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Diagnosing Plant Damage

  MG Manual Reference
Ch. 3, pp. 31 - 37

I. Symptoms appearing first or most severely on new growth (root and shoot tips, new leaves, flowers, fruits, buds)
A. Terminal bud usually dies. Symptoms on new growth.
1. Basal part of young leaves and internal tissues of organs may become necrotic.
One of the earliest symptoms is failure of the root tips to elongate normally. Terminal shoot meristems also die giving rise to a witch’s broom. Young leaves become very thick, leathery, and chlorotic; in some species young leaves may be crinkled because of necrotic spots on leaf edge during development. Young leaves of terminal buds become light green then necrotic and stem finally dies back at terminal bud. Rust colored cracks and corking occur on young stems, petioles, and flower stalks. "Heart rot" of beets, "stem crack" of celery... boron deficiency
2. Necrosis occurs at tip and margin of leaves causing a definite hook at leaf tip.
Calcium is essential for the growth of shoot and root tips (meristems). Growing point dies. Margins of young leaves are scalloped and abnormally green and, due to inhibition of cell wall formation, the leaf tips may be "gelatinous" and stuck together inhibiting leaf unfolding. Stem structure is weak and peduncle collapse or shoot topple may occur. Roots are stunted. Premature shedding of fruit and buds is common. Downward curl of leaf tips (hooking) occurs near terminal bud. ammonium or magnesium excess may induce a calcium deficiency in plants... calcium deficiency
Differentiating between calcium and boron deficiency symptoms: When calcium is deficient, there is a characteristic hooking of the youngest leaf tips. However, when boron is deficient, the breakdown occurs at the bases of the youngest leaves. Death of the terminal growing points is the final result in both cases.
3. Tissue breakdown – necrosis and firing of the tip and margins of the leaf.
The ammonium cation in itself may become phytotoxic and result in breakdown of the plant tissue (proteolysis -breakdown of plant proteins) initially producing a wet, dark-green, "steamed" appearance at the leaf tips and margins. This destroyed tissue eventually desiccates and becomes a light tan color. Excess ammonium may also induce calcium deficiency (abnormally dark green foliage, scalloped leaf margins, weak stem structure, death of terminal bud or growing point of the plant, premature shedding of the blossoms and buds)... ammonium excess
B. Terminal bud remaining alive. Symptoms on new growth.
1. Interveinal chlorosis on young leaves.
a. Interveinal chlorosis on young leaves with larger veins only remaining green. Necrotic spots usually absent; however, with extreme deficiencies, young leaves are almost white and may have necrotic margins and tips; necrotic spots may extend inward. potassium, zinc or copper excess can inhibit uptake of iron. High pH may also induce iron deficiency....iron deficiency
Iron deficiency symptoms are similar to those of magnesium deficiency, but iron deficiencies occur in young leaves first: Iron accumulated in older leaves is relatively immobile in the phloem.
b. Interveinal chlorosis with smallest veins remaining green producing a checkered or finely netted effect. Grey or tan necrotic spots usually develop in chlorotic areas; the dead spots of tissue may drop out of the leaf giving a ragged appearance. Poor bloom-both size and color. potassium excess can inhibit uptake of manganese... manganese deficiency
c. Stunted new growth with interveinal chlorosis: Young leaves are very small ("little leaf"), sometimes missing leaf blades altogether, and internodes are short giving a rosette appearance... zinc deficiency
2. Interveinal chlorosis is not the main symptom on new growth.
a. Wilting and loss of turgor of young, terminal leaves and stem tips is common. Symptoms are highly dependent upon plant species. In some species younger leaves may show interveinal chlorosis while tips and lobes of older leaves remain green followed by veinal chlorosis and rapid, extensive necrosis of leaf blade... copper deficiency
There are no known reports of H2PO4-1 toxicity; however, plants may take up the phosphate anion in luxury amounts. Phosphorus excess is associated with impeded uptake and possible deficiency of copper and sometimes of zinc... phosphorous excess
b. Leaves light green, veins lighter in color than adjoining interveinal areas. Leaves over entire plant may become yellowish green, roots and stems are small in diameter and are hard and woody. Young leaves may appear to be uniformly yellow. Some necrotic spots... sulphur deficiency
In plant species where the sulfur is not withdrawn from the older leaves and retranslocated to the new growth, leaves matured prior to onset of sulfur deficiency remain green: This retention of green color of older foliage distinguishes sulfur deficiency in these species from nitrogen deficiency where the nitrogen is translocated from the older foliage into the new leaves. With nitrogen starvation, old leaves as well as new leaves turn yellow.
c. Shoot inhibition causing malformed and twisted tops with major injury at the tips and edges of the leaves... xylem-translocated "shoot-inhibiting chemicals"
• EXAMPLES OF TOXIC XYLEM-TRANSPORTED CHEMICALS include the thiocarbamate herbicides (symptoms on grasses and broadleaves) and alachlor and metolachlor (symptoms on grasses)
d. Young tissues discolored or deformed and injury may persist for several sets of new leaves...toxic phloem-translocated chemicals
• EXAMPLES OF TOXIC PHLOEM-TRANSPORTED CHEMICALS include the herbicides 2, 4-D, dicamba, picloram, glyphosate, amitrole, dalapon, sethoxydim and fluazifopbutyl.
