Diseases(causative agents)

(1)Biotic

Diseases

(2)Abiotic

Diseases

(I )PhysiologicalDiseases

(nutritional deficiency)

)II( EnvironmentalDiseases

(Temperature, pH, Rainfall)..,

Causative organisms:Virus, Bacteria ,

Fungi, Actinomycetes ,Nematodes

(i )Macronutrients(C, N, P, K, Mg, Ca)

(ii )Micronutients(Cu, Zn, B, Mo)

ABIOTIC DISEASES

Abiotic Diseases

(I) Physiological (nutritional deficiency)

(i) Macroelements (N, P, K, Mg, Ca)

Under nitrogen deficiency, chlorotic )yellow color( symptoms and light red cast can be seen on the veins and petioles of leaves: the older mature leaves gradually change from their normal characteristic green appearance to a much paler green. As the deficiency progresses these older leaves become uniformly yellow over the entire leaf including the veins. Leaves approach a yellowish white color under extreme deficiency. The young leaves at the top of the plant maintain a green but paler color and tend to become smaller in size. Branching is reduced in nitrogen deficient plants resulting in short, spindly plants )long week stem with poor root development(.

Nitrogen )N( – a major element needed by all green plants. It is transported from older growth to new growth.

Nitrogen (N)

The P-deficient leaves show some necrotic spots. A major visual symptom is that the plants are dwarfed or stunted: Phosphorus deficient plants develop very slowly in relation to other plants growing under similar environmental conditions but without phosphorus deficiency. Some species such as tomato, lettuce, corn and the brassicas develop a distinct purpling of the stem, petiole and the under sides of the leaves. Under severe deficiency conditions, there is also a tendency for leaves to develop a blue-gray luster. In older leaves under very severe deficiency conditions a brown netted veining of the leaves may develop.

Phosphorus )P( – an important mineral that stores energy in plants, also a flowering agent.

Phosphorus (P)

K-deficient leaves show marginal and tip necrosis )burn and dead(, others at a more advanced deficiency status show necrosis in the interveinal spaces between the main veins along with interveinal chlorosis. Under sever conditions, wilting and drying of the plant due to poor water uptake.

Potassium )K( – a nitrogen catalyst needed for enzyme manufacture.

Potassium (K)

The Mg-deficient leaves show advanced interveinal chlorosis, with necrosis developing in the highly chlorotic tissue. In its advanced form, magnesium deficiency may superficially resemble potassium deficiency. In the case of magnesium deficiency the symptoms generally start with mottled chlorotic areas developing in the interveinal tissue. The interveinal laminae tissue tends to expand proportionately more than the other leaf tissues, producing a raised puckered surface, with the top of the puckers progressively going from chlorotic to necrotic tissue. Later, leaf tips curl, entire plant turns yellow and dies. Magnesium is mobile and is transported from older to newer growth. Old growth is affected first. In some plants such as the Brassica, tints of orange, yellow, and purple may also develop.                                            

Magnesium )Mg( – is important in photosynthesis and the chlorophyll molecule where light energy is converted to chemical energy. Chlorophyll gives plants their green color.

Magnesium (Mg)

The very low mobility of Ca is a major factor determining the expression of Ca-deficiency symptoms in plants. The Ca-deficient leaves show necrosis around the base of the leaves )soft dead necrotic tissue at rapidly growing areas(, which is generally related to poor translocation of calcium to the tissue rather than a low external supply of calcium. Very slow growing plants with a deficient supply of calcium may re-translocate sufficient calcium from older leaves to maintain growth with only a marginal chlorosis of the leaves. This ultimately results in the margins of the leaves growing more slowly than the rest of the leaf, causing the leaf to cup downward. This symptom often progresses to the point where the petioles develop but the leaves do not, leaving only a dark bit of necrotic tissue at the top of each petiole.

Calcium )Ca( – helps form the structural parts of the plants )it is a major element in cell walls(.

Calcium (Ca)

Abiotic Diseases (cont.)

(I) Physiological (nutritional deficiency) (cont.)

(ii) Microelements (Cu, Fe, Zn, S, B, Mo)

These Cu-deficient leaves are curled, and their petioles bend downward. Copper deficiency may be expressed as a light overall chlorosis along with the permanent loss of turgor in the young leaves. Recently matured leaves show netted, green veining with areas bleaching to a whitish gray. Some leaves develop sunken necrotic spots and have a tendency to bend downward. Trees under chronic copper deficiency develop a rosette form of growth. Leaves are small and chlorotic with spotty necrosis.

Copper )Cu( is needed in only small amounts. This metal aids in plant metabolism and general health. It helps ward off disease and pests, aids in the utilization of iron and the manufacture of enzymes.

)Cu( Copper

The Fe-deficient leaves show strong chlorosis )yellow( at the base of the leaves with some green netting. The most common symptom for Fe-deficiency starts out as an interveinal chlorosis of the youngest leaves )low mobility of Fe(, evolves into an overall chlorosis, and ends as a totally bleached leaf )white(. The bleached areas often develop necrotic spots. Up until the time the leaves become almost completely white they will recover upon application of iron. In the recovery phase the veins are the first to recover as indicated by their bright green color. This distinct venial re-greening observed during iron recovery is probably the most recognizable symptom in all of classical plant nutrition.

