Plant Pathology Science

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Vegetables are attacked by different pests and diseases during their growth. Some fruits such as cucumber have sprayed regularly against diseases like Pseudoperonosporacubensisspecially in humid areas, and after a short time the cucumber are picked up and the amount of pesticide residue are measured and comparing with Maximum Residue Level (MRL). In this research the amount of Mancozeb fungicide has quantified in cucumber samples in Sari district. So in this order 26 samples of cucumber are collected and were carried to the laboratory. Samples of fruits are weighted and washed by using of n- hexan in Suksoleh device. After concentrating the obtained solution put in spectrophotometer set for measuring the optical density with 280 nanometer wave lengths and the amount of uv absorbance of samples and standard solutions were checked. The obtained numbers was changed into all right amount of mancozeb by using Excel software and Cx= α. Cs / β. Vx equation. Results showed that the amount of remained mancozeb fungicide was less than maximum residue limit in 77 percent of samples and more than maximum residue limit in 23 percent of samples in these periods. Also it defined among the samples which have had residue higher than standard limit, just 2 samples have been increased more than 1 milligram per kilogram.

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Powdery mildew fungi are important obligate parasites of plants. They produce white cover on aerial parts of host plants, which then turn to yellow. The diseased plant becomes yellow and then wilted result in reducing quality as well as quantity of products in field and vegetable crops and also fruit trees. Among all of the control measures against powdery mildew, the most common method is application of the chemical fungicides, after disease incidence. Regarding to the hazards of agrochemicals and also considering the probability of incidence of resistant races of the pathogen, the biological control is considered as a safe and useful method for controlling the disease. The fungi Ampelomyces quisqualis, Pseudozyma flocculosa and Lecanicillium longisporum the bacterium Bacillus subtilis the mite Tydeus lambi and twenty-spotted lady beetle Psyllobora vigintimaculata , have been used effectively against the powdery mildew of cucumber, squash, tomato, pepper, grape and rose. Some of these biological agents have been introduced to the market, commercially.

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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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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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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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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.