Module 3: Phytophthora ramorum
Module 2: Disease Management
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Life Cycle

Disease Cycle

Seasonal Activity

Species Profiles

Virtual Dying Plant

Diagnosis

Practice Questions 

Glossary

 

What is Phytophthora?

Healthy plant (left) and plant showing symptoms of Rhododendron root rot (right) caused by Phytophthora cinnamomi.

Photo by R. Linderman.

Sporangium of Phytophthora parasitica releasing zoospores
Sporangium of Phytophthora parasitica releasing zoospores.

Photo by F. Schwenk.

Potato plants devastated by late blight, a Phytophthora disease.

Photo by N. Grunwald.

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.


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

Lithocarpus*

tanoak

Lycopersicon*

tomato

Malus*

apple, crabapple

Medicago

alfalfa

Microbiota*

Russian cypress

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

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

Aerial view of Marin Co. (CA) homes surrounded by trees affected by sudden oak death caused by Phytophthora ramorum.

Photo by Marin Co. Fire Department.

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.

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