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Definition of Plant Pathology

‘Plant pathology’ may be defined as ‘the study of microorganisms and of the environmental factors that cause disease in plants; of the mechanisms by which these factors induce disease in plants; and of the methods of preventing or controlling disease and reducing the damage it causes’. Plant Pathology or ‘Phytopathology’ is a branch of Biological Science, which exclusively deals with plant diseases and various aspects connected with plant diseases. The term Phytopathology had been derived from the Latin language. The Latin word ‘PHYTON‘ means plants; ‘PATHOS‘ means suffering and ‘LOGOS‘ means Science. So, Phytopathology means the Science dealing with suffering plants. Trees, shrubs, grasses etc., whether cultivated or wild are termed as plants and they form the majority of the earth’s living environment. These plants, which are so vital for humans, animals and insects are vulnerable to attack by numerous microorganisms, called ‘plant pathogens’. Some environmental factors are also detrimental to the normal growth of plants, resulting in diseases. Diseased plants exhibit some type of abnormalities and these abnormalities are termed as ‘disease symptoms’. Depending upon the pathogens attacking the plants or adverse environmental factors the disease symptoms vary markedly.

The Science of Plant Pathology has four main objectives:

1. To acquire knowledge about biotic plant pathogens and abiotic environmental factors, which are responsible for causing diseases.
2. To know more about the occurrence and mode of spread of the pathogens on a large scale.
3. To study the interaction between the host and the pathogen with respect to environmental factors.
4. To adopt suitable methods of preventing or controlling the diseases, thereby reducing the damage caused by the pathogens.

Systematic knowledge about Plant Pathology will be of much help in protecting the plants from attack by the pathogens, thereby allowing the plants to grow and attain maturity according to their genetic potential.

Agathi is a fast growing tree upto 10m. tall. Branching is drooping. The stem bark is deeply furrowing forming cork like plates. Leaves are compound, pinnae, 20 to 30 cm. long glabresence to pubescent. Leaflet oblong rounded 18 to 24 pairs of leaflets that are green to dark green mostly, bitter free. Inflorescence is a raceme 4 to 7 cm long and bearing 2 to 5 flowers that are white, red, yellowish and pink. Flowers are 5 to 10 cm long, 3cm wide, curved calyx bell shaped 2 cm long, 2 lobed, fine teeth like serration, fleshy. 

Physical Properties of Soil

Physical properties of soil have many important roles in the crop production. The basic requirements for germination, seedling emergence and root development are adequate moisture and aeration, optimum temperature and absence of mechanical hindrance. After germination the further development of plant is also governed by physical conditions of soil. Soil texture, soil structure, soil density, soil consistence, soil water, soil air, soil temperature and soil colour are main physical properties of soils also affecting mechanical behaviour of soils. These are described here.

1. Soil Texture

The weathering divides larger particles of rocks and minerals in to the smaller ones. This process goes continuously. Thus, particles of different sizes ranging from large fragments of rocks to the very fine clay minerals are found in soil. However, for texture particles less than 2mm diameter is considered. The relative amount of these particles indicates the soil type also. Such as sandy soils contain more amounts of coarse particles i.e., sand and clayey soils are rich in fine particles. The rock and mineral particles having diameter less than 2.0 mm and come between specifies size are called as soil separates. International society of soil science (ISSS) and United States Department of Agriculture (USDA) systems are Generally, followed to classify the soil separates. This classification is given in table 3.1.

CEREALS

Rice (Oryza sativa)

1. Rice blast
Pyricularia oryzae

‘Rice blast’ occurs in more than 80 countries across the world. The disease is more prevalent in Thailand, Japan, America, Guyana, Philippines and India. It is particularly serious in places, where rice is irrigated or receives heavy rainfall and where high levels of nitrogenous fertilizers are applied. Rice blast was first reported in China in 1637, in Japan in 1704 and in India in 1913. In India, it is found in all rice growing states, particularly in Telungu Desam, Karnataka, Maharashtra, Uttar Pradesh. Assam, Tripura, West Bengal, Bihar, Orissa, Himachal Pradesh, Punjab, Madhya Pradesh, Gujarat, Kerala, Tamil Nadu and Pondicherry and causes considerable damage to rice crop. In Tamil Nadu, the disease occurs in all rice growing districts and attains epidemic proportions in some seasons. Several rice blast epidemics have occurred in different parts of the world, causing yield losses ranging from 50 to 90 %. In Tamil Nadu, a serious epidemic of the disease occurred in 1919 in the Thanjavur Delta.

