Plant Pathology Science

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Management of soil borne disease of tomato with the aim of reducing the use of chemical pesticides and produce healthy products needs to provide appropriate policy. Grafting is one of the most effective control measures of soil borne pathogens result in healthy crop production and is an excellent substitute for chemical control. According to some investigations, grafting the commercial varieties on resistant rootstocks results in higher yield as well as the better quality. In addition to obtain higher product quality and optimize plant growth, disease management can be achieve by minimum application of pesticides.

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Powdery mildew is one of the most important diseases of alfalfa crops in Iran. Leveillula taurica which is the main causal pathogen on alfalfa, has a wide host range and distributed in warm and arid areas of the world. Planting the resistant and moderately resistant cultivars is the most appropriate method to control the powdery mildew of alfalfa. Based on field trials in Iran, the cultivars Codi and Gharehyonjeh are moderately resistant Bami is moderately susceptible Hamedani 121 and Hamedani 122 are susceptible and Mohajeran, Simertchenskaya, Diablorde and Ranger are highly susceptible to this disease. Accordingly, Codi and Gharehyonjeh which are the moderately resistant cultivars can be used for management of this disease.

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Fusarium yellows causing yellow, brown and wilted leaves. The gladiola bulbs rot and reduce quantity, quality and marketability of flowers. Four Fusarium species, especially F. oxysporum. f.sp. gladioli cause this disease. These fungi can survive as microconidia, macroconidia, clamydospore and mycelium, in the soil. Bulb discoloration is the most common symptom of the disease. In storage, the diseased corms get softened, mummified and wrinkled. Many researches have been carried on to investigate the methods of disease control, but all were unsuccessful so far. Disease management is based on resistant varieties, chemicals, cultural and biological measures. However, a combination of several methods provides a better opportunity to manage this disease.

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Powdery mildew disease of beet, cause by the fungus Erysiphe polygoni, is highly epidemic on table beets which are mostly cultivated in cooler regions of Isfahan province. Severity of the disease in fodder beets is lower than that of table beets. Studies on this fungus revealed that there is ascocarp formation as yellow spots which then turned to light to dark brown and then to black spherical bodies on upper, lower and petiols of the leaves, at the end of the season. Asci contain ellipsoid ascospores . Disease symptoms, characters of the pathogen and methods of disease management are described.

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Nanotechnology is the science of identification, production and use of materials at nanometer (10-9 m) scale. Regarding to application of this technology in plant pathology, this technology provides power to organize producing biological nanosensors for rapid detection of pathogens, production of nano silver and nano silica-silver to control bacteria and fungi, and the preparation of pesticides as nano capsules, at molecular level. Metal nanoparticles, the inhibitor of plant pathogens are derived from some fungi, bacteria and some plants including sunflower, alfalfa, Indian mustard, magnolia and Japanese persimmon.

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Contamination of the soil in limited arable areas with toxic substances has increased during recent decades. Phytoremediation is a technology in which plants are applied to remove, degrade or reduce the hazardous effects of toxic substances in the soil. Symbiotic fungi in symbiosis relation with plant roots efficiently increase the plants ability to remove the toxic substances from contaminated soils. The ectomycorrhizal fungus Hebeloma mesophaeum which is in symbiosis with norway spruce, beech, alder, willow and pussy, can uptake the heavy metals of soil and disable them in its hartig net in the root epidermis. It helps plants to maintain optimal growth and establish in contaminated soil. Arbuscular mycorrhizal fungi Funneliformis mosseae, Claroideoglomus etunicatum, Septoglomus deserticola, Glomus versiforme and G. intraradices help to deploy for optimal growth, and more absorption of nutrients in contaminated soils to cadmium, lead, zinc, arsenic and petroleum in maize, soybean, clover, subterranean clover, tomato and eucalyptus. These fungi can fix and disable these elements and pollutants in intraradical hyphal net. Accordingly, mycorrhizal fungi are powerful biological restoratives in contaminated lands.

