Plant Pathogens- Detection & Diagnostics
Plant pathogens are organisms those cause plant diseases. They are considered as plant pests and may include bacteria, fungi, viruses, nematodes, parasitic plants. Plant pathogens spread in various ways, including by insects, water, soil, air, people and other animals. Although relatives of some plant pathogens are human or animal pathogens, most plant pathogens only harm plants. Organisms that cause plant diseases reduce the ability to produce food and support the economy. Some plant pathogens make immuno-depressed people also sick. All plants from citrus to grains to ornamental plants are susceptible to plant diseases. Plant diseases cause billions of dollars worth of direct and indirect losses every year. Emerging plant pathogens require preparation and planned, scientifically-based response to lessen the impact on our farmers and the economy.
All material of plant origin in sludge, soil and biowaste may harbour the potential risk of being contaminated with plant pathogens. The composition of plant pathogens is dependent on the type and species of plant material included in the waste. Sludge which has undergone biological, chemical or heat treatment, long-term storage or any other appropriate process so as significantly to reduce its fermentability and the health hazards resulting from its use. Biowaste is in the broadest sense any biodegradable waste. Surface soil from construction or building sites may be infected with plant pathogens, and may thus cause a potential risk for spread of disease. However, most of this soil is not spread on agricultural land.
Soilborne plant pathogens can significantly reduce the yield and quality of crops. Soilborne pathogens can be defined as pathogens that cause plant diseases via inoculum that comes to the plant by way of the soil. The most familiar diseases are probably rots that affect below ground tissues and vascular wilts initiated through root infections. Soilborne pathogens can be divided into soil inhabitants which are able to survive in soil for a relatively long time and soil transients which are only able to survive in soil for a relatively short time. The most important soilborne plant pathogens belong to very different classes: Plasmodiophoramycetes, Oomycetes, Zygomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes. Fewer plant diseases are caused by soilborne bacterial pathogens.
Quite a lot of plant-pathogenic fungi establish a long-term feeding relationship with the living cells of their hosts, rather than killing the host cells as part of the infection process. These pathogens are termed biotrophic .Typically; these fungi grow between the host cells and invade only a few of the cells to produce nutrient-absorbing structures termed haustoria. By their feeding activities, they create a nutrient sink to the infection site, so that the host is disadvantaged but is not killed. This type of parasitism can result in serious economic losses of crop plants, and in natural environments it can reduce the competitive abilities of the host; indeed, a few biotrophic pathogens have been used successfully as biological control agents of agricultural weeds.
The ability to accurately detect and identify a potentially plant pathogenic organism is fundamental to plant pathogen diagnostics and plant disease management. Conventional methods to detect plant pathogens have often relied on interpretation of symptoms, morphological identification, usually following isolation and culturing of the organism and, sometimes, further characterization based on pathogenicity tests. Although these methods are fundamental to diagnostics, the accuracy and reliability of these methods largely depend on skilled taxonomical expertise. In addition, diagnosis requiring a culturing step is time consuming and labor intensive. Finally, these techniques rely on the ability of the organism to be cultured in vitro. This latter aspect is a serious limitation of the applicability of these techniques since possibly less than 1% of the microorganisms in an environmental sample may be cultured in vitro.
Bacteria pathogenic for plants are responsible for devastating losses in agriculture. The use of antibiotics to control such infections is restricted in many countries due to worries over the evolution and transmission of antibiotic resistance. The advent of genome sequencing has enabled a better understanding, at the molecular level, of the strategies and mechanisms of pathogenesis, evolution of resistance to plant defense mechanisms, and the conversion of non-pathogenic into pathogenic bacteria.
Accurate identification of fungal phytopathogens is essential for virtually all aspects of plant pathology, from fundamental research on the biology of pathogens to the control of the diseases they cause. Technological advances in PCR-based methods, such as real-time PCR, allow fast, accurate detection and quantification of plant pathogens and are now being applied to practical problems. Molecular methods have been used to detect several pathogens simultaneously in wheat, and to study the development of fungicide resistance in wheat pathogens. Information resulting from such work could be used to improve disease control by allowing more rational decisions to be made about the choice and use of fungicides and resistant cultivars. Molecular methods have also been applied to the study of variation in plant pathogen populations, for example detection of different mating types or virulence types. PCR-based methods can provide new tools to monitor the exposure of a crop to pathogen inoculum that are more reliable and faster than conventional methods. This information can be used to improve disease control decision making.
Plant diseases need to be controlled to maintain the quality and abundance of food, feed, and fiber produced by growers around the world. Different approaches may be used to prevent, mitigate or control plant diseases. Beyond good agronomic and horticultural practices, growers often rely heavily on chemical fertilizers and pesticides. Such inputs to agriculture have contributed significantly to the spectacular improvements in crop productivity and quality over the past 100 years. However, the environmental pollution caused by excessive use and misuse of agrochemicals, as well as fear-mongering by some opponents of pesticides, has led to considerable changes in people’s attitudes towards the use of pesticides in agriculture. Today, there are strict regulations on chemical pesticide use, and there is political pressure to remove the most hazardous chemicals from the market. Consequently, some pest management researchers have focused their efforts on developing alternative inputs to synthetic chemicals for controlling pests and diseases. Among these alternatives are those referred to as biological controls.