DAMAGE: I. DEFINE THE PROBLEM
Ch. 5, pp. 6 - 7
PLANT IDENTIFICATION AND CHARACTERISTICS - GROWTH
AND APPEARANCE OF THE "IDENTIFIED" PLANT -NORMAL?
Determine that a real problem exists. It is essential that the
plant be identified (genus, species and cultivar or variety) so
that the normal appearance of that plant can be established either
by personal knowledge or by utilizing plant reference books. Many
horticultural plants, or structures on those plants such as
fruits-seeds, lenticels, etc. may appear to be abnormal to the
person who is not familiar with the specific plant. For example,
the Sunburst honey locust might appear to be suffering
from a nutrient deficiency because of its chlorotic yellow-green
leaf color, but it was selected because of this genetic
characteristic. It is not abnormal for this plant.
Therefore, it is not a problem.
Figure 1. Normal vs. Abnormal Needle Drop or Leaf
Drop from Evergreens
|Nondeciduous plants normally retain their
leaves for several years but eventually they fall. This drop is
usually gradual and production of new leaves obscures loss of
A. Normal - If drop is confined to older leaves, alarm is
unnecessary because it is a normal response to a condition of
stress (e.g. drought). Unfavorable growing conditions, such as
drought, may accelerate leaf fall so that it becomes apparent
and of concern.
B. Abnormal - If newly produced leaves are lost, it is a
problem. Drop of current year's leaves may result from pathogen
or insect attack or from chemical deficiencies or toxicities.
Always compare the typical diseased plant with a
healthy or normal plant, since normal plant parts or seasonal
changes are sometimes mistakenly assumed to be evidence of
disease. Examples are the brown, spore-producing bodies on the
lower surface of leaves of ferns. These are the normal propagative
organs of ferns. Also in this category are the small, brown, club
like tips that develop on arborvitae foliage in early spring.
These are the male flowers, not deformed shoots. Small galls on
the roots of legumes, such as beans and peas, are most likely
nitrogen-fixing nodules essential to normal development and are
not symptoms of root-knot nematode infection. The leaves of some
plants, such as some rhododendron cultivars, are covered by
conspicuous fuzz-like epidermal hairs. This is sometimes thought
to be evidence of disease, but it is a normal part of the leaf.
Varieties of some plants have variegated foliage that may resemble
certain virus diseases. These examples illustrate the importance
of knowing what the normal plant looks like before attributing
some characteristic to disease. In describing the plant "abnormality",
distinguish between symptoms and signs: Symptoms
are changes in the growth or appearance of the plant in response
to living or nonliving damaging factors. Many damaging factors can
produce the same symptoms; symptoms are not definitive. Signs
are evidence of the damaging factor (pest or pathogen life stages,
secretions; mechanical damage; chemical residues; records of
weather extremes or chemical applications; damage patterns). Patterns
of damage are excellent signs and are definitive diagnostic
Examine the Entire Plant and Its Community
In defining a plant problem, it is essential to determine the
real primary problem. There are foliage symptoms that may occur
due to root damage. The primary problem would be root damage, not
chlorosis of the foliage-examine the roots. In general, if the
entire top of the plant or entire branches are exhibiting abnormal
characteristics, examine the plant downward to determine the
location of the primary damage. Look for the factor causing the
damage at the periphery of the plant damage.
Some pathogens and insects as well as nonliving factors
are only damaging if the plant has been predisposed by other
primary factors. For example, borers generally only attack trees
that are already predisposed by moisture or other physical stress.
Premature dropping of leaves by foliage plants (i.e. Ficus
benjamina ) and of needles by conifers frequently causes alarm.
Evergreen plants normally retain their leaves for 3-6 years and
lose the oldest gradually during each growing season (Figure 1).
This normal leaf drop is not noticed. However, prolonged drought
or other stress factors may cause the tree as a whole to take on a
yellow color for a short period and may accelerate leaf loss. If
the factors involved are not understood, this often causes alarm.
The leaves that drop or turn yellow are actually the oldest leaves
on the tree, and their dropping is a protective mechanism which
results in reduced water loss from the plant as a whole.