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Metabolomics or analysis of all cellular metabolites is a new and powerful tool that provides possible quick view to the large number of small molecules (metabolites) within the cell and indicates dynamics of these molecules under different conditions. Quantitative and qualitative measurements of large number of cellular metabolites provide a broad view of the biochemical status of an organism that can be used to monitor and assess gene function. Today, metabolomics is widely being used in agriculture for classification of plants, studying phytochemical diversity of medicinal plants, assessment of the changes which occur in the biochemical composition of foods occurring, for example, during the pasteurization of Basmati rice for long-term storage or the boiling for direct consumption. In plant pathology, metabolomics has been mainly used to study plant responses to a wide range of biotic or abiotic stresses including resistance of plants to pathogens and also as a powerful tool for functional genomics studies. Profiling of the transcriptome and proteome has received some criticism due to their inability to predict gene function but profiling of the metabolites is promising as it provides instantaneous large amounts of data from cell physiology. Study of plant genetic resistance is one of the most important applications of metabolomics. Since metabolites are final products of gene expression and all changes in gene expression is reflected in metabolite profiles, hence metabolite profiles produce a more comprehensive understanding of plant defense mechanisms against stresses such as pathogen challenge. On the other hand, breeders are looking for rapid, simple and accurate tools for identifying metabolites associated with resistance as biomarker for screening cultivars resistant to diseases. In addition, understanding resistance mechanisms at the level of metabolome may help breeders for better understanding of resistant gene function and pyramiding suitable resistant gene in elite cultivar. This article is a review of the science, its applications in plant pathology, methods of study metabolites and their administrative problems.

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Development of human civilization is closely associated with agricultural crops. The major threat to crops posed by fungal diseases results in the use by growers of enormous amounts of chemicals. Fungicides are compounds, which are toxic to fungi. These materials have been applied for a long time to reduce losses and increase the quality and yield of the agricultural products. Today in addition to improving the quality and quantity of agricultural products and protecting plants by fungicides, the fungicide resistance problem must also be considered. The appearance of resistance has an important factor in limiting the efficacy and useful lifetime of fungicides. In general, systemic fungicides have been associated with resistance problems to a much greater extent than have non-systemic (protectant) fungicides. However, there are some exceptions. This paper discusses the resistance mechanisms to fungicides of some phytopathogenic fungi (e.g. altered target site, reduced uptake of fungicide, removal, detoxification or metabolism of fungicide) at the molecular level and describes methods used in the molecular detection of fungal resistance (e.g. RFLP-PCR, and allele specific real-time PCR) to fungicides.

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Plant diseases are the most important limiting factors in agricultural production. Currently the major control method of plant diseases is based on the use of chemicals that raises serious concerns about food quality, environmental hazards and development of resistance to agrochemicals. These concerns have increased the need for other alternative disease management techniques. Macro- and micronutrients are normally applied to increase crop production and improve general plant health and quality. They can also increase the disease tolerance or resistance of plants, however there are some opposing reports. Although our knowledge on the impact of mineral nutrients on plant diseases, many other factors that control plant's response and dynamic interactions among plant, environment and pathogen is not sufficient, manipulating soil nutrients through amendment or modification is always an essential part in plant disease control as well as in sustainable agriculture. Mineral nutrients are generally the first and the most important line of defense against plant diseases which affect all parts of the disease triangle. Nutrients can satisfactory decrease diseases, or at least diminish them to a level at which additional control measures are more successful and less expensive. Here we review the most recent data regarding the influence of mineral nutrients on plant disease resistance and tolerance, plant histological or morphological structure and the virulence or capability of pathogens to survive.

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Yellow rust is a major disease of wheat in Iran. Airborne spores fungal pathogen are the primary source of contamination. Temperature, humidity, wind intensity and direction, reaction of growing cultivars to the disease, presence of wheat in summer, planting time and nutritional status of wheat are effective factors in epidemy of the disease. The importance of the disease in the world, symptoms of the disease , effective factors in disease spread, forecasting model and its administrative procedures, and suitable fungicides to control the disease have been described .

