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A general uniform pattern of damage occurring over
several plant species and over a relatively large area indicates a
nonliving factor such as a chemical phytotoxicity.
Questions-answers, records, the plant symptoms and knowledge about
the mobility within the plant of the common chemicals (nutrients
and pesticides) should help determine which chemical caused the
damage. Patterns of injury symptoms on an individual plant that
develop because of deficiency, excess or toxicity of a chemical
differ depending primarily upon whether the chemical causes damage
directly on CONTACT or is absorbed and distributed within the
plant through PHLOEM-TRANSLOCATION or through XYLEM-TRANSLOCATION.
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Symptoms from Direct Contact of Chemicals with the
Plant: |
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Shoot-foliage Contact: Symptoms from
shoot-contact chemicals occur over the general plant canopy. If
the toxic chemical is applied directly to the above ground parts
of the plant (SHOOT-FOLIAGE CONTACT CHEMICAL), the physical
pattern of application may be detected, i.e. spray droplet size,
etc. If the toxic chemical is spray-applied, the pattern of spray
droplets or areas where spray accumulated to runoff along the leaf
edges will show most severe damage. If it is a toxic gas (volatile
chemical acting as an aerial pollutant), the areas between the
leaf veins and along the leaf margins where the concentration of
water within the leaf is lower will be the first to show damage.
Injury from foliar applications of insecticides, fungicides and
fertilizers is primarily of the direct-contact type and is
typified by chlorotic-necrotic spotting, especially interveinally
and along leaf edges and other areas where chemical concentrates
and is least diluted by intercellular moisture. Examples of
SHOOT-FOLIAGE CONTACT CHEMICALS are foliar-applied fertilizer
salts and the herbicides paraquat, acifluorfen, dinoseb, and
herbicidal oils. |
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Root Contact: Toxic CONTACT CHEMICALS
in the root zone, including excess fertilizer, result in poor root
development. Symptoms from root-contact chemicals are localized
where the chemical contacts the root, but produce general symptoms
in the shoot. The shoots may show water and nutrient stress
symptoms, i.e. reduced growth, wilting, nutrient deficiency
symptoms. The injury symptoms on the shoot and foliage from root
damage by direct contact with toxic chemicals or excessive salts
resembles a drying injury -the roots are unable to obtain water.
Roots are injured and root tips may be killed. This will result in
a general stunting of the plant. In severe cases, wilting can
occur even though the soil is wet. LOWER LEAVES generally wilt
first and this is followed by a marginal drying of the leaves.
Many factors injuring or inhibiting root growth may produce
similar shoot symptoms: Nematodes, soil compaction, cold weather,
salinity, nutritional disorders and certain herbicides
(dinitroanilines, DCPA, and diphenamid) cause root inhibition. |
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Symptoms of Deficient or Toxic Translocated
Chemicals: |
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The effects of mobile chemicals absorbed by the
plant are dependent upon whether the chemical is transported in
the phloem or in the xylem. If transported solely in the xylem
system, the chemical will move upward in the plant in the
xylem-transpiration stream. |
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Toxic symptoms from xylem-translocated chemicals occur
primarily in the older foliage. Deficiency symptoms of
xylem-transported (phloem-immobile) nutrient ions will occur first
in the new growth. |
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If the chemical is translocated in the phloem, it may
move multidirectional from the point of absorption, i.e. it may
move from the shoot to the root or the reverse. |
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Toxic symptoms from phloem-translocated chemicals occur
primarily in the new growth and meristematic regions of the plant.
Deficiency symptoms of phloem-retranslocated nutrient ions occur
first in the older foliage. |
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Xylem ranslocated chemical move primarily upward in
the plant to the foliage. |
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Chemical is translocated upward in the xylem
(apoplastic movement) of the plant from the point of absorption.
Symptoms occur in tissues formed after the toxicity or deficiency
occurs. |
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- Toxic Chemicals – xylem translocated.
When toxic chemicals are translocated to fully expanded, older
leaves, the toxicity symptoms generally appear on the leaf
margins and terveinal areas. When toxic chemicals are
translocated to immature, young leaves, the toxicity symptoms
generally appear associated with the veins, especially the
midrib.
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Photosynthetic-Inhibiting Chemicals –
Injury from translocated toxic chemicals is primarily to the
foliage. Plant injury generally progresses from the lower, older
foliage to the top. Individual leaves show greatest injury
(chlorosis) along their tips and margins or along the veins.
Examples of xylem-translocated herbicides include the
photosynthetic inhibitors such as the triazine, urea and uracil
herbicides. |
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Shoot-Inhibiting Chemicals – Examples of
toxic chemicals absorbed by the roots and translocated in the
xylem to the shoots are the "shoot inhibiting herbicides".
The shoot inhibitors cause malformed and twisted tops with major
injury at the tips and edges of the leaves; looping of the leaves
may occur since the base of the leaf may continue to grow while
the leaf tips remain twisted together. Thiocarbamate herbicides
cause these symptoms on both grasses and broad-leaves. Alachlor
and metolachlor herbicides cause similar injury symptoms on
grasses. |
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- Deficient Nutrient Ions, xylem-translocated
(phloem immobile)
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Several nutrient ions after upward translocation in the
xylem and incorporation in plant tissue are immobile: They cannot
be withdrawn when deficiencies develop in the root zone and
retranslocated in the phloem to the new growth. Deficiency
symptoms of phloem-immobile nutrient ions develop on the
new growth. Boron and calcium are quite phloem-immobile which
means that if the external supply becomes deficient, the symptoms
of boron and calcium deficiency will appear first on the new
growth. And, with severe deficiencies, the terminal bud dies.
Iron, manganese, zinc, copper, and molybdenum are also relatively
phloem-immobile and are not readily withdrawn from the older
leaves for translocation through the phloem to younger leaves and
organs. Deficiency symptoms are most pronounced on the new growth. |
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Phloem translocated chemicals move
multidirectionally from point of application or source of the
chemical to the meristematic regions. |
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- Toxic Chemicals – Phloem translocated
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Injury from phloem-translocated toxic chemicals is
primarily to new leaves and roots because of translocation of
chemical to the meristems. Whether taken up by the roots or
shoots, these compounds are moved through the living plant cells
and phloem (symplastic movement) to both the root and shoot tips.
The young tissue (shoots or roots) will be discolored or deformed
and injury may persist for several sets of new leaves. Examples of
phloem-translocated toxic chemicals, whether absorbed by the roots
or shoots, include the herbicides 2, 4-D, dicamba, picloram,
glyphosate, amitrole, dalapon, sethoxydim and fluazifopbutyl.
These compounds move to the meristems and typically injure the
youngest tissues of the plant. |
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- Deficient Nutrient Ions – Phloem mobile
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If phloem movile nutrient ions become deficient
in the root zone, these ions may be withdrawn from the older plant
tissue and retranslocated in the phloem to the new growth. In such
situations, deficiency symptoms will first occur on the older
leaves. Elements that may be withdrawn from older leaves and
retranslocated in the phloem to younger leaves and storage organs
include nitrogen, phosphorus, potassium, magnesium, chlorine and,
in some plant species, sulfur. Sulfur: In plant species where
sulfur can be withdrawn from the older leaves and translocated to
the newer growth, deficiency symptoms may initially occur on the
older leaves or over the plant in general. In plants where sulfur
is not readily re-translocated, the older leaves may remain green
and the sulfur deficiency symptoms occur only on the new growth. |
