Phytophthora Root and Crown Rot
R. C. Lambe, Blacksburg, Va.
Reprinted with permission from the Plant Disease Handbook,
a publication of the Extension Service, Virginia Polytechnic Institute.
Root and crown rot of several different woody ornamentals caused by the fungus Phytophthora is a common disease in poorly-drained soil or of plants that have been set too deeply in the soil. The most common ornamentals affected in Virginia are azalea, rhododendron and yew.
The fungus Phytophthora cinnamomi, the cause of Phytophthora root and crown rot, is moved from one area to another through the movement of diseased plants or infested soil and once established in the soil, can remain active for many years. The fungus is also native to some soils in the southeastern United States. When a susceptible host plant is present, the fungus invades the roots of the plant killing root tissue and eventually the whole plant.
In general, symptoms consist of retarded growth and "off-color" yellow foliage which is followed by sudden or gradual death of the entire plant. The most characteristic symptoms of the disease are-the roots die and become dark in color before they decay and dark streaks extend up into the wood of the lower stem. The lusterless foliage is commonly smaller in size than normal.
The fungus is distributed widely in the state and every precaution should be taken to prevent further distribution. Only healthy appearing plants should be selected. If susceptible plants like azalea, rhododendron and yew are planted in infested soil they will die, and if they are replaced with another susceptible plant, the fungus will readily affect the new one as well.
The soil should be well-drained and special care should be taken with yews, and most other woody shrubs to see that they are not planted too deep. The soil line should not be more than one inch over the upper roots. After planting the soil should not be mounded up around the stem since this too encourages injury from the root rot fungus.
Root rot may be checked by drenching the soil around the plant with the fungicide Dexon (70% W.P.) at a concentration of 1 teaspoon per 4 gal. of water. This quantity of diluted material is sufficient to treat an area of 20 square feet. Repeat applications are required at 10-14 day intervals throughout the normal growing season; failure to do so will lead to disappointing results.
Where plants have died and been removed there are several choices available to the home owner. If practical, remove all the infested soil from the location where the plant has died and replace with clean soil. The site can also be fumigated with one of the following fumigants.
Vorlex: Remove as much of the root system as possible and break up all clods and loosen soil thoroughly by cultivation before treatment. Be sure the soil is moist. Consult the container label for information on dosage rate, soil temperature at time of application, sealing of soil surface after chemical application, fertilization, and the aeration period needed prior to transplanting into fumigated soil.
Methyl bromide: The soil should be free of clods, loosened to a depth of 12" and in good seedbed condition. The soil temperature should be 40 degrees F. or higher at a 6" depth, with adequate moisture for seed germination. The chemical is introduced as gas under a gas-tight cover over the area being treated. Follow manufacturers' instructions on dosage rate and the aeration period needed prior to transplanting in the fumigated soil.