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Alfalfa downy mildew caused by Peronospora trifoliorum de Bary, is one of the factors decreases alfalfa yield. It distributes as epidemic and causes falling and yellowing the leaves in favorite conditions especially in spring and first cutting of alfalfa. In order to select alfalfa resistant ecotypes to downy mildew disease, standard greenhouse and field experiments can be used. In greenhouse tests 7-day-old seedlings are inoculated with spore suspension and after incubation period, the percentage of the symptomless seedlings is compared with resistant control as soon as disease symptoms appear. Field experiments are carried out with minimum three replications of alfalfa ecotypes in an appropriate statistical design at some locations in which natural condition for disease occurrence exists. A susceptible ecotype is used as spreader in order to help disease distribution. Resistance of ecotypes is evaluated based on percentage of the leaves infection in five different classes (1-5) scoring system. Nowadays large number of researches has been done on alfalfa resistance to downy mildew and several resistant cultivars have been reported worldwide. For example KS224 and Saranac have been known as resistant cultivars to downy mildew. Some cultivars and ecotypes such as Kiseverdai, Nikshahri, Gharghlooogh and Malek-Kandi show good tolerance to disease in a few researches carried out in Iran. Results of field and greenhouse experiments are relative similar and show good correlation. It seems that using these ecotypes in different regions of Iran especially cold regions in which disease is frequently observed and considering other management methods such as appropriate cutting can be considerably decrease the occurrence of the disease and the crop loss.

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Salicylic acid is a kind of phenolic acids and a derivate of salicylate hormone that produced in plants in response to the large numbers of plant pathogens and is necessary for inducing systemic acquired resistance. Vast numbers of studies have done by researchers on the salicylic acid function in inducing plant resistance genes specially those that are responsible for pathogenesis related protein encoding. Results indicated to the effective role of this hormone in inducing innate resistance against pathogenic agents in plants. Changes that block the salicylic acid production in plants increase the disease susceptibility to plant pathogens. Transcription factors belonging to the WRKY, TGA and MYB families are involve in salicylic acid dependent resistance to plant pathogenic agents. Salicylic acid induces the pathogenesis related proteins through the NPR1 dependent pathway during hypersensitive response and systemic acquired resistance reactions. Signaling pathway, independent from NPR1 is a different form of salicylic acid dependent signal transduction and is require for specific resistance against pathogens. Salicylic can also induce the RNA-Dependent-RNA Polymerase gene expressions and play an important role in RNA silencing. Totally, with having knowledge about the salicylic acid biology and its signal transduction pathways in plants it is possible to increase the crop yields with increasing the resistances potential to stresses.

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Most of the symbiotic fungi can improve the growth of the host plants by increasing in water and nutrients absorption. Among the symbiotic fungi, Piriformospora indica or Piri have a good potential in symbiotic relation with plants and well known for its effect on host plants by increasing adventitious roots and growth stimulation in above and below ground parts of the plants. The effects of this fungus on vegetative growth of some medicinal plants and accumulation of the secondary metabolites have been discussed.

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Plant-parasitic nematodes are one of the most important pests worldwide and cause considerable economic loss to many of agricultural products. Some of soil inhabited nematodes are affected by some of antagonistic bacteria, so they can be used in biological control. Nematodes can be affected by bacteria in different ways such as direct suppression, promotion of plant growth, and facilitation of rhizosphere colonization. In overall, regarding to effect of soil inhabits bacteria on nematodes they can be classified as toxin producing, antibiotic producing and enzyme producing as well as plant growth promoting groups. Based on the recent researches, bacteria are divided to six groups including: parasitic bacteria (nematophagous bacteria), opportunistic parasitic bacteria, rhizobacteria, endophytic bacteria, symbionts of entomopathogenic nematodes and cry protein-forming bacteria. Combination of bacteria with some other antagonistic microorganisms was successful in control of plant parasitic nematodes.

