ROANOKE TIMES 

                         Roanoke Times
                 Copyright (c) 1995, Landmark Communications, Inc.

DATE: THURSDAY, November 17, 1994                   TAG: 9411170079
SECTION: CURRENT                    PAGE: NRV-12   EDITION: NEW RIVER VALLEY 
SOURCE: JOE HUNNINGS
DATELINE:                                 LENGTH: Long

ABIOTIC PROBLEMS CAN BE MURDER FOR WOODY SHRUBS

Most often when we think of plant disease, we think of problems caused by fungi, bacteria viruses or maybe nematodes. However, just as common are factors referred to as abiotic, or non-living, factors. These agents cause abnormal and harmful physiological processes in the host plant. Abiotic factors also predispose the plant to some of the biotic - or living - disease-causing organisms.

Let's take a look frequent abiotic problems affecting woody plant material. If you're having trouble with trees or shrubs and haven't seen any signs of organisms, such as fungi or insects, maybe this list of the 17 most common abiotic problems in woody shrubs can help.

1. Improper pH can lead to chlorosis (i.e., yellowing) of foliage or death of foliage and twigs, if the stress is prolonged. Some woody plants have stricter requirements for soil pH then others, especially such acid-loving plants as azalea, rhododendron, andromeda, bayberry, the hollies, inkberry, camellia, gardenia, juniper, wax myrtle, and yew. Usually the problem is high soil pH, but growers sometimes lower pH too much, and excessively low pH can be a problem as well. Keeping pH between 5 and 6 is adequate for healthy growth of these plants.

2. Tag wires can eventually girdle the twig or trunk they encircle, if not removed at planting time. Death of all plant parts above the girdle may occur long after planting. Abnormal swollen growth around the wire is usually evident.

3. Underfertilization leads to chlorosis, stunting, and unthrifty growth.

4. Overfertilization can burn roots, leading to impaired water transport to leaves. The obvious aboveground symptom is leaf scorch

5. Excess soil salt levels from runoff of deicing salt lead to symptoms resembling overfertilization.

6. Poor drainage can cause symptoms similar to root rot diseases. Insufficient oxygen in the soil causes root death and impaired water transport to leaves. Symptoms are wilt and death of sections of the plant or the whole plant. Problems with poor drainage are common in Virginia's clay soils.

7. Deep planting can result in symptoms similar to those caused by poor drainage. When roots are too deep in the soil, they don't get enough oxygen. Evidence of deep planting can often be seen when the plant is uprooted. A new root system may have formed some distance above the original root system as the plant tries to compensate for poorly functioning roots. Some plants such as boxwood and Bradford pear are especially intolerant of deep planting.

8. Improper mulching can lead to death of shrubs and trees. If mulch is piled up against the base of the trunk, it causes prolonged moisture retention on the bark and leads to decay and girdling of the trunk. This problem is very common in our landscapes. Mulch, which is primarily for soil moisture retention and cooling of roots, should be applied in a ring around the trunk and should not contact the trunk. Generally a two- to three-inch layer is adequate; deep mulch may encourage root formation within the mulch. These roots are more susceptible to drought injury.

9. Drought and heat stress cause a water deficit in leaves and can result in wilt, leaf scorch or both. Prolonged drought may cause death of many roots, and plants may be unable to recover even after they're watered.

10. New transplants may suffer transplant shock as a result of a sudden change in moisture, temperature or both. Until roots have established themselves in surrounding soil, the plant needs frequent watering to prevent wilt, leaf scorch or dieback. Beware of overwatering in clay soil, however.

11. Transplant shock can be especially severe in plants that are rootbound, since roots do not grow readily into surrounding soil. Outer roots of plants grown too long in one container in the nursery tend to grow around in a circle and plants become rootbound. If rootbound plants are used, loosen outer roots by scraping them away from the rootball before transplanting.

12. Cold or frost injury can cause a watersoaked appearance to leaves. In some species, leaves eventually turn black (e.g., aucuba); in others, they turn brown (e.g., viburnum) or straw- colored (e.g., boxwood). Cold injury can cause death of whole plants that have not had time to acclimate to cold temperatures.

13. Improper light can lead to either leaf scorch, if shade-loving plants are planted in full sun, or chlorosis, if sun-loving plants are planted in shade.

14. Chemical injury is common on woody landscape plants in lawns where herbicides are used. The most frequent culprits are 2,4-D, a broadleaf weed killer, and Roundup, a non-selective herbicide. Both can damage plants they were not intended for. 2,4-D causes distortion of new growth: Strap-shaped leaves, vein proliferation, and a thick, leathery texture to leaves. Roundup causes burn of contacted foliage and, when it contacts bark of trees or shrubs, new growth the following spring may be chlorotic or distorted.

15. Natural gas leaks in the root zone of any plant can cause sudden death. Soil develops a crumbly texture, turns gray, and has a foul odor.

16. Oedema, a physiological condition resulting from uneven water supply, is a common problem on yew. Corky, brown blisters form on the lower surface of needles.

17. Holes in leaves of spiny plants such as holly or grape holly may be caused by leaves poking each other in a wind. This condition is called spine spot.

Joe Hunnings is the Virginia Cooperative Extension agent for agriculture in the Montgomery County Extension Office in Christiasnburg. If you have questions, call him at 382-5790. 


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