Abiotic Diseases Of Woody Ornamentals
Gary W. Moorman, Professor of Plant Pathology
Of the two major types of disease (BIOTIC, those in which fungi, bacteria, viruses, nematodes, etc. are involved; and ABIOTIC, those in which non living things are involved), ABIOTIC diseases are by far the most important ones on landscape and nursery plants. Abiotic diseases are the result of the interaction, over an extended period of time, between the plant and one or more factors such as lack of space for root growth, the presence of chronic or acute levels of air or water pollutants, or the presence of extremes of moisture, heat, light, soil pH, and nutrients. Most abiotic diseases cause generalized symptoms to develop over a large portion of the tree or shrub. Wilting, yellowing, the development of smaller than normal leaves, slowing of growth, branch death, water sprout (epicormic shoot) formation, premature autumn leaf coloration, and heavier than normal seed production are some of the symptoms characteristic of abiotic diseases. Sometimes very similar symptoms develop in a location on very different species of plants. This is another indication that abiotic factors rather than living pathogens are involved in damaging plant health. Below is a brief review of abiotic diseases other than those caused by pollution.
MOISTURE EXTREMES: Plants require sufficient moisture to grow but not so much that the roots drown. Related to this is the fact that soil compaction by heavy equipment or foot traffic in the plant's root zone reduces the pore space in the soil. Moisture holding capacity and pore space for air in the soil are reduced. Also, percolation of moisture through the soil profile is disrupted so that the desirable air/moisture balance is not maintained. Roots die from a lack of oxygen. To diagnose problems caused by moisture extremes, refer to local weather records to determine if rainfall has been lacking or excessive. Determine if the plants are being irrigated, how the water is applied and how frequently it is applied. Use a soil probe or shovel to determine if the root zone and soil profile are wet or dry. Check the drainage pattern in the vicinity. Runoff may be being directed toward the plant. Or excavation may have changed the pattern in a way that now prevents moisture that had once been available to the plant to now be directed away from the site. If a plant has been recently transplanted or if nearby excavation has greatly pruned away roots, drought symptoms can develop even if the surrounding soil is moist. There may not be enough roots present to support the above-ground portion of the plant. Drought is one of the most important abiotic stresses on plants in the landscape and nursery.
NUTRIENT EXTREMES or IMBALANCES and pH: Plants need nitrogen, phosphorus, potassium, magnesium and many other major and minor nutrients in sufficient quantities to grow but not in excess such that one or more becomes toxic to the plant. There must also be a balance among the available nutrients. When there is an imbalance among certain elements (such as calcium/magnesium) the plant selectively takes up one to the exclusion of other element, thereby causing a deficient or toxic level in the plant. Symptoms can vary by species of plant affected, the time when the deficiency or toxicity began, and other growing condition factors. The soil pH plays a very important role in determining whether a given element is tied up in the soil or is available to the plant. If the soil pH is either too high or too low, some elements will be unavailable while others may be available in toxic quantities. Positive diagnosis of nutrient problems must be based on the analysis of representative leaf and soil samples.
HEAT/LIGHT EXTREMES: The effects of light and heat are difficult to separate. Plants in locations with very high light intensity are exposed to high temperature conditions. Both high light intensity and high temperature can result in the breakdown of chlorophyll in the leaves. Leaves on the sunny side of the plant or those receiving the high temperature may exhibit death of the tissue at their margins or in large areas between the veins. High light intensity and temperature can scald and kill the cambium, the growing cell layer just under the bark, of thin-barked trees, trees newly transplanted or trees freshly pruned. The wood shrinks as it dies and dries. The bark cracks open and predisposes the damaged area to attack by canker causing fungi. Eventually, a branch dieback develops. Freezing is one of the most important abiotic factors in the landscape. Uneven light absorption by tree trunks in the winter results in uneven thawing of water in the plant. If the temperature in the trunk drops rapidly, the quick freezing and expansion of the water splits the bark of the tree opening it to attack by wood rotting and canker-causing fungi. Even if bark splitting does not occur, the damage to the wood may result in branch death, delayed bud break, and lack of flowering. In extreme cases, the plant may be killed to the soil line. Late spring frosts often kill young, tender foliage and flowers. This is diagnosed by correlating local weather records to the onset of symptoms. Succulent tissue may turn brown or black after freezing. Often, a large numbers of the outer twigs are affected. However, the death of the tissue occurs over a short period and does not continue if frost is the cause. Usually, vegetative buds farther back on the twig develop.
INHIBITION of ROOT DEVELOPMENT: Plants need space for their roots to grow. As roots mature, their cell walls thicken and become suberized and lignified to provide structural support and probably to resist attack by insects, fungi, bacteria, nematodes, and other organisms in the soil. The young portions of the roots are responsible for most of the uptake of water and nutrients. If the tree does not produce new root tips because root growth is restricted, plant growth slows. In some cases, this is beneficial because the plant will take much longer to outgrow its location. In the interior landscape or outdoor patios for example, it is not desirable to have a plant grow rapidly because it will overwhelm its setting, require increasing amounts of water and nutrients, and need to be repotted frequently or replaced by a smaller plant. Trees that normally attain a large size may be placed under a great deal of moisture stress if their roots are restricted. This "drought" stress predisposes them to root rots, cankers, and branch diebacks. Containerized plants can be checked for root circling in the container. Similarly in plants in restricted locations such as parking lot islands, a soil probe or shovel can be used to determine where the roots are located. An examination of the weeds or lack of plant growth around a plant can indicate how much foot traffic occurs. Some weeds grow primarily where foot traffic damages its less-resistant competitors. The force needed to insert a soil probe or metal rod into the root zone soil can be compared to the force required to penetrate the soil in an area away from the root zone as an indication of soil compaction.
GENERAL DIAGNOSIS: These are a few examples of abiotic diseases. Some requirement of the plant is not met and the result is a decline in plant health. Determining exactly which requirement is not being met demands that all the symptoms be noted and that the plant site be carefully examined. Rule out the possibility that any organisms present on the plant are the primary cause of the disease. Sending a sample to a plant disease clinic is necessary in order to obtain a positive identification of the organisms found. It is known that certain organisms ONLY cause disease when the plant is under some type of stress or when the plant has been damage first by some other factor. In general, canker-causing fungi most readily damage trees that are under drought stress. The stress must be relieved. Keep in mind that, as noted above, drought may be caused by lack of moisture or damage to roots that results in lack of uptake of available moisture. This example shows the complexity of abiotic disease diagnosis.
Because ABIOTIC diseases are the ones that occur most frequently on plants in the landscape, it is critical that landscapers, nurserymen, and grounds maintenance personnel develop the skills and experience needed to diagnose them.
Epicormic shoots (water sprouts)
Early fall leaf color
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