Phytophthora Root and Crown Rot

Too much water can result in a lack of vigor or sudden death by Phytophthora root and crown rot. Phytophthora is Greek phytón (plant) and phthorá (destruction), so the name means the plant-destroyer.

Raspberry plants showing symptoms of Phytophthora root and crown rot (H. J. Larsen) CC BY 3.0 US

What is Phytophthora root and crown rot?

Phytophthora [Fie-TOF-ther-uh] is a family of water molds called oomycetes. Oomycetes fall somewhere between fungi and algae in the web of life. There are many different types of Phytophthora molds. They generally attack stems and roots. Stem damage occurs at or just above the crown, at the soil line, though it can appear elsewhere on a plant. These molds cause many plant diseases, including sudden oak death, potato blight, damping-off disease, and crown rot. Phytophthora root and crown rot can kill a tree or shrub if the soil remains wet for too long or when planted too deeply. [Moist soil around the trunk is never a good idea.]

Host plants

Nearly all fruit and nut trees, including cherries and kiwifruit, are susceptible to Phytophthora root and crown rot. But so are members of the nightshade and cabbage families. So, tomatoes, eggplant, and potatoes are vulnerable, as are cauliflower, Brussels sprouts, and cabbage. And all because of too much water.

Symptoms of Phytophthora root and crown rot

Plants affected by Phytophthora root and crown rot look drought stressed. Unfortunately, because the natural response is to provide more water - the last thing you want to do when Phytophthora is present. Symptoms start in just one branch or area of the affected plant before spreading. Leaves may turn purple or reddish. Plants may die suddenly or linger poorly for years before dying.

Symptoms can vary greatly, depending on the type and age of the plant, the plant’s genetic resistance to infection, overall health, soil temperatures, and moisture levels. The bark around the crown and upper roots of infected plants is dark. A dark sap or gum may ooze from damaged areas. Using a sharp knife, cut away an area of bark. Infected plants will show reddish-brown streaks or patches. Water-soaked areas on roots may also be visible. If white threads are visible between the bark and the inner layer or around the root system, it is Armillaria root rot.

​Preventing Phytophthora root and crown rot infestation

Proper water management is the best way to prevent and control Phytophthora root and crown rot. Never allow standing water to remain around tree and shrub trunks. Also, don’t let sprinklers hit tree trunks. These other tips can help you manage Phytophthora in your garden or landscape:

• Only use certified pest- and disease-free plants.
• Install plants at the proper depth.
• Avoid over-watering.
• Allow soil to dry out between waterings.
• Ensure proper  drainage.
• Quarantine all new plants for 4 to 6 weeks, including water that may drip from the containers.
• Sanitize tools, boots, and hands after handling potentially infected plants.
• Sanitize shoes with bathroom cleaner after walking through an infected area
• Do not add infected plants to the compost pile.
• Employ crop rotation to interrupt the disease triangle.

You may be able to maintain an infected plant with proper irrigation and good cultural practices, but it will never be the same. Phytophthora can stay in the soil for many years, so prevention is far easier than control.

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NOTE: One new-to-us variety, Phytophthora tentaculata, is on the Dept. of Agriculture’s watch list. If it appears in your garden or landscape, contact your local Cooperative Extension Office. They may have helpful advice to protect your plants and need to know where this disease is spreading.