Dissemination of Plant Pathogens by air and water __________________________________________________
A plant disease may spread from plant to plant and from one locality to another. When the disease is caused by the plant pathogen; the plant pathogens are being carried by one means or another to uninfected plants and uninfected localities. The spread of such diseases of plants, depends on some agency of dissemination such as air, water, animals, birds, insects and man.
Dissemination by air: Of the several agencies by which plant pathogens are being disseminated, the air serves the largest number of pathogens and permits the most rapid and widespread distribution of them. Strong wind gusts are effective in dislodging and carrying the masses of fungal spores, bacteria, pollen, etc., to a reasonable height, and thus making them available to the convection currents of wind which carry them to great height. Thus the air in circulation carries these minute spores to long distances.
Dissemination of plant pathogens by air may take place in two general ways—(1) by wind distribution of diseased plant parts and (2) by wind dispersal of spores and vegetative cells. Spores of many phytopathogenic fungi are well adapted to aerial dispersal. They are very small and light in weight and may be carried easily by wind from one place to another. Common examples are uredospores of rust fungi, conidia of Albugo, Peronospora and Phytophthora, chains of conidia in powdery mildews, the conidia of many Ascomycetes and Deuteromycetes, and the teliospores of some smuts. Certain adaptations favour the spore dispersal. For instance the forcible discharge of basidiospores in Hymenomycetes and ascospores in many Ascomycetes.
For effective air dissemination, high spore production is also essential, because a large number of spores are wasted during this process. For example, rusts and smuts produce large number of spores. A single wheat kernel being converted into a bunt sorus by Tilletia caries contain 6-12 million spores. Ustilago maydis—the corn smut produces above six billion spores in a cubic inch of smut gall. Reproduction in bacteria is even more prolific. Because of the enormous production of plant pathogens, in spite of great wastage, during dissemination, they survive and develop during favourable conditions.
Numerous examinations of the spore content of the upper air have been made by aeroscopes, and the spores and pollen grains were found to be fairly abundant upto 11,000 feet, above which they become scarce. Nearly all the wheat grown in India is found as a winter crop in the plains. In these areas rust does not survive the intense heat of the summer, and the wheat crop is infected a new each year from rusted cereals grown in the Himalayan foot hills and Nilgiri and Palni hills of Peninsular India. The uredo-stage is abundant when the hill crops are harvested in May and June, and then persists on stubbles and self-sown wheat plants until the new hill crops are sown in the autumn. Inoculum is thus available in time to account for the first outbreak in the plains of Northern India which usually occur in January-February. The uredospores have been trapped in the air by means of aeroscopes before the rusts have been found on the crops in the plains.
Dissemination by water: Water may serve for dissemination of plant pathogens in two ways—(1) as a medium in which actively motile organisms or spores may swim about and (2) by the mechanical action of run off of rain; the flowing of irrigation water or by stream flow. Actively motile cells may be produced by some bacterial pathogens, the chytrids, the white rusts and few downy mildews. Such forms depend wholly or partially upon liquid moisture as a medium in which they may develop and through which they may migrate by the movement of their flagella.
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October 23, 2019 at 2:11 PM
Plant disease symptoms caused by plant viruses
Plant viruses can be recognized easily by the symptoms they produce on the host. Sometimes the same virus can cause widely different symptoms on different host plants, and the symptoms may vary in the same plant according to its age, nutrition and other environmental factors. Symptoms may sometimes be produced by a mixture of two or more viruses on the same plant. A few more common and easily detectable symptoms are given here:
Chlorosis. Due to the presence of plant viruses, the chlorophyll of the green parts disappears at places, leaving yellowish spots, this is known as chlorosis. The presence of yellow spots at places, in the green tissue appears like a 'mosaic' pattern and therefore, the diseases with such symptoms are known as 'mosaic' diseases. The viruses causing most mosaic diseases are mechanically transmitted and usually have aphid vectors in nature. Generally they do not stop flowering or affect the dormancy of buds.
Yellows. When the chlorophyll disappears completely (chlorosis, yellowing, bronzing or reddening) from the host tissue, the organs turn yellow and the symptoms are known as yellows. These viruses are generally transmitted by leaf-hoppers, and they are relatively sensitive to heat treatment.
