or compact, Cypress mulch.
Avoid over watering. Excessive irrigation improves the movement of Phytophthora spores throughout the soil and may increase plant stress by restricting oxygen in the soil.
Root collar excavation. Pull back soil or mulch if it is touching the root collar of trees or shrubs.
Fungicide Control of Phytophthora
Preventative fungicide soil drench treatments are the most effective means to protect trees or shrubs from Phytophthora disease. Periodically treating susceptible tree or shrub species that are growing in wet soil conditions can minimize the possibility of new infections.
Mixed results have been obtained when trying to control Phytophthora on infected trees or shrubs. If Phytophthora is suspected as the cause of plant decline any nearby Phytophthora susceptible plants should receive a preventative fungicide treatment.
Lawson's Cypress killed by Phytophthora.
Phytophthora disease has infected the Pine on the right. Photo: Clemson University - USDA Cooperative Extension Slide Series, http://www.forestryimages
TREES & SHRUBS INFECTED BY PHYTOPHTHORA
Phytophthora disease infects a host of common landscape trees and shrubs including:
Andromeda
Arborvitae
Avocado
Azalea
Beech
Bottlebrush
Boxwood
Camellia
Cedar
Citrus
Cypress
Dogwood
Eucalyptus
Falsecypress
Fir
Forsythia
Hemlock
Japanese Holly
Juniper
Madrone
Malus spp.
Mountain Laurel
Oak
Pieris spp.
Pine
Prunus spp.
Red Maple
Rhododendron
Sugar Maple
Yew
Zelkova
SYMPTOMS
Symptoms vary depending on the host species, which Phytophthora species has infected the plant, the host plant location in the landscape, and even prevailing environmental conditions. Symptoms of Phytophthora infection include:
Sparse crown
Discolored roots
Wilting of foliage
Lack of fine roots
Reduced shoot growth
Twig or branch dieback
Smaller than normal leaves
Yellow or discolored leaves
Arborists usually rely on these plant symptoms and a process of elimination to identify Phytophthora as the cause of plant decline. The microscopic fruiting bodies would require sending the entire plant to a laboratory to determine the specific Phytophthora species infecting a plant.
Leaf blight caused by Phytophthora. Photo: Fk.
Fir tree killed by Phytophthora. Photo: John H. Ghent, USFS, Bugwood.org
