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Diagnosing Plant Damage

  MG Manual Reference
Ch. 5, pp. 8 - 9

Figure 2. Patterns on plant canopy.

Figure 2
A. Entire or major portion of top dying. If all or major portion of tree or shrub dies, suspect a problem with the roots. LOOK FOR DAMAGING FACTOR AT JUNCTION OF NORMAL AND ABNORMAL PLANT TISSUE.

Gradual decline of entire plant or a major portion of it is caused by living factors such as Armillaria root rot, Verticillium wilt and rootweevil.

Sudden decline is generally caused by a nonliving factor such as toxic chemical is soil or drastic climatic changes such as freezing or drought.

B. Single branch dying. If only scattered damage occurs in the plant canopy, suspect that the primary problem is related to the foliage or aerial environment - not the roots.

Gradual death of branch: If scattered branches start to decline and eventually die, suspect a living organism such as a canker pathogen, a shoot blight or borers.

Sudden death of branch: If branch dies suddenly, and especially if affected branches are concentrated on one side of the plant, suspect a nonliving factor such as weather (wind, snow, etc.), animal damage, or chemical drift.
Here is where we start making the distinction between living and nonliving factors that cause plant damage.
Nonuniform Damage Pattern (living Factors) vs Uniform Damage Pattern on Plant Community, Plant, Plant Part (nonliving Factors).
Living Factors: There is usually no discernible widespread pattern of damage. Living organisms generally produce no uniformly repeated pattern of damage on a planting (Figures 2-4). Damage produced by living organisms , such as pathogens or pests, generally results from their using the plant as a food source. Living organisms are generally rather specific in their feeding habits and do not initially produce a widespread, discernible damage pattern. Plants become abnormal: Tissues are destroyed or removed, become deformed, or proliferate into galls.
Living organisms are specific, i.e. damage may be greatest on or limited to one species of plant.
Living organisms multiply and grow with time, therefore they rarely afflict 100 percent of the host plants at one time. The damage is progressive with time. Likewise, the damage, generally, is initially limited to only one part of the plant and spreads from that initial point of attack with time.
Living organisms usually leave "signs", i.e. excrement, cast skins, mycelium, eggs...
Nonliving Factors: Damage patterns produced by nonliving factors such as frost or applications of toxic chemicals (Figure 5) are generally recognizable and widespread: Damage will appear on all leaves of a certain age (for example on all the leaves forming the plant canopy at the time a toxic spray was applied) or exposure (i.e. all leaves not shaded by overlapping leaves on the southwest side of a plant may be damaged by high temperatures resulting from intense sunlight). Damage will likely appear on more than one type or species of plant (look for similar damage patterns on weeds, neighboring plants, etc.) and over a relatively large area.
Figure 3. Shoot Dieback

Figure 3
A. Shoot Dieback caused by nonliving factors: Sudden dying back of a shoot usually indicates nonliving cause such as climate or chemical damage – not a living factor. Damage caused by nonliving factors usually results in a sharp line between affected and healthy bark.
If dieback is more gradual and there is also cracking of the bark and wood, suspect winter injury.
B. Shoot Dieback (Blight) caused by living factors: Gradual decline of shoots and retention of dead leaves may indicate a living factor.
The margin between affected and healthy tissue is often irregular and sunken.
There may be small, pinlike projections or bumps over surface of dead bark: these are spore-producing structures of pathogenic fungi.
However, small, woody, bumps radiating from all sides of twigs of Dwarf Alberta Spruce are pulvinus – woody projections where needles were attached. This is a taxonomic identifying characteristic of spruce.
Figure 4. Needle Damage

Figure 4
Death of the tips of conifer needles producing a uniform pattern usually indicates a nonliving factor such as toxic chemical or unfavorable climatic condition. Air pollutants frequently cause tip burn on conifers as do certain soil-applied herbicides or excess fertilizer. Drought and freezing may have similar effect. In these cases all needles of a specific growth period are usually affected, and usually the same length on each needle is affected. Margin between the affected tissue, usually reddish brown, and healthy tissue is distinct.
Damage by living organisms such as fungi and insects to needles usually occurs in a random, scattered pattern and rarely kills all needles of a particular growth period. Needles are usually affected over varying lengths and often appear straw yellow or light tan in color. Black fruiting bodies of causal fungus may be present on diseased needles.
Figure 5. Foliar Chemical Spray Injury Pattern on Leaf

Figure 5
Spots are usually uniformly and evenly distributed over the leaf surface, and generally will be of uniform size. Color is usually uniform across the spot.
The margin between affected and healthy tissue is usually sharp. Injury pattern does not spread with time or move to previously undamaged plants.

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