Symptoms. The disease, though chiefly a foliage disease, attacks other parts of rice plants, such as leaf sheath, rachis, joints of the culms and even the glumes. On the leaves, the initial symptoms appear as small bluish or grayish specks, which remain circular in older leaves. On young leaves, they enlarge and become eye-shaped, diamond-shaped or spindle-shaped spots, extending over several centimeters in length and about 1.0 cm. in width. The central portion of the lesion appears pale green or grayish green and water-soaked in the beginning. In older spots, the center becomes white to gray with a dark-brown margin surrounded by a yellow hallow. The lesions may coalesce, cover large area of the leaves and eventually the entire leaves are killed. The shape, size and color of the spots vary depending upon the variety, growth stage of the plants and environmental conditions. Similar spots may be formed on the leaf sheath also. During severe incidence, large areas of the leaves dry and die and the entire crop presents a burnt appearance. Hence the disease is known as ‘blast disease’. Leaf blast is more severe during the seedling and maximum tillering stages. In the nursery, the seedlings show numerous spots on the leaves, which wither and die. In case of severe infection, large number of seedlings may die. Blast also affects the leaf collar, which results in the death of the entire leaf.

The disease attacks the stem-nodes at the heading stage leading to production of ‘white panicles’ or the stem may break at the infected node. Rarely the internodes are affected. Further, at heading the fungus also attacks the panicle neck, which is girdled and becomes shining black and shrivelled. This is the most destructive phase of the disease and is known as ‘neck blast’, ‘neck rot’ or ‘panicle blast’. If infection of the panicle neck occurs early, the grains become chaffy and the panicle remains erect. If panicle neck infection occurs late, the grains become partially filled and because of the weight of the grains, the panicle breaks at the neck- node and droops. Sometimes, only parts of the panicle and joints of the culms and some glumes are infected and develop brown to black spots (Fig. 1).

Under conditions highly favorable for the pathogen, even the varieties resistant to the fungus under normal conditions are also attacked and small, round or short elliptical spots develop. However, the spots normally do not enlarge as typical blast spots.

USA ranks 17th in per capita cut flowers consumption.

India ranks 2nd in flower cultivation next to China.

India stands 14th in the world in exporting floricultural products, while the Netherlands and Columbia rank 1st and 2nd, respectively

Location

Bundelkhand is a geographical and cultural region and also a mountain range in central & North India. The hilly region is now divided between the states of Uttar Pradesh and Madhya Pradesh with the larger portion lying in the latter state. Bundelkhand region is situated between Vindhya range in south and Indo-Gangetic plain in north and lies between 23º10’ and 26º27’ N Latitude and 78º4’ and 81º34’ E Longitude, covering an area of 70,747 sq km. It spread over two states viz. Uttar Pradesh and Madhya Pradesh and comprises of 14 districts. It is spreading over of seven districts of Uttar Pradesh viz. Jhansi, Lalitpur, Jalaun, Hamirpur, Mahoba, Banda and Chitrakoot and similarly seven districts of Madhya Pradesh viz. Sagar, Panna, Tikamgarh, Chhatarpur, Damoh, Datia and recently formulated Niwari district (Fig. 1). The language spoken in Bundelkhand is Bundelkhandi and Hindi. The total population of Bundelkhand as 2011 census is 1,83,35,044.

Vegetables are rich source of vitamins, minerals than other than nutrients. In India, vegetables occupy only 2.8% of the total cropped area. Vegetables are (except mushroom) grown for their succulent and edible parts such as the roots, stems, leaves, young tops, fruits or seeds which are used in culinary preparations either fresh or preserved in the fresh state.