II. Symptoms do not appear first or most severely on youngest leaves: Effect general on whole plant or localized on older, lower leaves.
A . Chlorosis general, no interveinal chlorosis. Effects usually general on whole plant.
1. Visible symptoms include yellowing and dying of older leaves. Foliage light green, growth stunted, stems slender, yellow...nitrogen deficiency
Plants receiving enough nitrogen to attain limited growth exhibit deficiency symptoms consisting of a general chlorosis, especially in older leaves. In severe cases, these leaves become completely yellow and then light tan as they die. They frequently fall off the plant in the yellow or tan stage.
2. Older leaves wilt. Entire leaf is affected by chlorosis, but edges and leaf tissues near main veins often retain more color (chlorophyll)...zinc excess
B. Vein-clearing, chlorosis-necrosis at leaf tips and margins, older-younger foliage...xylem-transported photosynthetic-inhibitors. When toxic chemicals are xylem-translocated to older, fully-expanded leaves, the toxicity symptoms generally occur on the margins and interveinal areas of the leaf. When translocated to young, expanding leaves, toxicity symptoms are generally associated with the veins, especially the midrib.
• EXAMPLES OF XYLEM-TRANSLOCATED, PHOTOSYNTHETIC INHIBITORS include the triazine, urea and uracil herbicides.
C. Interveinal chlorosis. Interveinal chlorosis first appears on oldest leaves.
1. Older leaves chlorotic, usually necrotic in late stages. Chlorosis along leaf margins extending between veins produces a "Christmas tree" pattern. Veins normal green. Leaf margins may curl downward or upward with puckering effect. Necrosis may suddenly occur between veins. Potassium or calcium excess can inhibit uptake of magnesium...magnesium deficiency
When the external magnesium supply is deficient, interveinal chlorosis of the older leaves is the first symptom because as the magnesium of the chlorophyll is remobilized, the mesophyll cells next to the vascular bundles retain chlorophyll for longer periods than do the parenchyma cells between them. Leaves lose green color at tips and between veins followed by chlorosis or development of brilliant colors, starting with lower leaves and proceeding upwards. The chlorosis/brilliant colors (unmasking of other leaf pigments due to the lack of chlorophyll) may start at the leaf margins or tips and progress inward interveinally producing a "Christmas" tree pattern. Leaves are abnormally thin, plants are brittle and branches have a tendency to curve upward. Twigs are weak, subject to fungus infection, usually leaves drop prematurely; plant may die the following spring.
2. Smaller veins in older leaves may turn brown. Small necrotic spots in older leaves spread from the margins inwards, and finally desiccate the entire leaf blade. At severe, advanced stages, young leaves also display this spotting... manganese excess
3. Chlorotic areas (pale yellow) on whole plant; leaf edges curl upward... molybdenum deficiency
General symptoms are similar to those of nitrogen deficiency: Interveinal chlorosis occurring first on the older or midstem leaves, then progressing to the youngest. Sometimes, as in the "whiptail" disease, plants grown on ammonium nitrogen may not become chlorotic, but develop severely twisted young leaves, which eventually die. Other characteristic molybdenum deficiency symptoms include marginal scorching and rolling or cupping of leaves. With molybdenum deficiency, nitrogen deficiency symptoms may developing the presence of adequate levels off nitrate nitrogen in the root environment and high levels of nitrate nitrogen in the plant. Nitrate nitrogen must be reduced in the plant before it can be utilized. Molybdenum is required for this reduction, and if molybdenum is deficient, nitrate may accumulate to a high level in the plant, and at the same time the plant may exhibit nitrogen deficiency symptoms. Molybdenum differs from other trace nutrients in that many plants can develop in its absence provided that ammonium nitrogen is present. Molybdenum appears to be essential for the nitrate-reducing enzyme to function. Molybdenum deficiencies are commonly found in Northeastern Washington.
