Survival of Plant Pathogen in Soil

There is much evidence to show that plant pathogenic bacteria survive from season to season in soil, but it is difficult to explain exactly what is happening or how they surviving. Microflora in soil are believed to be in a quiescent state most of the time. They don not grow unless nutrients reach them. In this respect, soil is energy deficient. If for example , dry leaf debris happens to be incorporated, it and the pathogen it may carry soon become moist, nearby microorganism multiply, and decomposition follows.

Yet, pathogen do survive in soil. How is this to be explained. There are two possible reasons. The first has to do with the actual location of survivors. Clearly hypobiotic cell set deeply within hard to decompose tissues would not be readily accessible to degradation. Pathogen cells eithin a horny tomato stem would be an example. Therefore, practices that encourage the decomposition of crop residues, such as adding nitrogen and organic material, fragmentation and buying pathogen bearing debris, and allowing time for decomposition to take place during warm moiste season should reduce carry over. Also rotation with crop that don not permit pathogen increase would allow more time for decomposition of pathogen bearing debris.

The second way in which pathogen may survive in soil was first suggested in 1944 when it was reported that two leaf pathogen of tobacco overwintered in the rhizosphere of the living, nonhost weed plants. Since then, there have been many reports of isolating pathogen of the shoot from the root of host and non host plants, but determining the source of these bacteria is difficult. Whatever the source, it is now clear that pathogen may at least survive in association with apparently healthy plant. Thus survival  would be expected as long as living, suitable root of crop or wood plant were available for survival or for growth it this occurs. The soil organisms that have the potential to be plant pathogens include fungi, bacteria, viruses, nematodes and protozoa. Some pathogens of the above ground parts of plants (leaves, stems) survive in the soil at various stages in their life cycles. Therefore, a soil phase of a plant pathogen may be important, even if the organism does not infect roots. They survive in soil for long periods in the absence of a host, and inoculums levels in soil increase slowly over several years (crop cycles)

•           They all have a wide host range, except formae speciales of Fusarium oxysporum

•           They can be spread in:

– Irrigation water

– Soil carried on animals and humans

– Contaminated planting material (potato tubers, ginger rhizomes, seedling  transplants)

•           They are not usually dispersed by wind •      they are not usually dispersed by wind. Bacterial wilt pathogens can also be carried on seed. These pathogens are often overlooked because they are difficult to identify.

Fungi survive in the soil in the form of mycelium, spores, or sclerotia. Some plant pathogens are soil inhabitants and they are able to survive indefinitely as saprophytes. Soil inhabitants are generally unspecialized parasites that have a wide host range. Other fungi are soil transients, i.e., they are rather specialized parasites that generally live in close association with their host but may survive in the soil for relatively short periods of time. Nematodes usuallysurvive as eggs in the soil and as eggs. Disease-causing microorganisms and soil animals are a natural component of the soil community. The organisms are normally present in relatively low numbers. An outbreak of disease commonly follows either an increase in the abundance of the pathogen or a change in the susceptibility of the host to the pathogen.

Bacterial Endospre

Certain bacteria can form endospores in dry environments in a process called sporulation. They are called endospores because the spores form within the cell. Endospores offer great advantages to bacterial cells because they are extremely resistant to a number of harsh environments, including: heat, desiccation, radiation, chemicals, acids, and drying. The discovery of bacterial endospores was important to microbiology because it provided knowledge on proper methods for sterilization of culture media, foods, and other perishable items. Many organisms form spores, but the bacterial endospore is unique in its heat resistance capabilities. Endospores are bacterial structures (resting cells) formed by a few groups of bacteria as intracellular structures, but ultimately they are released as free endospores. Biologically, endospores are a fascinating type of cell. Endospores exhibit no signs of life, being described as cryptobiotic. They are highly resistant to environmental stresses such as high temperature (some endospores can be boiled for hours and retain their viability), irradiation, strong acids, disinfectants, etc. They are thought to be the most durable cell produced in nature. Although cryptobiotic, they retain viability indefinitely, such that under appropriate environmental conditions, they germinate back into vegetative cells. The primary function of most endospores is to ensure the survival of a bacterium through periods of environmental stress.

They are therefore resistant to ultraviolet and gamma radiation, desiccation, lysozyme, temperature, starvation, and chemical disinfectants.

Endospores are commonly found in soil and water, where they may survive for long period. An endospore is a dormant, tough, non-reproductive structure produced by a small number of bacteria from the Firmicute family. Endospores are formed mainly by two genera of Gram-positive bacteria: Bacillus,the aerobic sporeformers, and Clostridium, the anaerobic sporeformers. Both genera contain pathogens, and the endospores produced by these bacteria invariably play some role in the toxicity, transmission or survival of the pathogen.

Mushroom Cultivation; Spawning

In case of mushroom cultivation, spawn means the propagative material used by the mushroom growers for propagating bed is called spawn. The spawn is sort of seed as in higher plants. It actually is the mycellium of the  mushroom. Spawn is of different types depending upon its production. 