These crops are further classified into different groupings according to similarities in edible parts, growth habits, methods of culture and botanical family.

Aloe, known for laxative property is one of the old natural remedies that have emerged as modern miracle drug. Popularly known as ‘nature’s sun screen’ has growing use as skin tonic in cosmetic industry and as a nutraceutical. Aloes are also quite popular among succulent ornamental plants because of their stiff, harsh and rugged habit and are commonly grown in deserts and other dry situations. Genus Aloe is indigenous to African continent and Mediterranean countries such as Greece and Southern Italy, and got naturalized in arid tracts of India. In India, Aloe barbadensis is cultivated on a small scale in Rajastan, Andhra Pradesh and Gujarat (CSIR, 1998).

Botany

Genus Aloe belonging to Liliaceae comprises 275 species distributed in Africa, Asia and America. Forty-two species belong to Madagascar region, 12-15 to Arabian Peninsula and rest are distributed over tropical South Africa. Aloe barbadensis, A. ferox and A. arborescens are used as herbal drugs. In India, only four species occur and of these, Aloe barbadensis is the most widely naturalized species. These taxa comprise several varieties viz., officinalis, chinensis, littoralis and they cross freely and several intermediary forms occur in semi wild state.

1. INTRODUCTION

India is principally an agricultural country with seventy per cent of its population living in villages and fifty nine per cent of the total work force dependent on agriculture for its daily bread. Almost 20% of the gross GDP is contributed by agriculture. A steep surge in production since late 1960s has made country self-reliant in food grain production with buffer stocks full to the brim. Since independence, 0.66% growth/ year in gross agricultural area combined with 2.67%/year increases in food grain production led to a net increase in per capita food grain availability from 395 g/ day to 475 g/ day. An outstanding achievement, as it happened despite an unabated population build up at a rate of 2.15 %/ year or 13 million more mouths to be fed every year. Not only in food grain production, the country has made revolutionary progress in milk, fish, eggs, fruits and vegetable production also (NAAS, 2004).

Needless to say that the economic growth of our country depends largely on the prosperity of the agriculture. The agricultural sector acts as a bulwark in maintaining food security as well as national security. The first and the foremost role of agricultural is its promotion of economic progress in the country by enlarging the savings of the rural sector and making them available for the development of industries. Gandhiji, the father of the nation called “Agriculture as the soul of India”. Also, agriculture has been termed as “The mother of all industries and the maintainer of human life standing at the need of all sciences and arts, indeed, at the head of civilization and progress”. The significance of agriculture in India arises also from the fact that the development in agriculture is an essential condition for the development of the national economy. Thus any change in the agriculture sector- positive or negative- has a multiplier effect on the entire economy (Rekha & Rangappa, 2010).

Over 70 years ago, Mahatma Gandhi made the following observations while addressing youth who wanted to serve rural India:

‘The fact is that the villagers have lost all hope. They suspect that every stranger’s hand is at their throats and that he goes to them only to exploit them. The divorce between intellect and labour has paralyzed our agriculture. The worker should enter a village full of love and hope, feeling sure that where men and women labour unintelligently and remain unemployed half the year around, he working all the year round and combining labour with intelligence cannot fail to win the confidence of the villagers’.

This message is as relevant today as it was 60 years ago. Integrating brain and brawn in farm enterprises should hence be the purpose of agricultural education. (ICAR, 1997)

FRUIT CROPS

Banana (Musa species)

1. Panama disease of banana
Fusarium oxysporum f.sp. cubense

‘Panama disease’, ‘Fusarium wilt’ or ’vascular wilt’ of banana is one of the most serious and destructive diseases of the crop. The disease was first reported from Australia in 1874. Now, it is widely distributed all over the world, including America, Africa, Sri Lanka, Burma, Thailand, Malaysia, Indonesia, Hawai, Fiji, the Philippines, India, Australia and New Zealand. In India, it is prevalent in the states of Assam, Bihar, Karnataka, Kerala, Tamil Nadu, Telungu Desam, Maharashtra and West Bengal.