4. Foliar marginal necrosis is the most common symptom of fluoride toxicity along with Chlorosis along and between the veins occurs in fluorine-sensitive plants. With many plants, the marginal necrosis is preceded by the appearance of gray or light-green, water-soaked lesions which later turn tan or reddish-brown. Injury generally occurs at the tips of the leaves first, then moves inward and downward until a large part of the leaf is affected... fluoride excess
D. Leaf chlorosis is not the dominant symptom. Symptoms appear on older leaves at base of plant.
1. Plant dark green
a. At first, all leaves are dark green and growth is stunted. Purple pigment often develops in older leaves, particularly on the underside of the leaf along the veins...Leaves drop early... phosphorous deficiency
Phosphorus deficiency is not readily identified by visual symptoms alone. Visual symptoms of phosphorus deficiency are not always definite, but many phosphorus deficient plants exhibit off-color green foliage with purple venation, especially on the underside of leaves, and plants are stunted and remain stunted even when fertilizers supplying potassium and nitrogen are applied. Older leaves assume a purple-bronze color. Small growth, especially root development; spindly growth with tips of older leaves often dead. Phosphorus is phloem retranslocated from older leaves to new growth.
Aluminum appears to affect root growth in particular: Root tips blacken, no longer lengthen, but become thickened. Excess aluminum accumulation in roots reduces their capacity for translocating phosphorus. Amelioration involves suppression of aluminum activity, for example by liming to bring the medium’s pH above 5.5, and not by addition of phosphorus. The toxic amount of aluminum in a soil will depend upon other soil properties such as pH and phosphorus content and upon the plant grown. Media amendments such as perlite may release toxic quantities of aluminum if the media pH is extremely acid...aluminum excess
b. Leaves are thick and brittle and deep green. In acute toxicity, older leaves wilt and scorch from the margins inward... nitrate excess
2. Necrotic spots develop on older leaves
a. Margins of older leaves become chlorotic and then burn, or small chlorotic spots progressing to necrosis appear scattered on old leaf blades. Calcium excess impedes uptake of potassium cations.... potassium deficiency
Potassium deficiency symptoms first appear on the recently matured leaves of the plant (not on the young, immature leaves at the growing point). In some plants, the first sign of potassium deficiency is a white specking or freckling of the leaf blades. With time, the symptoms become more pronounced on the older leaves , and they become mottled or yellowish between the veins and scorched at the margins. These progress inward until the entire leaf blade is scorched. If sodium cations are present and taken up in place of K+1, leaf flecking (necrotic spots scattered on leaf surface) and reduced growth occur. Seed or fruit is shriveled. Potassium is phloem retranslocated from old leaves to new growth.
b. Tips and edges of leaves exhibit necrotic spots coalescing into a marginal scorch. Symptoms proceed from the plant’s base upwards with older leaves being affected first. In advanced, severe toxicity, necrotic spots with a pale brown center also appear in the inner parts of the leaf blade... boron excess
c. Mottling and necrotic spots primarily on margin and interveinally may occur on leaves due to excessive amounts of fertilizers or pesticides applied either as foliar sprays... direct-contact of toxic chemical with shoot & foliage
• EXAMPLES OF SHOOT DIRECT-CONTACT TOXIC CHEMICALS INCLUDE the shoot-foliage applied herbicides paraquat, acifluofen, dinoseb and the herbicidal oils produce this type of symptom.
3. Reduced growth and wilting of older leaves with development of chlorotic and necrotic spots. Roots become stunted in length and thickened, or club-shaped, near the tips: The shoots remain normal but may show nutrient and moisture stress. Under severe conditions, root tips may be killed causing general stunting of the plant, wilting followed by marginal drying of the lower leaves first... direct-contact injury by toxic chemicals or other factors in the root zone, i.e. low temperatures; nematodes; root weevils.
• EXAMPLES OF ROOT DIRECT-CONTACT TOXIC CHEMICALS INCLUDE excess salts or presence of toxic chemical such as the herbicides DCPA, dinitroanilines, diphenamid.
Leaves eventually become bronze colored....chloride deficiency
4. Marginal scorching that may progress to general leaf scorching. Generally no spotting... excess salt or sodium excess
5. Intense yellow or purple color in leaves. Molybdenum excess or toxicity in field-grown plants is rarely observed. Plants appear to tolerate relatively high tissue concentrations of molybdenum. Isolated reports of symptoms from excess molybdenum include development of intense yellow color in tomato leaves and intense purple color in cauliflower leaves... molybdenum excess
References, Laboratory Analyses
If you have identified the plant and have narrowed the probable cause down through the various categories, (i.e. distinguished between living and nonliving – then if living, distinguished between pathogens and animal factors – then if pathogen, distinguished between fungal and bacterial organisms), you will probably need assistance in identifying the specific responsible organism or nonliving factor. But, by now you know what specialist to contact (plant pathologist, entomologist, physiologist...) and what specific reference book would provide further assistance in narrowing down the search for the specific factor causing the observed plant damage. Laboratory analyses and examination may be required to further narrow the range of probable causes.
V. Synthesis of Information to Determine Probable Causes of the Plant Damage
The detective work to find the "signs" (residues of the living, damaging organism or nonliving factor, records, etc.) is time consuming and methodical. But, without this process of elimination and synthesis, probability of making a correct diagnosis is low.

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