IN THE GARDEN PLANTS CAN GROW, BUT THEY CAN'T HIDE FROM PESTS.

Byline: JOSHUA SISKIN

No matter how durable, it seems that every species and variety of plant is discovered and attacked, sooner or later, by pests.

Close to 20 years ago, when I first started watching plants in Valley gardens, there was one ubiquitous shrub I quickly got to know that seemed invincible. I learned and easily remembered its name: heavenly bamboo (Nandina domestica). Although no relation to true bamboo, its multicolored, feathery foliage and spreading growth habit more than entitled heavenly bamboo to its name. I saw it thrive in sun and shade, in dry, fast-draining soil and in soil that was always moist.

A few years ago, for the first time, I suddenly noticed a thin layer of white covering the foliage of heavenly bamboo plantings in shady locations. The sight of its foliage afflicted with a white fungal plague was difficult to accept. In fact, throughout horticultural literature and the Internet, it is still difficult to find references to a white heavenly bamboo fungus, despite its appearance throughout Southern California.

This malady certainly looks like powdery mildew, one of the most common plant diseases. When the top side of a leaf appears to be dusted or covered with white powder, the disease in question is frequently powdery mildew. Powdery mildew is commonly found on roses, euonymus shrubs, zinnias, cabbages, cucumbers, California sycamores and crepe myrtles.

Organic pest control advocates promote application of baking soda or fine horticultural oil to prevent and combat powdery mildew. There is also an abundant supply of more toxic anti-powdery mildew chemicals available at any garden center.

Two increasingly popular container/hanging basket plants that I have planted extensively over the past two years have also shown pest susceptibility this summer. The first is bacopa (Sutera cordata), which sends out long slender chains of white or violet flowers. Early this summer I noticed that certain shoots, planted last fall, had suddenly turned black, apparently the victim of sooty mold fungus. Immediately, I pruned out the infected shoots. Because of its vigorous growth, bacopa must be cut back severely on an annual or semi-annual basis. A thatch of brown stems builds up as the plant grows and these stems must be removed for bacopa to maintain good health.

A second container plant called zinnia ``Profusion,'' highly recommended for pest resistance, suddenly showed a collection of severely munched leaves a few weeks ago. Closer inspection revealed a large population of skipper butterflies living among the zinnias. Unlike the taller common zinnia (zinnia elegans) that nearly always develops powdery mildew, zinnia Profusion grows less than a foot tall and is mildew-proof. It is available in orange and rose-pink, colors that look remarkably good together. I had thought that nothing could affect this plant until I saw the skippers, those somewhat drab-looking butterflies that implacably open and close their wings as they sun themselves on your flowers, and then hop away the moment you try to put a finger on them. Skipper larvae munch ``Profusion'' zinnias. Control them with a product known as BT, a bacteria in powder form that, when ingested by butterfly larvae, kills them.

TIP OF THE WEEK: New Guinea impatiens can last for several years if they are not overwatered. With striking foliage and larger flowers than regular impatiens, they are worth trying in containers or planters in the shade. If possible, soak them with a hose only when their soil is dry.