If pH is grater than normal pH increase the chances of different fungus diseases such as potato scab. Nutritional behavior also impact of fungus diseases, their quantity high or low. Infection also appeared due to lack of few parameters of environment. Sunken, swollen, or discolored areas in the fleshy stem or bark may indicate canker infection by a fungus or bacterium or injury caused by excessively high or low temperature.Bacteria grow in many different microenvironments and specific niches in the soil. Bacteria populations expand rapidly and the bacteria are more competitive when easily digestible simple sugars are readily available around in the rhizosphere.

  • Soil pH
  • Soil type
  • Soil Fertility  

Role of soil and environmental factor in the pathogenesis of fungi:

Many fungi produce “signs” of disease, such as mold growth or fruiting bodies that appear as dark specks in the dead area.Soil pH impact on growth of Fungus, such as that pH higher than the 7.0 increase the scab of potato and different fungus diseases. Soil pH, a measure of acidity or alkalinity, markedly influences a few diseases, such as common scab of potato and clubroot of crucifers (Plasmodiophora brassicae). Growth of the potato scab organism is suppressed at a pH of 5.2 or slightly below (pH 7 is neutral, numbers below 7 indicate acidity, and those above 7 indicate alkalinity). Scab is not normally a problem when the natural soil pH is about 5.2. Some farmersadd sulfur to their potato soil to keep the pH about 5.0.

Different condition of soil  is mainly impact on spread of pathogen.

Soil type: Certain pathogens are favored by loam soils and other by clay soils. Phymato trichumroot rot attacks cotton and some 2,000 other plants in the southwestern United States. This fungus is serious only in black alkaline soil pH 7.3 or above that are low in organic matter. Fusarium wilt disease, which attacks a wide range of cultivated plants, causes more damage in lighter and higher (topographically) soils.

Pathogens differ in their preference for higher or lower temperatures. Some fungi grow much faster at lower temperatures than others and there may be significant differences among races of the same fungus.Temperature affects the number of spores formed in a unit plant area. Moisture affects fungal spore formation, longevity, and particularly the germination of spores, which requires a film of water covering the tissues. In many fungi, moisture also affects the liberation of spores from the sporophores, which, as in apple scab, can occur only in the presence of moisture. Several diseases are known in which the intensity and the duration of light may either increase or decrease the susceptibility of plants to infection and also the severity of the disease. Wind helps prevent infection by accelerating the drying of the wet plant surfaces on which fungal spores or bacteria may have landed.

Hafiz Muhammad Rizwan Mazhar

M.Sc (Hons) Plant Pathology Department of Plant Pathology, University of Agriculture Faisalabad-Pakistan

Role of soil and environmental factor in the Pathogenses of bacteria:

Bacteria grow in many different microenvironments and specific niches in the soil. Bacteria populations expand rapidly and the bacteria are more competitive when easily digestible simple sugars are readily available around in the rhizosphere. Root exudates, dead plant debris, simple sugars, and complex polysaccharides are abundant is this region. About 10 to 30 percent of the soil microorganisms in the rhizosphere are actinomycetes, depending on environmental conditions. . Wind-blown rain helps release spores and bacteria from infected tissue and then carries them through the air and deposits them on wet surfaces of plants, which, if susceptible, can be infected immediately. Wind also injures plant surfaces while they are blown about and rub against one another or wind-blown sand; this facilitates infection by many fungi and bacteria and also by a few mechanically transmitted viruses. Moisture: Most bacterial diseases, and also many fungal diseases of young tender tissues, are particularly favored by high moisture or high relative humidity. Bacterial pathogens and fungal spores are usually disseminated in water drops splashed by rain, in rainwater moving from the surfaces of infected tissues to those of healthy ones, or in free water in the soil. Bacteria penetrate plants through wounds or natural openings and cause severe disease when present in large numbers. Light: The effect of light on disease development, especially under natural conditions, is far less than that of temperature or moisture. Temperature: Several diseases solanaceous plants caused by Ralstonia solanacearum, are favored by high temperatures and are limited to hot areas, being particularly severe in the subtropics and tropics. Soil PH and soil structure: The pH of the soil is important in the occurrence and severity of plant diseases caused by certain soilborne pathogens. For example, the clubroot of crucifers caused by Plasmodiophora brassicae is most prevalent and severe at about pH 5.7, whereas its development drops sharply between pH 5.7 and 6.2 and is completely checked at pH 7.8.

