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ID-50: Stress and Decline in Woody Plants

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ID-50: Stress and Decline in Woody Plants University of Kentucky College of Agriculture, ID-50 Food and Environment Cooperative Extension Service Stress and Decline in Woody Plants Nicole Ward Gauthier and Cheryl Kaiser, Plant Pathology, Jamie Dockery, Delia Scott, and Kristin Goodin, Horticulture, and Jeremy Williams, Agriculture and Natural Resources oody plant stress has many causes that Affected woody plants may survive over a might ultimately lead to plant decline. long period of time (in some cases, decades) in TreeW and shrub degeneration is often referred a gradual state of decline. Alternatively, plants to as a “complex,” meaning the condition is usu- may rapidly decline over one or two years ally caused by multiple factors. Typically, one or they may appear to die suddenly. Sudden or more primary stresses cause deterioration of death is most common when already weakened plant health, followed by secondary pathogens woody plants are subjected to a particularly and/or insects that further decline or destroy severe environmental stress, such as excessive plants. Determining causes of decline requires heat or drought. careful examination of plants and growing sites, Some of the most common plant stresses are as well as knowledge of site history. Neverthe- addressed in this publication. A wider range less, diagnoses may be difficult, as the original of possible causes of plant stress and decline cause(s) of plant stress may be obscure or no should be considered during evaluation of longer present. woody plant material. Figure 1. Norway spruce on the left suffers from environmental stress, while the specimen on the right is not affected. Cooperative Extension Service | Agriculture and Natural Resources | Family and Consumer Sciences | 4-H Youth Development | Community and Economic Development Figure 2. Verticillium wilt, shown here on catalpa, is a vascular disease that causes die- back in a broad range of hosts. Plant Ecology and Physiology (including xylem and phloem), move these vital products from source (where they are produced) Each part of a woody plant’s anatomy performs to sink (where they are stored or utilized). Dam- unique functions, all of which are necessary for age to vascular tissue inhibits water, nutrient, and healthy growth. Interruptions in any of these carbohydrate transport, resulting in deficiencies functions may lead to stress and ultimately, to that equate to starvation. plant decline (Figure 1). Leaves Roots Roots support plants, store energy, and absorb Photosynthesis, or food production, takes place water and nutrients from soil. The majority of tree in leaves. The leaves utilize water and nutrients and shrub roots are within the top 3 feet of soil taken up by roots, combine them with carbon (deeper in sandy soils), and more than 90 percent dioxide from the air and energy from the sun, of a woody plant’s small feeder roots are usually and produce carbohydrates that fuel entire plant located in the top 12 inches of soil. Roots extend systems. When an outside factor interferes with outward from trunks much farther than branches normal photosynthesis, plants produce less food or driplines. In fact, roots often extend 1½ to 2 and have fewer energy reserves. times the spread of woody plant canopies. Thus, Trunks root damage caused by grade change (suffoca- Woody plant trunks physically support branch- tion), cultivation (physical damage), or compac- es and leaves, conduct nutrients and water from tion (physical barrier) affects uptake of water and roots to leaves, and transport carbohydrates down nutrients. to roots. Transport systems, called vascular tissue 2 Figure 3. Scorching of leaf margins is often caused by restricted water uptake Figure 4. Severely declining or dying trees may (e.g. drought, root damage, cankers, etc.). re-sprout from roots, even though primary trunk is dead. Common Signs and Symptoms Causes of Plant Stress of Stress and Decline Plant stress may be caused by various exter- When plants are stressed by biotic or abi- nal factors that affect nutrient availability, gas otic factors, they begin to decline. During the exchange, or other critical life processes. Various early stages of decline or when symptoms appear levels of one or more of these stresses can affect gradually, it may be possible to alter conditions severity of plant decline. and improve plant health. Frequent observations, Poor Plant Selection for the Location including routine scouting, are key to identifying Ornamental plants thrive best in locations symptoms and managing stress. similar to the soil and climatic conditions of their Symptoms can include: natural habitats. Plants placed in environments • Thinning canopy unlike their native conditions are more likely to • Dead limbs, dieback (Figure 2) become stressed. For example, wetland plants will • Lichen growth on branches and trunks not perform well in sandy upland soils, and shade • Off-color leaves (pale green or yellow in sum- plants often suffer in sunny locations. mer) • Small or stunted leaves or shoots Physical Constraints • Stunted