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DIAGNOSING PLANT DAMAGE: IV. DETERMINE CAUSES [continued]

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  MG Manual Reference
Ch. 5, pp. 26 - 30
[Determine Causes: determine | symptoms and signs | distinguishing | chemical injury]


CHEMICAL INJURY PATTERNS ON AN INDIVIDUAL PLANT Top

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.
Symptoms from Direct Contact of Chemicals with the Plant:
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.
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.
Symptoms of Deficient or Toxic Translocated Chemicals:
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.
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.
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.
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.
Xylem ranslocated chemical move primarily upward in the plant to the foliage.
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.
  • 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.

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.
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.
  • Deficient Nutrient Ions, xylem-translocated (phloem immobile)

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.
Phloem translocated chemicals move multidirectionally from point of application or source of the chemical to the meristematic regions.
  • Toxic Chemicals – Phloem translocated

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.
  • Deficient Nutrient Ions – Phloem mobile

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.



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