Common Problems & Disorders

The best defense against tree problems and disorders is a well-maintained and healthy tree. However, should your tree develop problems, these tips may help.

Here are the basic concepts of “integrated pest management,” an effective and environmentally sensitive approach to pest management:

  1. Put the right tree in the right place. Don’t force something to grow under conditions it can’t handle. FUF helps you with that decision when you plant through us.
  2. Improve the “cultural conditions” for the tree (water, light, pollution, wind and soil).
  3. Know your pest. Is it a bug, disease, or something else? Find out as much as possible so your efforts aren’t wasted.
  4. Choose the least toxic method of pest control. If improving cultural conditions isn’t sufficient, then try a non-chemical method of control. Chemicals are a last resort.
  5. Know your chemical, if you must use one. What specifically does it kill? What are the risks to you or the environment around you?

Here are some common problems and possible solutions:

SymptomPossible CausesSolution
Low vigor, wilting or leaf drop, leaf dieback (leaves die from the tip back) or very little new growth.Water stressSee our watering page for info.
Yellowish leaves. However, the soil may be swampy or even smell bad. Overwatering/poor soil drainage Choose an appropriate species; amend soil with organic matter to increase drainiage; raise tree if possible.
Leaves to appear yellowish, or chlorotic. Nitrogen Deficiency Adding organic nitrogen fertilizer on a regular basis can help – but don’t overdo it.
Leaves will yellow, but the veins remain green. This is known as iron chlorosis.
Example image.
Iron deficiency Wind is drying, so try increased watering if the soil drains well. Windburn is usually not fatal if the tree is generally tolerant of conditions. This is usually more noticed on young trees.
In rare cases of frost, some plants will partially or fully die back. Frost DiebackDon’t prune off the frost-burned branches until you are sure no further frosts are expected. The outer foliage is keeping the inner foliage warm. To prevent frost burn, keep plants well watered. Cover with an old sheet or towel at night if frost is expected, trying to leave an air gap between the sheet and foliage. Uncover during the day. Outdoor rated string lights ("Christmas lights") in the tree at night can also help, especially if it is covered. Jacaranda (Jacaranda mimosifolia) and New Zealand Christmas trees (Metrosiderus excelsus) are some of the more common San Francisco street trees subject to frost damage.
The leaves generally wilt and brown within 1-3 days and the tree does not recover. Chemicals dumped into tree basin This could include but is not limited to: painting chemicals, motor oil, cleaning solvents, etc. Painting chemicals seem to cause the most sudden and dramatic tree death when dumped into tree basins.

Be sure that any contractors you hire understand that they are not to dump any toxins in the tree basins (including the neighbor’s!) and that they are responsible for the cost of removing and replacing any trees that die if they do. If you suspect chemical dump, gather some soil with a trowel and smell it. You may detect a chemical odor.

The most common bugs

Here we describe pests and their effects, and list some least-toxic methods of control. (Some text adapted from the Department of the Environment website; see links below)

Aphids, Ants and Sooty Mold

Although sooty mold is not a bug, it is generally associated with aphids.

Aphids are among the most common garden pests, especially in the spring and summer. It’s not cause for panic, but you may want to control their population.

Identification

Insects on a leaf

Winged adult cotton aphid next to several green peach aphids (photo by Jack Kelly Clark).

Aphids are small (less than 1/4 inch long), soft-bodied insects that suck sap from leaves, twigs, or roots. They may be green, yellow, brown, red, or black depending on the species and the plants they feed on. They are pear-shaped insects with long legs and antennae. Adult aphids can be winged or wingless. They are often clustered on new growth.

Although aphids are most common in spring and summer, some species mate and produce eggs in fall or winter, which provides them a more hardy stage to survive harsh weather. Under ideal temperatures, many aphid species can complete their life cycle in less than 2 weeks, and because of their prolific reproductive capacity, enormous populations of aphids can build up in a short time.

Although aphids seldom kill a plant, the damage and unsightly mold growth they cause sometimes warrant control. Aphids cause curling, yellowing, and distortion of leaves and stunting of shoots. Moreover, aphids serve as carriers of viral diseases on certain vegetable and ornamental plants. In urban environments, aphids can produce copious amounts of “honeydew” excretion, which often turns black with the growth of a sooty mold fungus. Sooty mold is usually just unsightly, coating leaves with a black residue, but can kill a small tree that is already under stress.

Check plants frequently for aphids, including the undersides of leaves. Look for curled green leaves and/or wilted buds. Many species of aphids cause the greatest damage when temperatures are 65-80 F. The presence of ants often indicate aphids, because ants act as “farmers,” protecting the aphids, in order to harvest their honeydew excretions.

Treatment

First, is it a big deal? If there is little damage, some aphids are OK. Even in our urban environment, aphids have natural predators that will keep the population under control. We actually want some aphids in order to sustain the predators so they can reproduce to eat more aphids! The key here is not to panic if a few aphids are feeding on a tree.

The easiest thing to do is to wash off the aphids with a strong jet of water one morning per week (morning is best, allowing the leaves to dry during the day). Doing it more than once a week helps keep the population down. You can wash leaves when you’re watering your young tree, washing your car, or watering plants.

