Infectious and Non-infectious Plant Diseases

Infectious and Non-infectious Plant Diseases

What is non-parasitic disease?

Plant diseases in which no foreign organism or parasite is associated with the cause are known as non-parasitic disease. They differ from virus diseases in being non-infectious.

Also read: Phytoplasma Plant Diseases Examples

Infectious and Non-infectious Plant Diseases


Such diseases are induced by certain disorders in the physiology of plants due to unfavorable environments including the soil conditions as listed below.

Also read: Causal Organisms of Plant Diseases

Role of low temperature in plant diseases

Damage to plant is much more by low temperature than by high temperature. Warm weather plants are more susceptible to low temperature.

Examples: Freezing injury in potatoes and frost injury to winter crops are common examples. Subfreezing temperature in causes ringlike necrosis of vascular elements.

The frost injury to winter field crops is characterized by death of meristematic tissues the apical portions ans leaves showing the maximum damage. Low temperature usually kills the tissues by ice formation in and between the cells which rupture.

Role of high temperature in plant diseases

Generally the adverse high temperature effects are seen in conjunction with other abnormal environmental conditions such as excess of light, low oxygen supply, drought, high winds, etc. Sun scald of fruits and vegetables are common examples. The side of the fruits exposed to the sun shows necrosis of the skin may go deeper in the flesh. High temperature usually injuries the tissues by inactivating certain enzyme systems while accelerating others. This leads proteins, disruption of cytoplasmic membranes and release of toxic products into the cell.

Role of unfavorable light

When unfavorable excess light combines with high temperature during summer months it causes injury to plants. Low light retards chlorophyll synthesis and promotes slender growth. The leaves are pale, almost colourless if light is completely excluded. Generally dense plant stand or dense canopy of leaves excludes light from around the lower parts of the plants.

Role of unfavorable moisture

The excessive moisture in the filed may cause rotting or wilting of the plants, primarily due to lack of oxygen and accumulation of toxic materials around roots and base of the stem, and also due to non-availability of nutrients under conditions of poor aeration. On the other hand, the absence of moisture causes the plants to die or wilt as direct effect or due to rise in temperature.

Unfavorable oxygen

When there is excessive respiration in closed atmosphere the entire supply may be exhausted resulting in disintegration of cells due to enzymatic action.

Atmospheric impurities

Sulphur dioxide impurities

  • 0.3-0.5 ppm sulpher dioxide is toxic to plants.
  • Low concentration causes general chlorosis.
  • High temperature causes bleaching of interveinal tissues.

Nitrogen dioxide impurities

  • It is toxic at 2-3 ppm.
  • At low concentration it suppresses growth.
  • High concentration causes bleaching and bronzing of plants.
  • Leaf margins of dicots and leaf tips of monocots turn brown to dark brown, die and may separate from the leaves.


Ozone is toxic at 0.1 ppm. It cause following diseases in plants:

  • Stippling of leaves
  • Mottling of leaves
  • Chlorosis of leaves
  • Premature defoliation and stunting of plants

Toxic effects of decomposing organic matter

Damping off of plants
Root rot of plants
Wilting in plants
Other nutritional deficiency

Nutritional disorders

Deficiency of minerals, viz., N, P, K, Magnesium, Boron, Zn, Copper, Iron, etc., results in disorders in the plant metabolism and cause hunger signs in the crop. Excess of a particular mineral disturbs nutritional balance needed for good metabolism in the plants thus hindering the effect of essential elements. The deficiencies or excess of these minerals also reduce the resistance of plants to fungal, bacterial and other diseases.

Diseases due to lack of minerals

Deficiency of essential minerals in the soil or their non-availability to the plant even if not deficient in the soil develop hunger signs in the crop. This is especially true for certain crops like maize, soybean, rice, etc.

Nitrogen: In deficiency of nitrogen plants grow poorly and leaves are of light green colour. Lower leaves turn yellow of light brown and the stem are short and slender. Overall effect is reduced yield.

Phosphorus: P deficiency leads to poor growth, bluish green leaves with purple tint. Lower leaves turn light bronze with purple or brown spots. Shoots are short, thin, upright and spindly.

Potash: K deficiency leads results in thin shoots which may show dieback symptoms. Older leaves show chlorosis and browning of tips. Scorching of the margins and brown spots near margins are common.

Copper: Most of the diseases caused by copper deficiency develop symptoms of chlorosis. The leaves may fail to unroll and tend to appear wilted. Plants may be dwarfed and distorted. Citrus trees show dieback symptoms during summer.

Zinc: Due to zinc deficiency diseases like rosette of apple, mottle leaf of citrus, and little leaf of grapevine are caused. Leaves develop prominent veins, become pale yellow, and sometimes bear dead patches. The khaira disease of rice is caused by zinc deficiency.

Iron: It is mainly concerned with the process of photosynthesis. The chlorosis of leaves without any mottling is the most prominent effect.

Magnesium: Plants lacking this element show a progressive loss of green colour of leaves over the entire lamina surface. Usually this yellowing appears on the older leaves first because the magnesium present in these parts is drawn by the young growing parts.

Manganese: Chlorosis of spinach and bean, spots on potatoes and grey speck of oats are common diseases resulting from manganese deficiency. The symptoms on oats appear first as light green spots on leaves, the areas enlarging and changing to buff of light brown colour.

Boron: The necrosis of tissues is the most important symptoms of boron symptoms. In heart rot of beet, the disease is characterized by curling of central leaves which turn a blackish brown and die.

Calcium: In calcium deficiency the young leaves become distorted with their tips hooked back and margins are curled. Leaves may be irregular in shape with scorching and spotting. Terminal buds die. Calcium deficiency causes blossom end rot of many fruit trees.

Sulphur: Sulpher deficiency symptoms resemble those of nitrogen deficiency. Sulphur deficiency is not very common.

Molybdenum: In deficiency of Mo plants exhibit severe yellowing and stunting and fail to set fruits. Whiptail of cauliflower is caused by the deficiency of molybdenum.

Silicon: Non availability of silicon results in suppression of cereal powdery mildew, rice blast and rice sheath blight.

Major plant diseases

Black heart of potato

Causes of black heart of potato

  • Poor ventilation in store
  • High temperature during transportation
  • High temperature of soil during growth and maturation of tubers in the field

Symptoms and damage

Dark grey to purplish or inky black discoloration occurs in the central tissues of the tuber. In advance stage, the affected tissues may dry out and separate thus forming cavities. The discoloration may extend to the surface also. Large tubers are more susceptible than small one to black heart. Tubers are not consumable.

Black tip or mango neurosis

Also read: Mango cultivation

Cause: The disease is most common in orchards in the vicinity of brick kilns. The smoke of kilns with sulphur dioxide causes necrosis of tissues of the fruit.

Symptoms and damages

The characterized by necrosis of tissues at the distal end of the fruit. The first symptom is the development of a small etiolated area at the distal end which gradually spreads, turns nearly black and covers the tip completely. The tips flattens with the outer skin turning hard and sunken. Inner portion is soft and yields a dark brown liquid due to rotting induced by saprophytic bacteria.

Infectious and non-infectious plant diseases