Green Scene: El Niño could increase garden pests this spring

The El Niño effect: Rhododendrons are susceptible to root-rot diseases during periods of excessive moisture.
The El Niño effect: Rhododendrons are susceptible to root-rot diseases during periods of excessive moisture. (Sandra Leavitt Lerner For The Washington Post)
  Enlarge Photo    
By Joel M. Lerner
Saturday, January 30, 2010

El Niño is back. The National Oceanic and Atmospheric Administration Climate Prediction Center reports that El Niño has gained slightly above-average strength over the Pacific Ocean since December, signaling that its effects might persist into the spring. What that means to gardeners in the Washington area and the Southeastern states is the possibility of wetter weather than usual.

I'm sure that spring will be glorious, as it always is in this region. But a wet El Niño winter can cause more disease and insect problems when spring arrives. Aphids will no doubt get a head start because they can thrive in wet weather. A few more cold spells before winter's end would help control them. More pests perish when their life cycles are interrupted by severe cold. Boxelder bugs, for example, will emerge during warm spells in winter to feed and mate, so freezing temperatures will keep their numbers lower.

Plant pests can be viruses, fungi, bacteria, insects, birds, mammals, Mother Nature and even weeds -- anything that will severely injure or kill an ornamental plant. Here are some predictions for insect or disease problems that could occur on a variety of plants this spring:

Rhododendrons and some azaleas, mountain laurels, leucothoes and Japanese pieris might show signs of wilting, and you might even experience a loss of some shrubs as the season progresses. The problem is caused by one of several soil-borne fungi. These fungi are active during periods of excessive moisture and cool temperatures. In past years, I have helplessly watched rhododendrons wilt and die throughout the winter -- single branches at a time. I hope this conjecture is wrong, because there is no cure.

One possible way to avoid such root-rot diseases is to keep soil well-drained and airy. Dig a generous amount of composted leaves or other organic material into affected areas as soon as the soil is dry enough. Install shrubs in these areas high; simply set them on the ground and lay compost and soil over the roots. For plants already in the ground, create better drainage and aeration by digging leaf compost six to eight inches deep and wide in line with the edge of the plant's branch spread.

Dogwoods' greatest threats are spot anthracnose and root-rot diseases, both of which are worse during cool, wet springs. The anthracnose fungus appears as raised spots with red margins on leaves and flowers. If you are certain you've had a severe problem in the past, contact a landscape-contracting company that has a certified pesticide applicator before leaves emerge.

Root-rot diseases affecting dogwoods aren't curable, but they are preventable. Dig a wide planting hole prepared with generous amounts of leaf compost and plant the root ball high (with one-third above ground), mounding leaf mold over the roots above ground.

Hemlock: Woolly adelgids are a seriously distressing nemesis to hemlocks, and the reason I've been hesitant to recommend these trees in landscape designs for almost 20 years. These insects hatch in February and March during normal winters. Perhaps there will be another cold snap as they are emerging. You'll know if your hemlocks have adelgids if you see a white waxy substance at the base of their needles. But they can be controlled safely by using horticultural oil or insecticidal soap. Apply once in May and again in June. Insecticidal soap will wash off much of the white substance exuded by the adelgids. Application of one more spray of horticultural oil should be done in September. Total coverage of the tops and bottoms of leaves is necessary for effective treatment.

Lilacs are susceptible to a moth called the lilac borer. It winters as a caterpillar in the wood and isn't affected by cold weather, so its problems shouldn't be any different this year than last. But you might see the onset of powdery mildew earlier this year. Borers decimate a plant subtly. The infestation isn't visible until the borers are advanced several years. They're usually found in thicker trunks; so the best practice to control lilac borer is to cut large older wood to the ground when bark begins to have a rough character. Pruning large branches at the base also keeps the growth manageable without need for shears or a ladder.

Spruces' most devastating problem is spider mites. Some are far more susceptible than others. Although damage is most obvious in summer, it starts in April or as early as they hatch and start feeding coming out of winter. Therefore, a mild winter would probably mean earlier feeding. Horticultural oil applied early in March is one of the best controls for spider mites, but it should not be used on blue or dwarf Alberta spruces because it will ruin the color. A miticide might be necessary if these spruces begin to turn brown from damage. As brownish cast occurs and needles are still flexible, shake some branches over a white sheet of paper. If reddish-brown specs move on the paper, use a miticide according to labeled instructions. The problem usually presents itself in late spring early summer.

Oil is an excellent universal remedy for many insect problems. It's effective for smothering mites, adelgids, aphids and scale. It should be used only on an as-needed basis because it also harms beneficial insects. Don't apply it when temperatures are below freezing. A couple of products to look for are Sunspray Ultra-Fine Oil and Volck Oil Spray.

Horticultural soaps are effective controls for aphids, adelgids, whiteflies, mealy bugs and other soft-bodied insects. They are sold under a variety of names under the category of insecticidal soap. Follow labeled instructions for these materials, all of which are widely available at garden and home-improvement centers.

Forecasting the future isn't easy, especially when weather conditions are constantly changing. Come summertime, we'll see whether my predictions were correct.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md.

© 2010 The Washington Post Company