Tree Pests and Diseases

Listed below are a few common pests and diseases that you may encounter with your trees. If you are concerned about the health of a street tree, contact the City of Palo Alto Tree Section at 650-496-5953. If you are concerned about the health of a private tree, contact a certified arborist (refer to the Arborist List). The information below is from the Sunset “Western Garden Book”. Consult this book for further information.

Pests:

Whiteflies are small winged insects that fly up in clouds when you touch an infected plant. Signs of infection include yellow stippling at first, then leaves that curl and turn brown. To prevent whitefly infestations, carefully inspect indoor and greenhouse plants before planting them in your garden. Natural enemies of the whitefly include lacewing larvae, Delphastus pusillus (a species of lady beetle), Encarisa formosa and Eretmocerus californicus (both species of parasitic wasps.) You can also handpick heavily infested leaves, use yellow sticky traps, hose off the plant with a jet of water or use insecticidal soap, azadirachtin, horticultural oil or pyrethrins.

Aphids are small insects ranging from pinhead to matchhead size. They may be black, white, pink, or pale green. They cluster together on young shoots, buds and leaves. They pierce the leaves and stems of a plant and suck out its juices. Some may also transmit viral diseases. The best control tactic is to do nothing and leave the pests to natural controls. There are a number of creatures that prey on aphids including lady beetles, lacewing larvae, soldier beetles, syrphid flies, predatory midges, parasitic wasps, and even lizards and some birds. Aphids can also be blasted with strong jets of water or sprayed with insecticidal soap. Pesticides should only be used when the infestation is severe.

Thrips are almost microscopic. They feed by rasping soft flower and leaf tissue and sucking plant juices. They may also spread plant disease. Symptoms of a heavy infestation include flowers and leaves that are discolored and fail to open normally, and look twisted or stuck together. Leaves may take on a silvery or tan cast. Numerous small, black, varnishlike fecal pellets on leaf undersides indicate the presence of thrips. Natural enemies include lacewing larvae, minute pirate bugs, predaceous thrips, mites, and spiders. Insecticidal soap, horticultural oil, acephate and malathion can be used to control infestations.

Scales are related to mealybugs and aphids, but have a waxy, shell-like covering. They have a number of natural enemies including lady beetles. Scale-infested plants can also be hosed off frequently. Scales can be handpicked off plants or scraped off with a scouring pad. With deciduous trees, adult scales can be killed in the winter with horticultural oil. Horticultural oil, inseticidal soap and insecticides are effective against scales when they are in the crawler stage.

Diseases:

Anthracnose is caused by fungi and appears early in the growing season. It seldom kills plants. Symptoms vary but include sunken, gray or tan to dark brown spots on leaves, stems, fruit or twigs. Leaves may wither or drop. The spores that cause anthracnose are spread by rain and garden sprinklers. To discourage the disease, avoid overhead watering and use mulch to decrease splashing. Plant resistant varieties whenever possible. Remove infected leaves, fruit, twigs and branches and destroy them.

Fireblight is the result of a bacterial infection and attacks only members of the rose family that produce pomes. This includes (but is not limited to) apple, crabapple, hawthorn and pear species. It causes shoots to blacken and die suddenly. Plant resistant varieties whenever possible. Once the disease has appeared, prune out and discard diseased branches. Disinfecting tools can help keep the disease from spreading.

Oak Root Fungus results from an infection by the fungus Armillaria mellea. It destroys trees and other woody plants by gradually decaying their roots, eventually girding the plant. The first symptoms may be dull or yellowed leaves and/or sparse foliage. Leaves may then wilt and branches die. If a tree is infected with oak root fungus, you will see a layer of whitish fungal tissue under the bark of the trunk or large roots. Clumps of tan mushrooms may appear around the base of infected plants in late autumn or early winter. Plant resistant varieties whenever possible. A list is available in the Master Gardeners of Santa Clara County’s website. There is no chemical control.

Powdery Mildew is a group of diseases caused by fungi that infects leaves, buds, flowers and/or stems. It first appears as small, white or gray circular patches on plant tissue, then spreads rapidly to form powdery areas of fungus. New growth may be stunted and blossoms may fail to set fruit or may produce fruit covered with fungus. Plant resistant varieties whenever possible. To control the fungus once present, spray infected plants with jets of water early in the morning. Pick off and destroy infected leaves and flowers.

Verticillium wilt results from a fungus that invades and plugs the water-conducting tissues in the roots and stems of plants. A common symptom is the wilting or death of one side of the plant. Leaves turn yellow or brown, then die. Entire branches die as the disease progresses. Infected mature trees may linger on for long periods. The fungus can survive in the soil for years, even in the absence of host plants. Plant resistant varieties whenever possible. Mildly affected trees may recover. Aid recovery with deep but infrequent irrigation.

Sudden Oak Death Syndrome, or SODS is a forest disease caused by Phytophthora ramorum, a fungus-like pathogen. It can be fatal to some plants, including coast live oak, black oak, Shreve oak and tan oak. P. ramorum affects different species in a variety of ways, and is spread through a wide number of host species, including the California bay laurel and rhododendron varieties. The pathogen itself is spread by wind-blown rain. Read a full description of SODS.