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Oak Ailments: Identification and Treatment
Native California oaks can generally fight off most infections and pests, especially when growing under natural conditions. They are subject to problems in an irrigated setting or a disturbed area. Knowing which problems to treat will allow you to achieve success with the least toxic treatment. Trees in a state of decline cannot afford any leaf loss and any problem must be treated. Root collar diseases should almost always be treated. But healthy, vigorous trees can usually endure a cycle of insect infestation without damage to their long–term health. Proper management of cultural conditions (soil, water,and air) is the best preventative.
FUNGAL INFECTIONS
Crown Rot (Phytophtora, Pythium): Too much moisture around the base of the tree stimulates this water mold fungus, which attacks roots. Infected trees decline slowly and foliage becomes increasingly sparse. Live oaks are most susceptible. Don’t plant under oaks with plants that require summer water, and don’t raise soil level around the crown (base of trunk). Both practices encourage crown rot. If symptoms indicate crown rot, stop all watering between trunk and drip line (the ground directly below the outer edge of the leaf canopy). Pull back soil to expose infected parts to air.
Oak Root Fungus (Armillaria mellea): Our oaks experience little damage from this disease under natural, dry summer conditions. When oaks are watered in the summer or weakened by other impacts, the tree can suffer damage from the fungus. Symptoms shown by an infected oak include die-back of branches and yellowing, smaller leaf size, and thinning of foliage. The fungus itself may appear as honey colored mushrooms at the base of the tree or a white, fan-like growth under the bark. Uncover the infected root crown and leave it exposed to air. Stop watering within the drip line. It is usually best to consult a certified arborist.
Branch Dieback (Diplodia): Diplodia is encouraged by dry, hot summers. Branches from ½” diameter to 2” diameter are infected and die. The attached leaves turn brown and remain on the tree. Diplodia can kill large parts of the canopy. Treatment requires removal of diseased wood and spraying with 4 pounds of Fungo-Flow in 100 gallons of water in the spring.
Twig Dieback (Cryptocline or Discula): These fungi can destroy up to 90% of new growth in wet years. 2-3” long twigs and the attached leaves die, turning a straw brown color and remaining on the tree. Dieback of leaves and shoots appears scattered throughout the canopy with the heaviest infestation occurring in the lower canopy. Pruning out dead twigs minimizes damage but can’t prevent it. No treatments have been found that eradicate either twig blight pathogen. Some control can be achieved by pruning infected twigs and applying benomyl fungicide as a foliar spray. The treatment resulting in the most effective control is: (1) prune out all blighted twigs in fall (October- November); (2) spray with benomyl within 7 days of pruning; (3) spray with benomyl again at budbreak (February-March). Oak twig dieback occurs on coast live oaks and valley oaks. The infection pattern among nearby trees is unpredictable; trees untouched by the disease may grow right beside ones that are severely afflicted.
Insect Infestations
Oak Moth, California Oak Worm (Phryganidia californica): Adult moths lay eggs in live oaks twice a year, in pring and fall. Young larvae feed on the surface of leaves till about half grown, and then eat through the entire leaf. Severe infestations can almost defoliate oaks. The larva has a brown or reddish head and an olive green brown body with black and yellow stripes running lengthwise on back and sides. In the spring look for little green droppings from feeding larvae; May through June and again in September and October. The oak moth is subject to attack by many natural enemies, including parasites, predators and a wilt disease. These natural predators may be important in limiting the damage done by oak moth larvae. If leaf damage is unacceptable, Bacillus thuringiensis is an effective control when applied to coincide with the appearance of young larvae.
Pit Scale (Asterolecanium): Often associated with oak twig blight, pit scale attacks oak twigs. The pinhead-size insects that cause pit scale are green, golden, or brown. They live immobile, in a depression in the bark. By sucking the sap, they reduce an oak tree’s vigor. Poor growth and dieback of twigs are common results from infestation. Dieback is most noticeable in the summer and early fall. Affected twigs of deciduous oaks retain their leaves throughout winter. Severe infestation delays leafing out of deciduous oaks in spring. Control mature pit scale with sprays of horticultural oil in January.
Oak Twig Girdler (Agrilus angelicus): The oak twig girdler attacks live oaks, infecting twigs that are usually less than ½” in diameter. When the twig is girdled, moisture and nutrients are cut off and patches of dead leaves quickly appear. When the twig bark is peeled back, girdler burrows are exposed. The burrows are spiral-like and filled with brown, powdery frass (boring dust). The adult beetle is ¼” long, slender and brownish bronze. Mature larva are ¾” long, legless and white. The life cycle covers two years. Adults emerge during May through July, feed on leaves, mate, and then lay eggs. The eggs hatch in 2-3 weeks and the young bore into the twig. This means there is only a one-month period in which the adult insect is outside the twig and can be reached with a contact spray.
Sycamore Bark Moth (Ramosia resplendens): The adult is a black, clearwing moth with yellow bands, like a yellow jacket. Larvae are ¾” long, white with flat heads. They feed primarily in the phloem tissue of older trees or drought stressed younger trees. Symptoms appear as broken, crosschecked bark, usually near the base of the trunk. Removal of bark exposes ½” diameter tunnels in phloem tissue. Broken pupal cases may be seen at exit holes.
Western Oak Bark Beetle (Pseudopithiphorus agrifoliae): Adults are cylindrical, brown beetles 1/16” long. Eggs are laid in the spring or early summer. The larvae tunnel into the phloem tissue. Look for 1 to 1- ½ diameter black, wet-looking spots with a white dot (egg laying sites) on the underside of branches and trunk. If left uncontrolled, they can be a serious pest. Most commonly found on newly transplanted, large specimens, or distressed trees. Treat when adults are present.
Gall Insects (Cynipid wasps): Galls on oaks are common on twigs, leaves, flowers and roots, and in acorns. They assume different sizes, shapes and colors, from rosy, pea-size blobs to apple-size goiters, depending on the insect that makes them. Cynipid wasps are seldom a serious menace to trees, although certain species puncture leaf tissue to deposit eggs, causing leaves to turn yellow or brown.
Insect infestations in otherwise healthy trees are generally little more than nuisances. Coupled with other stresses, however, they can develop into serious problems. For more information see A Field Guide to Insects and Pathogens of California Oaks published by the USDA, available online as a PDF file.
