Thursday, May 24, 2012

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.

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