Gardening : Solving Mystery of Invisible Plant Killer
“Do you think it’s snails?” she asked, turning her mournful gaze from the moribund azalea to her gardener.
Mrs. E. was from Long Island and her understanding of California gardening was simple. To keep her dichondra lawn, Algerian ivy, camellias and azaleas happy in this arid land she poured on the water.
And when her prized azaleas began to wilt, turn yellow and shed their leaves even though the soil was wet, her cure was the usual--more water. But the decline continued and there was no visible culprit to blame it on, least of all snails, whose slimy tracks give them away.
Mrs. E’s azaleas were victims of the great invisible enemies that stalk southern California gardens--water-mold funguses. These secretive killers knock off full-grown trees and shrubs as easily as they do tiny seedlings while scarcely leaving a visible clue to their identity. Even that sinister parasite, oak-root fungus, occasionally produces clumps of honey-colored mushrooms to announce its deadly presence in the garden, but learning to diagnose root rot caused by water molds is more difficult.
Infected plants appear stunted or wilted as if they were lacking water which, in a sense, is true. Even though the soil may be saturated, roots rotted by water molds can’t send enough water to the rest of the plant to keep leaves from wilting and falling on. Wet, poorly drained soils are literally swimming with the spores of funguses known technically as Phytophthora and Pithium but generally lumped together as water-molds or root-rot funguses.
They begin by attacking the stems or trunks of plants at the surface of the soil. On small seedlings this can be clearly visible as the stem discolors and shrivels just above the ground level, causing tiny plants to topple. This is usually called “damping-on,” but the funguses that cause it are the same ones that cause root-rot and crown-rot of full-grown shrubs and trees.
Infected woody plants may show some dark discoloring of the trunk near the soil-line or cankers on the branches, but the most common signs are sparse foliage, slow growth, undersize leaves, wilting, leaf drop and branch die-back. All too often these symptoms are misinterpreted as lack of water.
When more water only makes the problems worse, many gardeners seek the advice of David Lofgren, horticultural consultant at the Los Angeles State and County Arboretum in Arcadia. Having fielded countless inquiries from mystified residents, Lofgren notes, “The incidence of root-rot is phenomenal. The classic symptom is branches dying one at a time as the roots associated with that branch die. Root-rot among azaleas is so prevalent you can almost bet any plant you buy will die if it’s over watered.”
The fungicide marketed under the trade-name Subdue controls both Phytophthora and Pithium, although a plant already severely damaged probably would not recover. At one time, Subdue could be had for a steep price at local nurseries, but now its use is restricted by law to agriculture.
Having lost their only chemical weapon, gardeners have little choice but to remove badly infected plants. To avoid spreading the disease, trimmings should go to the dump rather than on the compost pile. It may help to remove soil from around the trunk of an ailing avocado or citrus tree to expose and aerate the infected area.
Replanting a flower-bed or orchard that has been ravaged by root-rot poses a problem. Fumigating the soil will kill the funguses but, because their spores are virtually everywhere, the soil can easily be reinfected. Replanting with species resistant to water-molds would be a good idea if only there were more than a very short list of such plants.
A better answer is learning when not to water. Subtropical fruits like citrus and avocados often fall prey to root-rots because of the mistaken belief they need constantly moist soil.
What they really need is consistent watering--deep, thorough soakings at intervals that allow the soil to become fairly dry (every 10 days to three weeks depending on the local soil, climate and weather).
To avoid over watering, you need to know your soil’s water-holding capacity. Rather than buying an expensive soil-probe that you might only need to use a few times, David Lofgren recommends the following technique. When you think it’s time to soak your trees dig down 8 to 12 inches. If the soil is still moist, you can put off watering for at least another week.
Most of the sick azaleas and camellias you see are not hurting for water--they’re starving for air. These popular flowering shrubs for shady spots share a need for well-aerated soil and suffer from over-watering, which forces air from the tiny pores of the soil. That’s why azaleas should be planted in the chunkiest peat moss you can get (the bag should say “extra coarse”), rather than the finely ground kind that comes in a bag marked “medium,” Camellias are somewhat less prone to root-rot, and the small-flowered, fall-blooming sasanqua types definitely show better disease resistance than the more popular large-flowered japonicas.
Many plants native to Mediterranean climate areas, including some of our own California natives, must be kept dry during the summer. They have evolved to withstand summer drought, and have no resistance to root-rot funguses that thrive in wet, warm garden soil.
Our native fremontias are famed equally for their display of huge, golden flowers and for their tendency to die suddenly if watered during summer. Almost as susceptible is the bush morning-glory (Convolvulus cneorum) from southern Europe, more admired for the silvery sheen of its leaves than for its often brief life-span in the summer-watered garden.
The key to keeping a wide variety of plants happy in one garden is separating them on the basis of their water needs. Plants that can’t stand summer watering need a place of their own away from those that require summer watering.
Also be aware that if you mix deep-rooted shrubs and trees with shallow-rooted plants you are almost certain to over water the deep-rooted plants, making them susceptible to root-rot. A half-dead orange tree in the middle of a lush green lawn is an unhappy sight that can be avoided by good planning.