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In addition to the full-length viral DNA genome, various types of smaller specific DNA molecules have been isolated from plants infected by DNA viruses. These DNAs are usually derived from viral genomes by different ways or have non-viral genome sequences. Some of these DNA have no significant effect on the virus cycle and on the incidence and progression of the disease, while some of them inducing the viral disease symptoms. These components that are known as satellite, defective and defective interfering DNAs, depend on helper viruses for replication, encapsidation and movement in plants. Satellites have no significant homology with the helper virus genome. However, they are required for inducing disease symptoms. While defective and defective interfering DNAs exhibit high homology with the genome of helper viruses, only defective interfering DNAs have ability to interfere with virus replication and with disease symptom induction and development. In this paper, the characteristics of these subviral DNAs and the possible mechanisms by which they are generated and transmitted in virus infected plants are discussed.

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Phytoplasmas are some plant pathogens that establish and propagate in plant phloems. They have transmitted by sucking insects. Phytoplasmas have a different lifecycle as compare to bacterial pathogens. They have ability to infect different hosts two different kingdoms, planta and animalia (insects). They systemically infect their hosts. Phytoplasmas have various approaches for adaptation to their hosts. Some of adaptation mechanisms include: changes in the level of gene expression, variation and recombination in extrachromosomal DNA and potential mobile units, production of effectors and suppression of defense signaling pathways. These approaches enable them to establish, propagate and infect various hosts. Recognizing these strategies would be a major step on the effective management of these pathogens.

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Food production and food security is an essential precursor to sustainable development in agriculture. Currently, more than 800 million people, generally in Africa and Asia, suffer from hunger and agriculture is considered as the main source of food for them. One of the application of nuclear technology is reducing the damages of plant pest and diseases. The application of nuclear techniques in plant pathology can be grouped in three categories including disease tracing, mutagenesis induction and radiation of crops to induce resistance and destruction of pathogens. As a new method to induce defense responses to biotic and abiotic stresses, nowadays, gamma radiation is used to improve the growth in the way to induce the plant resistance to environmental tensions and plant pathogens as well. Use of this potential, especially in management of seed and seedling diseases is very important to reduce a big portion of crop losses caused by plant pathogens in the first weeks of seedling growth.

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Downy mildew, caused by Pseudoperonospora cubensis, is the most important disease of cucumber production in humid areas and greenhouses in Iran. The yield loss of cucumber due to downy mildew may reach to 100%. Because the chemical control is the main method of disease control, especially in susceptible cultivars, establishing a forecasting program which is based on estimation of pathogen population, recording the temperature and relative humidity of field and greenhouse, can reduce the risk of disease epidemic. So by proper application of a protective or systemic fungicide can reduce the disease incidence, production costs and the most important, the amount of fungicide residue.

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Abiotic stresses are major environmental factors that affect agricultural productivity worldwide. Depending on the crop, the yield losses associated with abiotic stresses can reach 50 to 82 percent. Extreme temperatures, drought, salinity, flooding, freezing, ultraviolet light, heavy metals, nutrient deficiency, unsuitable pH, air pollutants and mechanical damage are the most basic stressors. Because biotic stresses cause metabolic toxicity, membrane degradation, reduction of photosynthesis, decrease of nutrient uptake, changes in levels of phytohormones and ultimately affect the plant growth and its productivity, therefore reducing the effect of these stresses, is essential. Plant growth promoting rhizobacteria play an important role in plant disease management and have a high potential in alleviation the abiotic stresses.

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Ghazi-Mohseni V., Sabbagh S. K. & Esmaili Bahabadi S. 2015. Application of chitosan in plant diseases management. Plant Pathology Science 4(1):54-63. Chitosan is a biodegradable natural compound derived from the bark of crabs and shrimp which have antimicrobial role against fungi and bacteria. Chitosan has directly effects on morphology of treated pathogens which reflect its fungistatical and fungicidal activity. It has been shown that chitosan increases production of glucanohydrolase, phenolic compounds and specific phytoalexin synthesis with antifungal activity and reduces enzymes such as polygalacturonase, pectin methyl-esterase that related to soft rot . In addition, chitosan can develop structural barriers via lignin synthesis. Therefore chitosan is considered as a new non-toxic biological material, inducer resistance of plants against diseases.