Role of soil and environmental factors in the pathogenesis of virus:

Viral diseases, such as mosaics and yellows, are sometimes confused with injury by a hormone-type weed killer, unbalanced nutrition, and soil that is excessively alkaline or acid. Nearby plant species are often examined to see if similar symptoms are evident on several different types of plants. The effect of temperature on virus diseases of plants is much more unpredictable. In virus inoculation experiments in the greenhouse, temperature determines not only the ease with which plants can become infected with a virus, but also whether a virus multiplies in the plant and, if it does, the type of symptoms produced. The severity of the disease may vary greatly in various virus–host combinations depending on the temperature during certain stages of the disease. Reduced light intensity generally increases the susceptibility of plants to virus infections. Plants kept in the dark for 1 or 2 days before sap inoculation with a virus produce more local lesions (i.e., infections) than plants kept in the normal light–dark regime. This has become a routine procedure in many laboratories.

 The occurrence of suspended solid particles, such as clay minerals, in natural waters probably exerts some influence on the behavior of viruses. s the soil environment is a more diverse habitat for viruses than aquatic environments, viruses in soils have great potential to play roles comparable in quantity, which are unique in quality, to those in aquatic environments. 

This encompasses the effects of viruses on beneficial bacteria and soil-borne plant pathogens, adsorption of viruses to soils, soil factors influencing viral inactivation and survival in soils, and horizontal gene transfer in soils. The soil environment is a more diverse habitat for viruses than aquatic environments. Therefore, viruses in soils have great potential to play roles comparable in quantity, but unique in quality.

Role of soil and environmental factors in pathogenesis of Nematode

Soil temperature, moisture and relative humidity effect on survival and pathogenicity of steinernema carpocapsae and S glaseri were tested in the laboratory . Survival and pathogenicity of S.carpocapsae were significantly greater at lower temperature (5-25c) than the highest temperature. S .carpocapsae and S.glaseri survival at best at low moisture of 2 and 4% respectively. Above ground symptoms of many root problems look alike. They include stunting of leaf and twig growth, poor foliage colour, gradual or sudden decline in vigour and productivity, shoot wilting and dieback, and even rapid death of the plant. The causes include infectious root and crown rot; nematode, insect, or rodent feeding; low temperature or lightning injury; household gas injury; poor soil type or drainage; change in soil grade; or massive removal of roots in digging utility trenches and construction.Abnormal root growth is revealed by comparison with healthy roots. Some nematodes, such as root knot (Meloidogyne species), produce small to large galls in roots; other species cause affected roots to become discoloured, stubby, excessively branched, and decayed. Bacterial and fungal root rots commonly follow feeding by nematodes, insects, and rodents. Diagnosis of a disease complex, one with two or more causes, is usually difficult and requires separation and identification of the individual causes. Survival of infective juveniles of Steinernema carpocapsae and Steinernema glaseri gradually declined during 16 weeks of observation as the tested soil pH decreased from pH 8 to pH 4. Survival of both species of Steinernema dropped sharply after 1 week at pH 10. Survival or S. carpocapsae and S. glaseri was similar at pH 4, 6, and 8 during the first 4 weeks, but S. carpocapsae survival was significantly greater than S. glaseri at pH 10 through 16 weeks. Steinernema carpocapsae and S. glaseri that had been stored at pH 4, 6, and 8 for 16 weeks, and at pH 10 for 1 or more weeks were not infective to Galleria mellonellalarvae. Steinernema carpocapsae survival was significantly greater than that of S. glaseri at oxygen:nitrogen ratios of 1:99, 5:95, and 10:90 during the first 2 weeks, and survival of both nematode species declined sharply to less than 20% after 4 weeks. Survival of both nematode species significantly decreased after 8 weeks as the tested oxygen concentrations decreased from 20 to 1%, and no nematode survival was recorded after 16 weeks. Steinernema carpocapsae pathogenicity was significantly greater than that of S. glaseri during the first 2 weeks. No nematode pathogenicity was recorded at oxygen concentrations of 1, 5, and 10% after 2 weeks and at 20% after 16 weeks.

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