Vein clearing and vein banding. The disappearance of chlorophyll along the veins of the leaves is known as vein clearing and when chlorophyll surrounding the veins disappears the symptom is known as vein banding.
Necrosis. The brownish spots due to the death and ultimate drying of the tissue are known as necrotic spots and the phenomenon is known as necrosis. Necrotic spots are also known as lesions.
Ring spots. These symptoms are characterized by the appearance of chlorotic or necrotic rings on the leaves and sometimes also on the fruit and stem. Most ringspot—causing viruses are being transmitted by nematodes.
Other symptoms. Bunchy top, galls, hypertrophy, atrophy, rolling (leaf roll of potato), curling (leaf curl of potato), crinkling of plants are various other symptoms usually produced on the host plants by viruses.
Plant disease symptoms caused by mycoplasma
The most important symptoms developed by mycoplasma like organisms (MLO's) are little leaf and yellows.
Little leaf. This denotes reduction in the leaf lamina, typical of the mycoplasma infection in brinjals, tomatoes and cotton.
Plant disease symptoms caused by nematodes
The symptoms caused by nematodes appear on the roots as well as on the above ground parts of plants. The symptoms on the roots appear as hypertrophy, root knots or root galls, root lesions, excessive root branching, injured root tips and root rots.
The root symptoms are usually accompanied by the malformation and blistering of the above ground parts of the plants, twisting or distortion of leaves and stems, and abnormal development of the floral parts. Certain nematodes attack grains forming galls full of nematodes in place of seed.
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October 23, 2019 at 2:13 PM
Definition of Seed vigor and Seed viability Definition of Seed vigor
• Seed vigor is the measures of the physiological stamina and healthiness of seed.
• Seed vigor may be defined as those seed properties which determine the potential for rapid uniform emergence and development of normal seedling under a wide range of field condition.
• The organic powder of the seed is called seed vigor. On the other hand, the intensity of living force is called vigor of seed.
Definition of Seed viability
• Seed viability denotes the degree to which a seed is alive and metabolically active and possess enzymes capable of catalyting metabolic reaction need for germination and seed growth.
• Seed viability means that a seed capable of germinating and producing normal seedling.
• The term viability refers to the condition of seed, whether it is living, dormant or dead.
Dissemination of Plant Pathogens by air and water
__________________________________________________
A plant disease may spread from plant to plant and from one locality to another. When the disease is caused by the plant pathogen; the plant pathogens are being carried by one means or another to uninfected plants and uninfected localities. The spread of such diseases of plants, depends on some agency of dissemination such as air, water, animals, birds, insects and man.
Dissemination by air: Of the several agencies by which plant pathogens are being disseminated, the air serves the largest number of pathogens and permits the most rapid and widespread distribution of them. Strong wind gusts are effective in dislodging and carrying the masses of fungal spores, bacteria, pollen, etc., to a reasonable height, and thus making them available to the convection currents of wind which carry them to great height. Thus the air in circulation carries these minute spores to long distances.
Dissemination of plant pathogens by air may take place in two general ways—(1) by wind distribution of diseased plant parts and (2) by wind dispersal of spores and vegetative cells. Spores of many phytopathogenic fungi are well adapted to aerial dispersal. They are very small and light in weight and may be carried easily by wind from one place to another. Common examples are uredospores of rust fungi, conidia of Albugo, Peronospora and Phytophthora, chains of conidia in powdery mildews, the conidia of many Ascomycetes and Deuteromycetes, and the teliospores of some smuts. Certain adaptations favour the spore dispersal. For instance the forcible discharge of basidiospores in Hymenomycetes and ascospores in many Ascomycetes.
For effective air dissemination, high spore production is also essential, because a large number of spores are wasted during this process. For example, rusts and smuts produce large number of spores. A single wheat kernel being converted into a bunt sorus by Tilletia caries contain 6-12 million spores. Ustilago maydis—the corn smut produces above six billion spores in a cubic inch of smut gall. Reproduction in bacteria is even more prolific. Because of the enormous production of plant pathogens, in spite of great wastage, during dissemination, they survive and develop during favourable conditions.
