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DIAGNOSING PLANT DAMAGE: IV. DETERMINE CAUSES [continued]

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  MG Manual Reference
Ch. 5, pp. 21 - 25
[Determine Causes: determine | symptoms and signs | distinguishing | chemical injury]


DISTINGUISHING AMONG NONLIVING FACTORSTop

If patterns of damage in the field planting and on the individual plant are uniform and repeated, this indicates that a nonliving factor is the probable cause of the damage. We will now examine additional information and clues to determine whether the nonliving damaging factor was a mechanical, physical, or chemical factor.
Look for changes in the three categories of nonliving factors of the affected plant’s environment:
1) Mechanical Factors (Damage/Breakage) – plant damage caused by site changes –"construction damage," transplanting damage, "lawn mower blight", abrasion, bruising.
2) Physical Factors – environment or weather changes causing extremes of temperature, light, moisture-aeration.
3) Chemical Factors – chemical pesticide applications, aerial and soil pollutants, nutritional disorders.

MECHANICAL FACTORSTop

Close visual examination and questioning will often determine if the stems or roots have been broken or girdled or if the leaves have been bruised, punctured, or broken. For example, if a large Ficus elastica is dropped while being transplanted and the stem is broken, rapid wilting of the portion of the plant above the break will occur. Examine the plant site for signs of recent excavation, construction, paving, etc.

PHYSICAL FACTORS (Environmental Factors)Top

Primary sources of diagnostic information are damage patterns and weather records to pinpoint time and location of weather extremes. Records are "signs" of the factor that caused the plant damage.
Temperature Extremes:
Heat: The highest leaf temperatures will occur in the early afternoon when the sun is located in the southwest quadrant of the sky. Therefore, lethal leaf temperatures produced by solar radiation absorption will occur primarily on unshaded leaves on the outer surface of the plant canopy on the southwest side. Portions of leaves shaded by other leaves or leaves on the shaded northeast side may be undamaged. Most severe damage occurs on leaves most exposed and furthest from the vascular (roots, stem, leaf vein) source of water, i.e. leaves on outer perimeter of plant, leaf tips and interveinal areas. A recognizable pattern related to leaf tissue that would have the highest potential temperature and be most readily desiccated will occur uniformly over all plants in the area.
Cold: Damage will occur on the least hardy plants and will be most severe on the least hardy tissues of those specific plants. In fall acclimation, cold hardiness is first achieved by the terminal buds, and then with time the lower regions achieve hardiness; the branch crotches are often the last tissues to achieve cold hardiness. And, generally the root systems will not survive as low a temperature as will the tops -root systems are damaged at higher temperatures than are the tops. On-the-other-hand, after hardiness has been achieved, if warm temperatures induce deacclimation (i.e. in the early spring), the terminals (buds) are first to become less cold hardy.
Portion of plant damaged will indicate if low temperature damage occurred before plant achieved cold hardiness in the fall, or if it occurred after cold hardiness was lost in the spring: reverse patterns are produced.
On a given structure (i.e. leaf or bud), the damage will be death of exposed, non-hardy tissues in a recognizable (repeated) pattern. For example, frost damage to foliage, i.e. conifer needles, in the spring will uniformly kill all needles of a given age from the tip of the needle back toward the stem a given distance on each needle.
Frost cracks are longitudinal separations of the bark and wood generally on the southwest sides of the trunk-most likely to occur because of daily, wide temperature fluctuations. Freezing death of dividing cells on outer portions of leaf folds inside the bud will cause distorted or lace-like leaf blade because of nonuniform cell division and growth during leaf expansion. Cold damage to the root system is primarily a concern with container-grown plants where the root temperature fluctuates more and can be expected to reach lower temperatures than would occur with the same plant if field-grown. Cold damage to the root system can be detected by examining the roots: Damage generally occurs from the periphery of the root ball (near the container edge) and evidence includes blackened or spongy roots with lack of new growth or new root hairs. Above ground symptoms generally will not be evident until new shoot growth in the spring; at that time leaf expansion may be incomplete (small leaf size) because of the restricted uptake of water and nutrients by the damaged root system. With increased air temperatures, the water loss from the shoots and leaves may exceed the root uptake capacity; the plants may defoliate due to this water deficit.
Plants Vary in their Cold Tolerance: The cold tolerance (hardiness) of various plants in the landscape has been rated by the USDA (see Plant Hardiness Zone Map, USDA-ARS #814). The "indicator plants" listed for the various cold hardiness zones on the map are useful in surveying a group of landscape plants, observing which ones show cold damage and then estimating how low the temperature dropped based on the damaged/undamaged indicator plants. Differences in cold tolerance of root systems (especially roots of plants grown in exposed containers) are presented by Green (Ornamentals Northwest Newsletter 12-5:3-15, 1988).
Light Extremes: Plants can acclimate to various conditions, but the primary requirement for acclimation is time. Plants respond adversely to rapid changes in the environment. Rapid change from low to high light intensity will result in destruction of the chlorophyll pigments in the leaf (yellowing and necrosis = sunburn). Rapid change from high to low light intensity will result in reduced growth and leaf drop; new leaves will be larger. "Sun leaves" are smaller, thicker and lighter green in color than are "shade leaves". Flowering will be reduced, delayed or absent under low light.
Oxygen and Moisture Extremes: Here we are primarily considering the root environment where oxygen and moisture are inversely related. Waterlogging (moisture saturation) of the root environment results in oxygen deficiency; without oxygen, root metabolism and growth come to a standstill. Consequently, uptake of water and nutrients is restricted with subsequent wilting and nutritional deficiency symptoms occurring on the above ground portions of the plant. Drought and water logging produce many of the same symptoms on the above ground portion of the plant: The first symptoms will be chlorosis and abscission of older leaves. Under severe, continuing moisture stress wilting and necrosis will occur on tips and interveinal regions of recently expanded leaves and new growth (Figure 6).