Iron )Fe(

Iron )Fe( – is an important constituent of enzymes and plays a role in photosynthesis.

The Zn-deficient leaves show an advanced case of interveinal necrosis. In the early stages of zinc deficiency the younger leaves become yellow and pitting develops in the interveinal upper surfaces of the mature leaves. As the deficiency progress, these symptoms develop into an intense interveinal necrosis but the main veins remain green, as in the symptoms of recovering iron deficiency. In many plants, especially trees, the leaves become very small and the internodes shorten, producing a rosette like appearance )short stem internodes(.

Zinc )Zn( – is needed in small amounts for growth and chlorophyll synthesis.

Zinc )Zn(

The S-deficient leaves show a general overall chlorosis while still retaining some green color. The veins and petioles show a very distinct reddish color. The visual symptoms of sulfur deficiency are very similar to the chlorosis found in nitrogen deficiency. However, in sulfur deficiency the yellowing is much more uniform over the entire plant including young leaves. The reddish color often found on the underside of the leaves and the petioles has a more pinkish tone and is much less vivid than that found in nitrogen deficiency. With advanced sulfur deficiency brown lesions and/or necrotic spots often develop along the petiole, and the leaves tend to become more erect and often twisted and brittle.

Sulfur )S( – is a building block of amino acids and proteins. Used in small amounts, it aids transpiration and transport of other elements.

Sulfur )S(

The B-deficient leaves show a light general chlorosis. The tolerance of plants to boron varies greatly, to the extent that the boron concentrations necessary for the growth of plants having a high boron requirement may be toxic to plants sensitive to boron. Boron is poorly transported in the phloem of most plants )affects new growth first(, with the exception of those plants that utilize complex sugars, such as sorbitol, as transport metabolites. The B-deficient causes "Heart rot" in beets and "stem crack" in celery.

Boron )B( – is needed in small amounts. Boron aids in cell division and in transporting sugars through cell walls. It also aids in forming the amino acids thymine and cytosine, important to DNA synthesis.

Boron )B(

The Mo-deficient leaves show some mottled spotting along with some interveinal chlorosis. An early symptom for Mo-deficiency is a general overall chlorosis, similar to the symptom for nitrogen deficiency but generally without the reddish coloration on the undersides of the leaves. At high concentrations, molybdenum has a very distinctive toxicity symptom in that the leaves turn a very brilliant orange. In the case of cauliflower, the lamina of the new leaves fail to develop, resulting in a characteristic “whiptail appearance” )long, narrow and severely twisted, but not tightly bunched(.

Molybdenum )Mo( – a catalyst needed in small quantities. It is involved in nitrogen fixation )assimilation( and in the manufacture of enzymes.

Molybdenum )Mo(

Abiotic Diseases (cont.)

(II) Environmental

(Sunlight, Humidity, Temperature, pH,

Chemical addition, Rainfall, Soil texture)

D. deremensis 'Warneckii' is subject to notching, a condition affecting marginal tissue near the leaf base. This condition is apparently due to high temperatures. Apical necrosis caused by rapid increases in temperature in foliage plants was first described in 1961. The tested plants were not damaged at 15.5 or 35ºC )60 or 95ºF(, but raising the temperature from 15.5ºC to 35ºC resulted in apical necrosis or blackening of new leaves.

High temperature

The symptoms are characteristically greasy-looking, water-soaked lesion, developing within 48 hours of exposure and affecting old leaves most severely.

Chilling temperature

Cold damage

Cold or cool environments )2-8ºC( can severely damage tropical plants. Temperatures that are safe for many other crops cause some injury in some foliage plant. Exposure to these temperatures results the development of white, sunken, irregularly shaped lesions. Cold injury is more severe in plants receiving high levels of nitrogen and potassium than in plants receiving lower levels of these nutrients.

Direct sunlight exposure

necrosis )burn and dead(

Drought can broadly be defined as a low water availability that limits or prevents growth. The severity of drought depends on the duration. Symptoms of drought stress begin with a bluish color, followed by leaf rolling and eventual browning. Drought stress also can be observed as localized dry spots that appear as irregular patterns of dead and dying leaves.

Drought Injury

When too much fertilizer is applied, the nutrient balances in grass plants or soil systems disrupt so, excessive growth is favored, which may result in scalping damage )“burn” or desiccation of leaf tissues(.

Excess fertilizer

Many pesticides used for weed control and turfgrass disease control, and other pest management chemicals, have growth regulating properties. Misapplication of pesticides may cause injury or irregular growth.Pesticide injury is easily differentiated from disease and insect injury. Generally the damage will appear in a pattern that corresponds to the method of application and includes broad to narrow streaks, corresponding to overlapped application or other regular patterns. Specific signs of this type of injury include lack of pest control, turfgrass chlorosis, leaf speckling and death. These symptoms can develop quickly after application or several weeks later.

Pesticide Damage