Symptoms. Usually the symptoms are manifested, when the banana plants are about 5 months old. However, under highly favorable conditions for the pathogen, even 2 - 3 months old plants show wilt symptoms. Initial visible symptoms appear as pale yellow streaks, lengthwise in the petioles of the oldest, lowermost leaves. Two types of symptoms follow this. In the first type, the old leaves turn yellow progressively and finally they break from the petiole and hang down. In the second type, the leaves break from the petiole and hang down, without becoming chlorotic. Gradually, all the leaves collapse and hang down, except the crown, which alone stands upright. The newly formed leaves show yellow blotches and wrinkling of the lamina. Often, the leaf sheaths covering the pseudostem show longitudinal splitting above the ground level. But, sometimes this symptom may not appear. The affected plants may die completely in about 4-6 weeks after the appearance of yellow streaks on the petioles.

The vascular strands show discoloration varying from yellow to dark- brown, which is the characteristic internal symptom of the disease. The discoloration usually appears first in the outer leaf sheaths and extends to the vascular strands of the pseudostem. Vascular discoloration is more pronounced in the corms, but is not common in the roots. When the rhizome of an infected plant is cut transversely, the discoloration of the vascular bundles is distinctly seen as numerous, pin-point like dots all over the cut surface. The roots of affected plants become black and rotten. The suckers growing from the diseased corms are systemically affected and they wilt and die very soon (Fig. 71).

The causal organism. The mycelium of the fungus is hyaline, septate, much branched and mostly intracellular. Few hyphae may also be present in the intercellular spaces. The hyphae are largely confined to the vascular bundles, often filling the cavity of the vessels. The fungus produces three types of spores viz., macroconidia, microconidia and chlamydospores. The sporodochia bearing conidiophores and conidia emerge through the stomata on the petioles and leaves. The conidiophores are vertically branched. Macroconidia are borne at the apical ends of the main and lateral branches and are formed in succession one after another. They are thin-walled, hyaline, pedicellate, sickle-shaped, with both ends somewhat pointed, 2 - 5 septate, mostly 3-septate and measure 22.0 - 36.0 x 4.0 - 5.0µ in size. Microconidia are produced from the branches of conidiophores or from the tips of hyphae in very large numbers. They are single-celled or two-celled, thin-walled, hyaline, ovate or elongated and measure 5.0 - 7.0 x 2.5 - 3.0µ in size. Terminal or intercalary chlamydospores are formed from the hyphae, as well as from the conidial cells. They are oval or spherical, thick-walled and usually occur in pairs. They measure 7.0 - 13.0 x 7.0 - 8.0µ in size (Fig. 71).

The fungus is a facultative parasite. It can invade the host mainly through wounds in the corms or roots. After entry, the fungus develops extensively in the vascular tissue of the corm and then proceeds systemically to the leaf sheaths and pseudostem through the vascular system. The fungus colonizes the vascular bundles and produces masses of mycelium, which bear conidia and chlamydospores, filling the cavity of the vessels. The plugging of vascular elements by the fungus obstructs the translocation of water and nutrients to the aboveground parts of the plant leading to wilting and death of affected plants.

Mode of survival, spread and epidemiology. The fungus is soil-borne in nature. It can survive saprophytically in the diseased corms and other plant parts for prolonged periods and cause fresh infection. The chlamydospores can also remain in a viable state in the soil and diseased plant parts for a long time. The disease can be easily spread through spores present in the infected banana trash and soil, which may be carried in surface flooded water. Suckers used for planting from diseased areas to other places spread the disease easily. Contact of the roots of healthy plants with the roots of diseased plants carrying the spores also leads to fresh infection. The entry of the pathogen is mostly through wounds in the roots or corms, especially deep wounds up to the xylem vessels, caused incidentally during cultural operations or by soil-inhabiting pests, particularly nematodes.

Light textured, sandy loam soils, with acid reaction and low soil moisture of 25 % favor the occurrence and development of the disease. Ratooning of infected banana plants increases the disease incidence to a great extent and leads to continuous accumulation of soil inoculum.