Biology Reference
In-Depth Information
Nonparasitic Diseases
Plants in poor health from one or more environ-
mental conditions far outnumber those afflicted
with diseases caused by parasites - bacteria,
fungi, and nematodes. When foliage turns yellow
from lack of nitrogen, or from unavailability of
iron in an alkaline soil, or from lack of oxygen in
a waterlogged soil, we call it a physiological or
physiogenic or nonparasitic disease. The adverse
condition may be continuing, as it is with
a nutrient deficiency, or it may be transitory, an
ice storm, perhaps, lasting but a day but with
resultant dieback continuing for the next two
years. It may be chemical injury from injudicious
spraying or fertilizing or from toxic substances in
the atmosphere. It may be due to a toxin injected
by an insect.
Trees and crops can be insured against hail,
hurricanes, lightning, and other acts of God, but
not the misguided zeal of gardeners. Years of
working in gardens in my own state and visiting
gardens in other states from coast to coast have
convinced me that plants often suffer more from
their owners than from pests and diseases. Aza-
leas die from an overdose of aluminum sulfate
applied to correct acidity, when the original cause
of ill health was a too-wet soil. Rhododendron die
when a deep, soggy mass of maple or other “soft”
leaves is kept around the trunks. Roses die when
the beds are edged with a spade and soil is
mounded up in the center, burying some plants
too deeply and exposing roots of others. Seed-
lings die from an overdose of fertilizer in hot
weather. Trees die from grading operations.
Spray injury is exceedingly common, with the
gardener thinking the red or brown spots are
fungus leaf spots and increasing the chemical
dosage until all foliage is lost. Weed killers take
their unexpected toll of nearby ornamentals.
Either a deficiency or an excess of plant nutrients
can cause a physiological disease. Greenhouse
operators and commercial growers in the field
must watch nutrition very carefully. The back-
yard farmer gets along pretty well by using
a “complete” fertilizer containing nitrogen, phos-
phorus, and potassium in large amounts and
minor elements in trace amounts. There are kits
available for amateur diagnosticians who wish to
check soil deficiencies and acidity, but you may
prefer to send a soil sample to your state experi-
ment station for a correct interpretation of nutri-
ents and soil acidity. Take a slice through the soil
to spade or trowel depth from several places in
the garden, mix those samples together, and send
a small sample of the mixture.
Acidity, Excess
Soil acidity or alkalinity is measured on a pH
scale that runs from 0 to 14. When the number
of acid or hydrogen ions balances the number of
alkaline or hydroxyl ions, we have pH7.0 or neu-
tral. Above pH 7.0 the soil is alkaline and may
contain free lime; below it, the soil is acid. Few
crop plants will grow below pH 3.5 or above pH
9.0. If the soil becomes very acid, roots are poorly