If aphids are causing sufficient damage to warrant further treatment, or if washing with plain water has not worked, we suggest one of two insecticides. Always follow manufacturer’s application instructions:

Safer Insecticidal Soap – A contact insecticide that is fully biodegradable. It works by smothering rather than poisoning.

Neem Oil – Works quickly by suffocating the aphids. Also serves as a repellant and a fungicide. Local nurseries, such as Sloat Garden Center, carry neem-based insecticides.

Scale

Scale insects are usually slow moving or don’t move at all. They may look like little bumps clustered on the plant. They are also often accompanied by ants. Their honeydew excretions can also cause sooty mold. There are many kinds of scale.

Treatment

Traps, such as Tanglefoot®, can be smeared on a stiff band of paper taped around the trunk and stakes, and can catch scale during its young “crawler” stage. Follow the manufacturer’s application instructions. Also, place it where pets or people will not brush up against it. Don’t leave anything on the tree long enough to strangle it as it grows!

Horticultural oils or soaps can also work if timed correctly. See more information and photos here.

The most common diseases/ailments

Fire Blight

Fire blight is a bacterial disease affecting plants in the rose family such as pear, cherry and apple. It’s characterized by branches looking black and wilted as if they have been burned by fire. See information and photos here.

Powdery Mildew

Leaves

Powdery mildew on crape myrtle shoot (photo by Jack Kelly Clark).

Powdery mildew appears as a fine white dusting on leaves, causing leaves to look whitish and a bit rumpled. It’s caused by cool, damp, foggy weather, of which San Francisco has plenty. Rain and direct sunlight inhibit powdery mildew. Often seen on London Plane/Sycamore trees in the city (Platanus acerifolia), it is generally cosmetic and not fatal. The ‘Yarwood’ variety of Sycamore is more resistant. See more information here.

Take control measures on highly susceptible species. Prune out infected tissue and dispose of it away from plants. Avoid excessive fertilization or irrigation – these promote susceptible new growth. Overhead sprinkling may reduce powdery mildew infection because spores cannot germinate, and some are killed, when plants are wet. Sprinkle plants in the mid-afternoon when most spores are formed; this allows plants to dry before nightfall, reducing the likelihood that sprinkling will promote other diseases. Fungicides are only effective in early stages, not once there is a lot of mildew. Fungicides should be specifically for powdery mildew, and only used as a last resort.

Chinese Elm Anthracnose

Most often seen on Chinese Elms (Ulmus parvifolia), this fungus causes branches to die back from the tips, causing a “frizzy hair” look to the tree (see photo). The ‘Drake’ variety of Chinese Elm is resistant to anthracnose, and is the most common variety. To control, prune out and dispose of infected twigs during the fall and winter. Fungicides have not been found to be effective.

Chinese Elms other than the ‘Drake’ variety may also get anthracnose canker, which looks like large wounds in the bark (see photo). Consult an arborist for control methods and consider replacing severely infected trees.

See more info about anthracnose here.

Shot Hole Fungus

Shot Hole Fungus affects Prunus species such as plum, almond and apricot. In San Francisco, it’s most often seen on Purple-Leaf Plum trees. It looks like little holes in the leaves. Our (relatively) warm, wet winters and wet springs encourage shot hole, but it is cosmetic and usually not fatal.

Sooty Mold

Leaves

Sooty mold on leaves (photo by Jack Kelly Clark).

Sooty mold is dark fungi that grow on plant surfaces that have been covered with insect honeydew. Leaves will look black and “dirty.” See aphids and scale info, above. Sooty mold in itself is generally harmless and can be ignored, except when it covers leaves so much they can’t get enough light. Fungicides are not effective; instead, wash leaves with a forceful stream of water and control the insects (usually aphids) that are producing the honeydew.

Other Pest Resources

Free Sick Plant Clinics

At Berkeley Botanical Garden, on the first Saturday of each month, 9 a.m. to noon, UC Berkeley plant pathologist Dr. Robert Raabe, UC Berkeley entomologist Dr. Nick Mills, and their team of experts will diagnose what ails your plants (a photo or bagged sample may help). The sick plant clinic is free with no reservations required. Get directions and info here or call 510-643-2755.

The San Francisco Botanical Garden sometimes has plant clinics. For more information, call 415-661-1316 x354.

Books

Pests of Landscape Trees and Shrubs, publication # 3359, published by University of California, has very good pictures and helpful tables matching symptoms to causes. It’s available online or by phone at 800-994-8849. You also might find it at the San Francisco Botanical Garden Bookstore.

Write for advice

Email Dr. Hort at home@sfchronicle.com or send a stamped, self-addressed envelope to Dr. Hort, c/o San Francisco Chronicle, 901 Mission St, San Francisco CA 94103.

Email the Plant Doctor at Strybing Arboretum at education@strybing.org or write to The Plant Doctor, c/o Leaflet, Strybing Arboretum Society, 9th Avenue at Lincoln Way, San Francisco, CA 94122.

Websites

For more information about other pests and their least-toxic control methods, visit these excellent sites:

San Francisco Department of the Environment

UC Davis Integrated Pest Management

UC Davis Integrated Pest Management En Español

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