Phytophthora Root Rot on Shrubs and Trees

By Gene Sumi

There is a fungal root disease that is widespread in our region, but is little known to the average gardener.  Its targets are our favorite shrubs and trees, including rhododendron, pieris japonica, Japanese hollies, boxwoods, cherry laurels and yews.  It is Phytophthora cinnamomi, a soil fungus that attacks the roots of woody plants such as shrubs and trees.  It resides mostly dormant in our soil, but becomes active when the soil temperature warms to a temperature above 59° F,  and when the soil surrounding the plant has been unusually wet.  This fungus is referred to as a “water mold”, as it needs water-saturated soil to become mobile and reach nearby plant roots. Then the dormant fungus spores become active, attacking the cambium and sapwood of the roots to extract sap.  The feeder roots are killed in this process, so the plant loses the ability to support all the green growth with water and nutrients.

The sign that you see of the root destruction occurring underground is the quick death of individual branches, until few or no healthy branches remain.

Treatment
The best control for this particular fungus is to apply a soil drench of fungicide mixed with water, covering the entire root area of the affected plant.  The fungicide you should use is Agri-fos, made by Monterey Chemicals.  Mix up the drench in a watering can at a rate of 1/8 teaspoon of Agri-fos per gallon of water.  After applying the drench, water it in thoroughly with more water, to ensure full coverage of the plants root system.  Do a follow-up application of the drench in 14 days. Remove all dead branches and monitor the plant’s progress.  You will know if the fungicide drench was successful when no new branches die and  new green growth appears.

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