GARDENING : SOUTHLAND TREE EXPERT FIGHTS FUNGI WITH A VENGEANCE.

Byline: Joshua Siskin

Robert Wallace probably knows as much about trees as anyone in Los Angeles. Since the age of 7, when he first assisted his arborist dad in sprinkling fertilizer under white oaks, he has been a student of trees. His mentors have been his father, Thomas R. Wallace, and Professor Demming Williams of Pierce College. In the course of his career, Robert Wallace has pruned, sprayed and cabled trees of every description.

Since becoming a consulting arborist eight 8 years ago, Wallace has pretty much put down his tools. He is one of only a handful of people in our city who make their living exclusively by solving the problems of trees. Not that he's gotten rich in the process or even earned the luxury of free time.

``In eight years, I've taken off a few three-day weekends, but that's it,'' Wallace said. ``Most days, I work from 7:30 in the morning until 10 o'clock at night. You spend a lot of time just talking to people and teaching them about trees. Not that I'm complaining. It is definitely an interesting life. You're always learning something new.''

``People tell me I have the ideal job,'' he continued. ``And to the extent that I can preserve trees or make them healthy again, this is true. Often, though, I have to play the role of coroner and tell homeowners that their trees are either dead or dying. Many folks call an arborist only after it's too late to save their tree. It's unfortunate, because our attachment to trees is intense. Men will sigh and look toward heaven beseechingly, and women will weep at the news that there is no hope for their tree.''

Wallace's specialty is oak trees. ``The feeling of people for these trees is incredible. People talk about their oaks as if they were more than trees. This relationship has to do with the massive magnificence of old oaks, their scarcity and their association with the Indians for whom the oak was a staff of life. To have an oak tree that is several hundred years old growing in your back yard is to add another whole dimension to your world.''

The biggest problem faced by oaks - and most other trees for that matter - is armillaria root rot, often called oak root fungus even though every species of tree may be susceptible to this disease. It can be triggered by water from sprinklers hitting the base of a tree, a soil level so high that it covers and suffocates bark at the base of a tree, ground cover growing around the base of a tree, or not enough sun reaching the base of a tree to keep it dry.

``Never build a structure or wall on the south side of a tree,'' admonished Wallace. ``The base of such a tree will never see the sun and the development of oak root fungus in such cases is almost assured.''

Over the years, lists of trees supposedly resistant to oak root fungus have been developed. Yet one by one, each of the species on such lists has proven susceptible to the disease. In Wallace's 40 years of experience, he has found the fern pine (Podocarpus gracilior) to be the species most resistant to oak root fungus, with only one case of infection reported.

The fern pine, which can grow to a height of 50 feet, is native to the tropical evergreen forests of Ethiopia, where rainfall averages around 50 inches a year. It is a habitat shared by the coffee plant (Coffea arabica). The fern pine, which may be trellised, shaped as a shrub or sheared as a hedge, is also moderately shade-tolerant and is the ideal replacement for a tree that died from oak root fungus, he said.

Other trees that offer stronger-than-average resistance to oak root fungus are the carrotwood (Cupaniopsis anacardioides), jacaranda, eucalyptus and eugenia (Syzygium paniculata), which is often grown as a shrub or tall hedge.

Symptoms of oak root fungus include honey-colored mushrooms growing around a tree base during fall or winter and white fungal fans that spread between bark and wood. Roots and trunks gradually rot as the fungus slowly girdles the tree. After a number of years, the tree dies and, in the end, it falls over.

Wallace explains that his way of looking at a tree has completely changed over the years. When he first started diagnosing tree problems, he would start at the leaves, then look at the branches and the trunk and finally the condition of the roots and the soil. Now, he goes about his investigations in reverse, starting with the soil and roots, and working up.

``Ultimately, the condition of the soil is usually responsible for the health of the tree,'' Wallace explained. ``When the soil is too wet or too dry, the tree becomes stressed, making it more susceptible to the entry of pathogenic fungi and bacteria.''

Sometimes, though, you find trees growing to perfection in conditions which should have devastated them. Driving north with Wallace on Coldwater Canyon from the Valley, just before Beverly Hills, we came to the most amazing collection of Canary Island pines in the city. They are the healthiest street trees anywhere, despite three conditions that you might think would have hindered their growth: They are growing in lawns (conifers are not recommended for turf); they are just inches away from the street (compaction of soil from adjacent asphalt is deleterious to root growth); they are very closely planted (resulting in competition for light and restricted air circulation). Wallace theorized that the health of these trees may be attributable to a high water table - isn't that what Coldwater Canyon is named for? - under the trees.

Wallace also showed me a tree that is his personal pride and joy. He has lived within a few minutes of it his entire life and has personally pruned it on two occasions. It is a white alder that is more than 50 years old, yet it is the picture of health. More often than not, white alders in Los Angeles either die within 20 to 30 years of being planted, or they persist in a dried-up, sorry-looking state. This is somewhat ironic, since the white alder is native to the canyons that surround us. However, it is a riparian, or riverbed tree. In most yards, alders simply don't get the water that they need.

This older alder is growing on the west side of Jumilla Avenue between Strathern Street and Roscoe Boulevard in Canoga Park. It was planted as an afterthought in an 8-foot-deep by 8-foot-wide pit. Wallace proposes that this large planting hole was central to the alder's success. He surmises that the tree was quickly able to reach its own groundwater source, which it's been drinking from ever since. He also thinks that the current prevailing practice of digging shallow planting holes, no deeper than a tree's root ball, could be a mistake. This practice may come back to haunt us in years to come - if and when shallow-rooted trees start falling over in the wind.

If you want to talk to Robert Wallace about trees, call him at [phone].