Numerous examinations of the spore content of the upper air have been made by aeroscopes, and the spores and pollen grains were found to be fairly abundant upto 11,000 feet, above which they become scarce. Nearly all the wheat grown in India is found as a winter crop in the plains. In these areas rust does not survive the intense heat of the summer, and the wheat crop is infected a new each year from rusted cereals grown in the Himalayan foot hills and Nilgiri and Palni hills of Peninsular India. The uredo-stage is abundant when the hill crops are harvested in May and June, and then persists on stubbles and self-sown wheat plants until the new hill crops are sown in the autumn. Inoculum is thus available in time to account for the first outbreak in the plains of Northern India which usually occur in January-February. The uredospores have been trapped in the air by means of aeroscopes before the rusts have been found on the crops in the plains.
Dissemination by water: Water may serve for dissemination of plant pathogens in two ways—(1) as a medium in which actively motile organisms or spores may swim about and (2) by the mechanical action of run off of rain; the flowing of irrigation water or by stream flow. Actively motile cells may be produced by some bacterial pathogens, the chytrids, the white rusts and few downy mildews. Such forms depend wholly or partially upon liquid moisture as a medium in which they may develop and through which they may migrate by the movement of their flagella.
Plant disease symptoms caused by plant viruses
Plant viruses can be recognized easily by the symptoms they produce on the host. Sometimes the same virus can cause widely different symptoms on different host plants, and the symptoms may vary in the same plant according to its age, nutrition and other environmental factors. Symptoms may sometimes be produced by a mixture of two or more viruses on the same plant. A few more common and easily detectable symptoms are given here:
Chlorosis. Due to the presence of plant viruses, the chlorophyll of the green parts disappears at places, leaving yellowish spots, this is known as chlorosis. The presence of yellow spots at places, in the green tissue appears like a 'mosaic' pattern and therefore, the diseases with such symptoms are known as 'mosaic' diseases. The viruses causing most mosaic diseases are mechanically transmitted and usually have aphid vectors in nature. Generally they do not stop flowering or affect the dormancy of buds.
Yellows. When the chlorophyll disappears completely (chlorosis, yellowing, bronzing or reddening) from the host tissue, the organs turn yellow and the symptoms are known as yellows. These viruses are generally transmitted by leaf-hoppers, and they are relatively sensitive to heat treatment.
Vein clearing and vein banding. The disappearance of chlorophyll along the veins of the leaves is known as vein clearing and when chlorophyll surrounding the veins disappears the symptom is known as vein banding.
Necrosis. The brownish spots due to the death and ultimate drying of the tissue are known as necrotic spots and the phenomenon is known as necrosis. Necrotic spots are also known as lesions.
Ring spots. These symptoms are characterized by the appearance of chlorotic or necrotic rings on the leaves and sometimes also on the fruit and stem. Most ringspot—causing viruses are being transmitted by nematodes.
Other symptoms. Bunchy top, galls, hypertrophy, atrophy, rolling (leaf roll of potato), curling (leaf curl of potato), crinkling of plants are various other symptoms usually produced on the host plants by viruses.
Plant disease symptoms caused by mycoplasma
The most important symptoms developed by mycoplasma like organisms (MLO's) are little leaf and yellows.
Little leaf. This denotes reduction in the leaf lamina, typical of the mycoplasma infection in brinjals, tomatoes and cotton.
Plant disease symptoms caused by nematodes
The symptoms caused by nematodes appear on the roots as well as on the above ground parts of plants. The symptoms on the roots appear as hypertrophy, root knots or root galls, root lesions, excessive root branching, injured root tips and root rots.
The root symptoms are usually accompanied by the malformation and blistering of the above ground parts of the plants, twisting or distortion of leaves and stems, and abnormal development of the floral parts. Certain nematodes attack grains forming galls full of nematodes in place of seed.
Definition of Seed vigor and Seed viability
Definition of Seed vigor
• Seed vigor is the measures of the physiological stamina and healthiness of seed.
• Seed vigor may be defined as those seed properties which determine the potential for rapid uniform emergence and development of normal seedling under a wide range of field condition.
• The organic powder of the seed is called seed vigor. On the other hand, the intensity of living force is called vigor of seed.
Definition of Seed viability
• Seed viability denotes the degree to which a seed is alive and metabolically active and possess enzymes capable of catalyting metabolic reaction need for germination and seed growth.
• Seed viability means that a seed capable of germinating and producing normal seedling.
• The term viability refers to the condition of seed, whether it is living, dormant or dead.