First type is the Virgin Spawn. In this type of spawn, the spores of the mushroom fall on suitable substrate and germinate and form a mat of mycellium. When the mat form, it is dug from here and use as spawn for the production of mushrooms. The second type is the Flake Spawn in which the spawn is produce when the beds are covered with mycellium before a crop of mushroom appear, then this composte is broken in small pieces, dried and used as spawn. The third type is the Brick spawn. Small bricks of horse and cow dung mixed with water and loam are used to made in this method. Mycellium is inoculated in a small hole in each brick and when mycellim grow, these bricks are dried out and sold as spawn. Grain Spawn is another type of spawn. This type of spawn is most popular all over the world. Another type of spawn is the Substrate Spawn. The spawn that is produce on substrate is known as substrate spawn.

Symbiosis Between Insects and Microorganisms

As earlier we discussed nature of symbiotic relationship among insects and microorganisms, now i want to explain some example of symbiotic relation between insects and microorganism. First example is the Ambrosia Beetles. In this relation, beetles feed on a white glistering material which produced by the fungus. Ambrosia mainly belong to the group Xyloterus and Xyloborus. Ambrosia beetles belong to the family Scoltidea.

The second example is the living of fungi on termites. Very little is known about the identity of fungi cultivating by the termites. Fruiting body of Xylaria and volvaria have been found associated with the termites garden.

protozoa living inside the digestive tract of termites is another example of symbiotic relation. A large number of protozoa are present inside the termites and help in the digestion. Protozoa are found in wood eating termites and it has been proved that these help the termites in digestion of cellulose. 

Nature of Symbiotic Relation among Insect and Microorganism

Many diseases are transmitted by insects especially viral diseases are very well known to be transmitted by insect vector. Insects belonging to different orders are involve in transmission of plant pathogen from the source of pathogen to the healthy plants. First of all we should know the actual definition of insect which is "any arthropod which has three pairs of legs and one or two pairs of wings on their thorax region".

A symbiotic relationship is developed between pathogen with its vector. this symbiotic relation is basically characterized by "mutualistic symbiosis" and is based upon physiological conditions but there is no a sharp line between mutualistic and parasitism so there is a need of introduction of another term which is "obligate" to define the symbiotic relationship among insect and plant pathogen. There is a theory to define this sort of relationship that the two members live symbiotically in which one member which is microsymbiote is the parasite but as time pass other member develop an immune system and as a result they start to live mutualistically.   

Microorganisms get nutrition and shelter from their insect host while there are several examples in which microorganisms are known to aid in the digestion of the insect by producing certain enzymes, vitamins or the other substances that the insect cannot produce itself. 

Rice Diseases

Rice crop can be infected by living pathogens(fungi, bacteria, virus and nematodes) which cause infectious diseases or may also be damaged by some physical factors (rise or decrease in temperature, unavailability of water ) which cause noninfectious diseases. Apart from the fact weather the disease is infectious or noninfectious, any disease causes reduction in yield, so you can't ignore the diseases at any stage. The most important diseases of rice in various rice growing area of world are brown spot, blast, stem rot, bunt, bacterial blight, false smut, ufra and khaira (zinc deficiency). Blight or Brown Spot diseases is cause by fungal pathogen Helminthosporium oryzae in this disease, small oval shaped spots appear on the leaves which after few days join with each other and form linear spots. Later on, the leaves become yellow and poor germination occur. Another sever disease is the Bunt of Rice or Smut caused by Tilletia barclayana. In this disease, the grains filled with black powdery mass of fungus. Mostly a few number of grains are infected in an ear and if these are not severely infected, they can germinate but the seedlings show stunt in growth. Rice Blast caused by Pyricularia oryzae is another infectious disease in which fungal pathogen cause small, water soaked whitish, greyish or bluish spots on leaves, grains and on nodes. Brown spot may also be developed on plant. when the mature stage of fungal development reach the spot blasted. Grain development affected. In Bacterial Blight of Rice cause by Xanthomonas oryzea water soaked stripes appear along the margin of leaf blades, which later on enlarge and turn yellow. These lesions may cover the entire blade, may extend to the lower end of leaf sheath. Similar symptoms may occur on glumes of green grains.In Stem Rot (sclerotium oryzae) of Rice two to three months old plants begin to wither and ultimately dry up; the sheaths soon turn somewhat dark and start rotting. Black dots (fruiting bodies of sclerotia) occur at the base of dried leaves and leaf sheaths. Stem begins to rot and become soft, plant falls down.

Rice Crop

Rice is the most edible grain in the world after maize but if we are talking about the human intake the Rice comes first as maize is not only use as human intake. It has great nutritional values as it has carbohydrate content in it and proteins but not all essential proteins are present in it. Rice can be cooked with different recipes and has a delicious taste.

Rice can grow in any area with hard clay soil which has hard pan because it requires plenty of water to grow so it should be cultivated in areas which has good water resources and also low labour cost as it also needs many labours especially during harvesting . Rice can be grown for the reclaimation of saline soil as it leaches the excess salts. After setting the seedlings in field, it is required to irrigate the field with plenty of water as this water reduces the chances of weed production. Rice are harvested when their seeds become dry and has water 25% water content. Harvesting can be done with machines but in most of the Asian countries it is done manually. After harvesting, the chaff (outer covering of seed) is removed by using rice huller. At this time the rice is known as brown rice but it still has husk which also have to remove so the product is white rice which are now ready to cook.