branches, short internodes Mature sizes of trees or shrubs must be consid- • Leaf scorch, brown leaf margins (Figure 3) ered before planting. Crown spread, plant height, • Premature leaf drop trunk diameter, and root spread are all impor- • Premature fall coloration (late summer) tant components of plant size. In open spaces, • Water sprouts or suckers growing from lateral woody plant roots may expand twice as far as the branches or bases of trunks (Figure 4) expected mature crown spread. However, build- • Presence of insects (egg masses, visible insects, ing foundations, paved roadways, sidewalks, and frass) (Figure 5), increased insect damage other physical barriers inhibit expansion of root • Presence of disease and/or signs of disease- systems, thereby reducing availability of water and causing pathogens (mushrooms, conks, bracket nutrients. fungi) (Figure 6) 3 Figure 5. Boring insects cause branch dieback, but Figure 6. Mushrooms and conks indicate colonization and decay by wood- they may be difficult to detect without removing bark. rotting fungi. Improper Planting Practices Poor Soil Conditions New plantings require proper installation so Planting sites for trees and shrubs should have that they can become well established. Young a minimum of 3 feet of permeable (deep) soil that plants should be set so that the root-trunk inter- does not have a hardpan or other root-restrictive face is at ground level and root flare is visible. Roots layer. These deep soils allow tree roots to grow eas- set too deeply suffer from oxygen deprivation and ily and permit water movement and gas exchange. trunk rot. Planting holes should be dug twice as Shallow soils, or those with restrictive layers, wide as root balls to loosen surrounding soil, and inhibit root growth as roots approach rocky, clay, fill should be the same soil that was removed from or compacted layers. In contrast to deep soils, the planting hole. See Transplant Shock: Disease or shallow soils have less “water-holding capacity,” Cultural Problem? (PPFS-W-OR-19) for informa- as soil can only retain certain amounts of water. tion on proper transplanting methods. Thus, water runoff or puddling increases with Newly installed trees and shrubs should have shallow soil during wet conditions, and drought healthy root systems that are evenly distributed conditions result during dry seasons. Shallow and are not encircling or rootbound. Upon plant- soils can result from historical erosion (typically ing, roots should be spread evenly across planting associated with greater slopes), removal of topsoil holes without curling or restriction. Lateral roots during construction, rocky soils, and/or naturally that encircle tree trunks or intertwine with other occurring restrictive layers (in Western Kentucky, main lateral roots (girdling roots) cause restriction fragipans). As a result, roots remain close to soil of water and nutrient flow. Often these symptoms surfaces and are vulnerable to drought, heat, and are not evident until plants are well-established (20 nutrient deficiencies. Additionally, shallow-rooted or more years old). Girdling roots may be partially trees are poorly anchored and are at risk to blow exposed at soil surfaces, forming a noose around over during strong winds. Refer to Evaluating trees. In other cases, girdling roots may be 6 to 12 Land Resource Potentials in Kentucky (in press) inches deep. for additional information. 4 Compacted soils affect root growth and develop- ment. For optimal plant growth, soil must be loose enough to provide spaces (soil pores) for air and water. Compaction reduces size and number of these spaces. Primary causes of soil compaction include foot traffic from people and animals (pets, livestock) and vehicular traffic (cars, equipment). Construction projects also cause compaction. For more information, refer to Trees and Compacted Soils (HO-93). Poorly drained soils hold excess amounts of water (Figure 7) and deprive roots of oxygen. Surface drainage is often easy to monitor, while digging or probing is required to analyze internal drainage problems (heavy or clay soils, compacted subsoil). Red or brown soils indicate good drain- age, while soils that are gray or dull-colored rep- resent poor internal drainage. Drainage mottles (spots or blotches of different colors within the “normal” soil color) also indicate poor drainage. Excess water buildup occurs in poorly drained soils during periods of heavy rainfall or improper irrigation practices. Water may also drain away slowly. Many root rot pathogens favor wet soils, so disease is another problem in poorly drained soils. For additional information, refer to the section on diseases, under Biological Stresses, below, as well Figure 7. Proper planting depth and good soil drainage prevent puddling and as the fact sheet entitled Wet Feet in the Landscape “wet feet.” (PPFS-OR-W-04). Dry soils do not hold adequate moisture for plant roots. Rocky or sandy soils and steep slopes are the most common causes of dry soil condi- More often, improper soil pH (too high or too tions, as they drain quickly and have low water- low) may reduce availability of nutrients to plants.
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