Phytophthora Disease
There are approximately 40 species of Phytophthora fungi which cause blights, wilts, cankers, root rots, and dieback on many plant species throughout the world. The name Phytophthora means "plant destroyer".
Some of the common plant genera which may be affected include: maple, azalea, rhododendron, dogwood, madrone, oak, avocado, eucalyptus, pine, bottle brush, holly, yew, juniper, boxwood, chamaecyparis, cedar, cypress, and juniper.
SYMPTOMS: When the belowground portions of the root system are invaded by the fungus, it causes root decline. Root decline leads to these symptoms: a sparse crown, smaller, discolored leaves, wilting of the foliage, and die-back.
Collar rot, crown canker, basal canker, and root rot are all terms used to describe large lesions that develop at the base or major roots of plants. The inner bark, cambium, and sapwood associated with cankers may be a discolored reddish-brown.
Unfortunately, Phytophthora produces no fruiting bodies and the reproductive structures are microscopic. Arborists must rely on plant symptoms to identify this problem. Specific diagnosis requires microscopic examination of affected plant material.
CAUSE: Phytophthora exists as spores in soil and dead plant material. Spores can be dispersed in soil, diseased plants, infected plant debris, and splashing/running water. Spores require free water for swimming and germination. These spores are attracted by amino acids and other chemicals that exude from roots or other plant parts. Wounds and other tender, succulent plant parts are susceptible infection sites.
Infection by Phytophthora is promoted by soil conditions which lower the disease resistance of plants. Excessive moisture, improper pH, and poor nutrient levels all lead to plant stress.
SOL UTION: The best treatment, as with most insect and disease problems, is to keep trees healthy with proper care to prevent the problem from occurring. Proper fertilization, irrigation, mulching, and pruning will maintain the health of the plant and support its natural defenses against insect and disease pests.
Fungicides applied to the foliage or as a soil drench would be beneficial in managing Phytophthora diseases.