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DIAGNOSING PLANT DAMAGE: III. DELINEATE DEVELOPMENT
  MG Manual Reference
Ch. 5, pg. 10


As already mentioned, another clue for distinguishing between living and nonliving factors causing plant damage is to observe the development of the pattern.
Living organisms generally multiply with time, produce an increasing spread of the damage over a plant or planting with time, are progressive.
Nonliving factors generally damage the plant at a given point in time, for example death of leaf tissue caused by a phytotoxic chemical is immediate and does not spread with time (Figure 5). There are exceptions. If a nonliving damaging factor is maintained over time, the damage will also continue to intensify with time: For example, if a toxic soil or air chemical is not removed, damage to plants within the contaminated area will continue to develop (Figure 6), but damage will not spread to plants in uncontaminated areas: NONLIVING FACTORS ARE NOT PROGRESSIVE. This again reemphasizes the necessity of piecing together multiple clues to identify the most probable factor causing plant damage.
Figure 6. Leaf Damage Pattern by Nonliving Factors
Figure 6
Nonliving factors include toxic chemical taken up through roots or from polluted air filtered through the leaf, or from moisture stress.

Injury from chemicals taken up by plants from soil through roots or from air through leaves usually results in scorching (necrosis) of leaf margins and interveinal areas. If severe, necrotic tissue may drop out giving a ragged appearance. Similar patterns are produced by moisture stress. if uptake of toxic chemical is to fully expanded leaf, toxicity is marginal and interveinal. If to unexpanded leaf, toxicity occurs in veins.



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