I'm a pretty upbeat person, but as I was reading over this column, I thought, "This is a real downer!" It's all about plant diseases and pests. Unfortunately, they are a fact of every gardener's life. So you might want to drink some chamomile tea and do some calming meditation before you tackle this column, rated D for depressing. Then afterwards, drive to the Idea Garden west of the hospital for a real spirit-lifter. On a brighter note, this knowledge might save some of your favorite plants.

I received a call from a friend who had noticed sploches on the leaves of their huge beloved oak tree and worried it was a goner. After a few minutes of research on the internet, I was able to call her back with reassuring news:

Oak leaf blister

Oak leaf blister is a fungal leaf spot disease common on all oak species. Severe leaf blister can result in widespread early defoliation. On well-established trees, early defoliation will not cause tree death, but will reduce tree vigor. Chemical control measures are usually not recommended.

Symptoms:

Early in the spring, small, rough (concave-convex) spots appear as the oak leaves expand. The spots become pale green in color and somewhat thickened. Older spots are brown or greenish brown. Leaves with numerous spots will fall prematurely. If well-established trees defoliate before midsummer, they will sometimes leaf out later in the season. When defoliation occurs in the late summer, leaf loss will have little impact on the tree's health.

Verticillium wilt

IDENTIFICATION — If the leaves of your annual flowers and vegetable plants seem to be yellowing and curling, your garden may be infected with verticillium wilt. Older, lower leaves fall off first, as younger, higher leaves become discolored and distorted. Only a few healthy leaves will remain at the top of the plant until it eventually dies.

To make sure wilt is the culprit, cut an infected plant stem near the soil line, then slice the stem lengthwise. Look for a narrow tan ring between the center and stem wall, or dark streaks shooting up the middle of the stalk.

CONTROL — Plants infected with verticillium wilt should be destroyed promptly because there is no control. And make sure to keep far away from your compost pile. The disease-causing fungus can live in the soil many years after the plant has died, so rotate your crops.

Protect your garden from wilt by buying resistant plants. Look for labels "VFN resistant," which indicates less susceptibility to verticillium wilt (V), fusarium wilt (F) and nematodes (N), which are all common pests.

Verticillium wilt also attacks some trees and shrubs. In large woody plants, the fungus causes leaves to yellow and curl and branches to die suddenly. If you suspect verticillium wilt has infected a tree or shrub, cut a twig from the infected area. Look for the ring in the cross section; there may also be streaking along the length of the twig if you peel back the bark. Before removing a valuable tree, consult with a plant pathologist to be sure of your diagnosis.

Fireblight

IDENTIFICATION – This aptly named infectious disease makes a tree look scorched, with leaves and stems withered to brown or black. Many stems curl over in a distinctive "shepherd's crook." Infected leaves stay on a tree long after healthy leaves have fallen off in autumn.

A sure sign of fireblight: Diseased wood under the bark is wet and rust-colored. When it moves into larger limbs, fireblight forms dark sunken cankers, which ooze a creamy to amber-colored liquid filled with millions of bacteria. The bacteria overwinter in the cankers and spread quickly as temperatures rise and rainfall increases in spring.

DAMAGE – Fireblight is extremely infectious. It affects more than 75 species, including pears, apples, hawthorns and roses. Bacteria enter through any opening, from blooming flowers to the smallest of cuts. The disease is spread by insects, birds, wind, rain and humans. It moves quickly, searing portions of a plant within weeks. Cankers in main limbs can girdle and eventually kill a plant.

CONTROL – Fireblight occurs most often in new growth. For prevention, remove unwanted suckers and waterspouts quickly, and don't overfertilize or overprune susceptible plants. When the weather is dry in spring, you can spray plants with streptomycin (Agri-mycin®) or other bactericides to help prevent infection. Apply copper compounds (like lime sulfur) during the plant's dormant period.

If a plant is showing symptoms, prune diseased areas right away. Cut 10 to 15 in. below the base of infection. Make sure you disinfect your pruners after each cut, using a 10 percent chlorine bleach solution (1 part bleach to 9 parts water). Burn or bury infected branches and leaves when you're done. If the damage has spread to the trunk or main stems, it might just be easier (and cheaper) to remove the infected plant and replace it with a more resistant cultivar.

Spider mites

Identification — Mite populations thrive in dog days of summer. These tiny brown-gray insects, no bigger than specks of dust, live on plants and suck the sap (and sometimes the life) out of leaves. You may notice yellowing, dropping or a light webbing on the leaves or stems. One way to tell if a plant is stressed because of lack of water or because of mite infestation is to place a piece of white paper under a damaged stem. Shake the stem and look closely at what falls on the paper. "Dust specks" that move are probably mites.

Control — Mites thrive on dusty plants, so use a strong stream of water to knock the dust and the mites off plants. Keep plants well watered; healthy plants are better able to replenish valuable fluids taken by the mites. Spray weekly with insecticidal soap (available in garden centers) for three consecutive weeks.

Powdery mildew

IDENTIFICATION – In mid- to late summer, you may notice a gray-white film on roses, phlox, zinnias, bee balm or lilacs, and the plant may begin to lose leaves.

This condition won't kill the plant, but it's not very attractive. Symptoms look the same, but specific fungi cause powdery mildew on different plants. In other words, the fungus that causes these symptoms on roses won't cause the same disease on zinnias.

FAVORITE CONDITIONS – Humid weather, with warm days and cool nights, encourages powdery mildew.

CONTROL – As with many plant diseases, the easiest way to control powdery mildew is to plant resistant cultivars. But there are a few things you can do to help your existing plants. The spores survive the winter in leaves and plant debris, so dispose of infected plants to get rid of as many spores as possible. Make sure your plants have enough sun and good air circulation (not too crowded).

Fungicides are very effective for preventing this fungus. Read the label to make sure the fungicide controls powdery mildew, not downy mildew, a different disease. You'll probably need to apply it more than once, starting before you see symptoms. You can't "cure" powdery mildew — prevention is key. Spraying horticultural oil on plants until the leaves are covered, top and bottom, cuts down on the problem. Test this on a few leaves first, to make sure the leaves don't burn. Wait for a cloudy day to spray anything on your plants — the combination of sun and sprays can damage foliage.

Hope you have survived this D-rated column!

If you have any horticulture questions, call the U of I Extension office 345-7034. Volunteer Master Gardeners are in the office on: Monday—2 to 4 p.m.; Wednesday and Friday—9-11 a.m.

This column is based on information and materials at the University of Illinois Extension office, located at 707 Windsor Road, Suite A., Charleston, 61920; phone 345-7034; or web site: www.extension.uiuc.edu/coles/