What is Phytophthora?

Phytophthora (Fy-TOFF-thor-uh) species make up a group of microorganisms that are important plant pathogens. The name Phytophthora derives from Greek and literally means “plant destroyer.”

Phytophthora species resemble fungi but are not. While they are most closely related to aquatic organisms, such as brown algae and diatoms, they can also exist on land. However, Phytophthora organisms are often referred to as water molds because they do need water to complete their life cycle.

Certain species of Phytophthora cause important diseases in vegetable crops, fruit and nut trees, and forest trees as well as in nursery crops. Phytophthora disease is the number-one disease of nursery crops nationwide.

Notorious Phytophthora diseases include rhododendron root rot, sudden oak death, and potato late blight.

Phytophthora diseases have a large economic impact on the nursery industry. Phytophthora syringae results in more than $1 million in annual losses to shade tree production in Oregon alone. Phytophthora diseases on Rhododendron may affect up to 10% of plants; if a nursery becomes infected with Phytophthora ramorum, it could lose $90,000 to $1.3 million in costs for crop destruction and cleanup plus loss of sales.

Plants Susceptible to Phytophthora Diseases

Genus Common Name
Abies* true fir
Alnus alder
Arbutus madrone
Arctostaphylos kinnikinnick
Brassica cabbage
Buxus* boxwood
Calluna* heather
Camellia* camellia
Chamaecyparis* Port-Orford-cedar
Cornus* dogwood
Cotoneaster cotoneaster
Daphne* daphne
Erica heath
Fragaria* strawberry
Fraxinus* ash
Gaultheria salal
Ilex* holly
Juniperus* juniper
Kalmia mountain laurel
Lycopersicon* tomato
Malus* apple, crabapple
Medicago alfalfa
Microbiota* Russian cypress
Notholithocarpus* tanoak
Picea* spruce
Pieris*  andromeda
Pinus* pine, mugo pine
Prunus cherry
Pseudotsuga Douglas-fir
Pyrus pear
Rhododendron* azalea, rhododendron
Rosa rose
Rubus* raspberry
Solanum* potato
Syringa* lilac
Tsuga* hemlock
Vaccinium* blueberry, cranberry
Viburnum viburnum

* Commonly infected by Phytophthora