Injury produced by certain pollutants along with sources is given as follows:
Pollutant Source Symptoms
Ozone Nitrogen dioxide and Four different kinds of
hydrocarbons emitted from symptoms can result from
automobiles, industrial ozone exposure. The most
combustion, oil refineries common symptom is localized
and many lesser sources thickening and pigmentation
react with sunlight to of the cell walls causing
form ozone. During sharply defined small dot-
electrical storms, ozone like lesions. General
is produced and can be upper surface bleaching is
brought down from the another common type of
upper atmosphere by injury. Large bifacial
strong down drafts. necrotic areas ranging from
white to red may develop
if all the tissue through
the leaf is killed. Some
species show only general
chlorosis or chlorotic
mottling or chlorotic
flecks.
Sulfur Dioxide Emitted during combustion Accumulation of sulfite
of many fuels, especially in tissues produces a
coal and petroleum. Also general chlorotic appear-
released during smelting ance of the leaf and a
operations. silvering or bronzing of
the undersurface. Acute
injury from absorption of
lethal quantities of sulfur
dioxide appears as marginal
or intercoastal areas of
dead tissue with a gray-
green watersoaked appear-
ance, which usually dries
to a bleached ivory color
but may turn brown, red or
black. The necrotic areas
may fall out and after much
of the leaf is affected, it
will shed.
Fluoride Released from manufacturing Necrosis is the character-
processes involved in the istic symptom of fluoride
production of aluminum, injury occurring on broad-
steel, ceramics and leaved species at the leaf
phosphorus chemicals and tips and margins where the
fertilizers. fluoride accumulates.
First sign of injury is a
dull-green watersoaked
discoloration of these
tissues within 24 hours
or several days depending
on concentration. These
watersoaked tissues turn
light to dark brown
within 48 hours during hot
weather. Cool tempera-
tures may delay symptoms as
much as several days.
Symptoms on needles of pine
and other conifers consist
of dead tissue beginning at
the tip and progressing
toward the base. Injured
tissue first is chlorotic
and turns buff to reddish-
brown.
Nitrogen oxides Produced by high Many plants develop a
Peroxyactyl temperature combustion. silvering of the lower leaf
Nitrates (PAN) surface with PAN. Leaves
of sensitive species
develop a slightly oily or
waxy appearance two to
three hours after exposure.
Glazed symptoms develop
gradually with the advanced
bronzing stage following
after two or three days.
Very young and the most
mature leaves are
resistant.
Many other substances may be released to the atmosphere and produce damage
to plants. These include ethylene (usually from incomplete combustion),
herbicides, chlorine gas, ammonia, particulates (such as heavy metals or
sulfuric acid mist) and hydrogen sulfide. These all produce
characteristic symptoms.
Plants vary in their susceptibility to different pollutants. This is
indicated in the following chart:
Ozone
Sensitive - alfalfa, beans, oats, onion, peanut, potato, radish, spinach,
tomato, petunia, grape, carnation and pine
Sulfur dioxide
Sensitive - alfalfa, bean, cotton soybean, sweet pea, verbena, zinnia,
apple, pear, pine okra, spinach, turnip and sunflower
Fluoride
Sensitive - apricot, gladiolus, grape, peach, pine and tulip
Nitrogen oxides
Sensitive - azalea, bean, hibiscus, lettuce and sunflower
Those observing and diagnosing air pollution injury should be aware that
many things may resemble symptoms produced by air pollutants. Care should
be taken in diagnosis.
Crops: broccoli, buckwheat, cabbage, carrot, cauliflower, celery, endive, flax, lettuce, onion, parlsey, potato, parsnip, pumpkin, red clover, salsify, spinach, strawberry and tomato.
Flowers: aster, anemone, calendula, Centaurea, China aster, chrysanthemum, Clarkia, cockscomb, Coreopsis, cosmos, delphinium, daisies, Gaillardia, hydrangea, marigold, Nemesia, Paris daisy, periwinkle, petunia, phylox, Scabiosa, snapdragon, statice, strawflower, veronica, and zinnia.
Weeds: cinquefoil, daisy fleabane, dandelion, horseweed, plantain, ragweed, thistle, wild carrot, and wild lettuce.
The mycoplasma overwinters in leafhoppers on perennial host plants. Leafhoppers can spread the mycoplasma 9 to 21 days after feeding on diseased plants. The mycoplasma multiplies in the insects and leafhoppers can spread the disease for 100 days or more after becoming infective. The ability of leafhoppers to transmit the organism is reduced when temperature is over 90 degrees F. Overwintering of the mycoplasma occurs more often in some plants than in others because leafhoppers prefer to feed on those host plants. Symptoms show in plants in 10 to 40 days after insect feeding. The disease can be serious when dry weather forces leafhoppers to migrate from wild weeds to irrigated fields of susceptible plants. The six spotted leafhopper is one of the most common vectors in Texas, but at least twelve different species of leafhoppers may transmit the organism to healthy plants.
Control recommendations include the following: (1) Obtain healthy seed, cuttings and plants. (2) Early control of leafhoppers on lettuce and carrots. (3) Spray weeds surrounding field with insecticide according to current recommendations. (4) Apply insect control before cultivation, weeding, and other field operations. (5) Control weeds during the growing season in the field, on irrigation ditch banks and in surrounding areas. (6) Avoid rotations where one susceptible crop follows another. (7) Destroy volunteer overwintering plants and avoid planting near established diseased crops. (8) Destroy affected plants in small areas as soon as they appear to be diseased. (9) Screen small plantings with wire mesh to exclude leafhoppers if practical. (10) Always keep in mind the relationship between cultivated hosts, insect vectors and wild or alternate hosts and practice proper sanitation and good husbandry throughout the year to insure optimum growth of crop, plants and proper control of weeds and insects.