Phytophthora Crown and Root Rot

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PHYTOPHTHORA CROWN AND ROOT ROT — Chuck Ingels, UC Farm Advisor

Now that the summer is really here, Master Gardeners are getting questions about why a whole tree is suddenly dying or dead. There could be many reasons why whole trees decline, including drought or overwatering, scale or other insects, root or trunk damage, or diseases.

When excess water is excessively applied to heavy soils, simple anaerobic conditions can kill roots. But often Phytophthora crown or root rot is to blame. IDENTIFICATION: There are many species of Phythopthora, and all are soil-inhabiting fungi. They attack many different types of plants, and fruit trees are particularly susceptible.

The organism requires saturated soil to reproduce and then to infect the host plant; that’s why the disease is usually associated with heavy soils and poor drainage. Symptoms include poor growth, general weakness, cankers, twig dieback, a sparse canopy, and early leaf fall. Almost positive identification can be made by digging some soil from around the trunk and cutting into the bark to the cambium layer. If there is a clear margin of dead tissue just below ground level, with healthy tissue above the margin, it’s probably Phytophthora.

PREVENTION: To prevent Phytophthora, adequate drainage must be provided, especially around the crown (the trunk at soil level). If possible, susceptible species should be planted on mounds so that water does not collect around the crown. For a previously planted tree do not add soil around the tree crown to create a mound or create a basin that causes water to settle at the crown area.

MANAGEMENT: Sometimes trees showing early symptoms can be saved if the canker hasn’t progressed all the way around the trunk. The technique is to remove a fairly large amount of soil from the area of the crown and upper roots for several weeks or months. This dries the wood and stops the spread of the disease, allowing the tree to recover and develop new conductive tissues. But unless the cause of the problem is remedied (improve the drainage), the disease will be back.