CHEMICAL FACTORSTop

FIELD PATTERNS OF PLANT INJURY RELATED TO CHEMICAL APPLICATIONS.
Look for application, drift, or runoff-accumulation patterns in the field (Figure 10): The pattern of plant injury in a field or other group of plants and date of injury appearance can be helpful in relating the damage to a specific chemical application.
Damage diminishing uniformly from one side to the other (Figure 10.A, Spray Drift): A pattern in a field, yard or on a group of plants that starts on one side and diminishes gradually and uniformly away from that area is typical of wind-drift of droplets.
Figure 10. Illustrations of patterns of plant damage related to chemical applications to field or bed plantings.

Figure 10
A. Drift of spray droplets.

B. Spots of injury from low temperature or accumulation of volatile chemicals or accumulation of chemical runoff in low areas of the field; or, injury associated with soil variables.

C. Stripes indicating overlapping application pattern, or one or more faulty applicator openings.

D. Plant injury at end of field due to double application.

E. Definite break between injured and uninjured sections of the planting: application discontinued or changed in applied chemical.

F. Increasing injury within an application band due to poor mixing or inadequate chemical agitation.
Damage in individual spots or irregular patterns (Figure 10.B): Low lying areas in a field where air masses settle would enhance the accumulation of fumes from volatile chemicals, would be frost pockets, and might enhance pathogens. These damage spots might also be related to differences in the soils texture, organic matter, pH or moisture. High pH spots might induce nutritional disorders such as iron deficiency, increase the toxicity of triazine herbicides, etc.
Damage in linear stripes at regular intervals, (Figure 10.C), indicates nonuniform application of a chemical. Regularly recurring stripes of damaged plants at intervals within the width of the application equipment (fertilizer applicator, pesticide spray boom, etc.) indicate an oversized or worn nozzle, improper setting on one applicator opening, or an overlap in application. Another cause may be carry over of a residual chemical from bands applied the year before -this pattern would match the row width and direction from the previous season.
Damage at ends of field, (Figure 10.D), may be due to double application of a chemical either the year before or the year the injury is observed.
Damage on one part of the field only with a definite break between the damaged portion and the remainder of the field (Figure 10.E):
1) Was the chemical applicator reloaded or recalibrated at the break-point? If so, mistake might have been made in chemical selected or in rate of application, or the applicator might not have been adequately cleaned of a toxic chemical: the toxic residue was removed in application of the first load of chemical. Check equipment-use records.
2) Check tillage methods, dates and soil conditions (moisture) –resulting differences in soil texture or depth of tillage may cause differences in dilution of carry over chemical residue, differences in volatilization and dilution of an applied chemical, etc.
Damage intensity increasing along a broad band, (Figure 10.F), indicates inadequate mixing or poor agitation of a wettable chemical powder in a spray tank resulting in increased concentration of the applied chemical toward the end of the tank load.


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