Diseases caused by begomoviruses (family Geminiviridae, genus Begomovirus) constitute a serious constraint to tropical and sub-tropical agro-ecosystems worldwide. In recent years, they have also introduced in temperate regions of the world where they have great impact and are posing a serious threat to a variety of greenhouse crops. Begomoviral diseases can in extreme cases reduce yields to zero leading to catastrophic losses in agriculture. They are still evolving and pose a serious threat to sustainable agriculture across the world, particularly in tropics and sub-tropics. Till recently, there have been no records on the occurrence of begomoviral disease in South Korea, however, the etiology of other plant viral diseases are known since last century. The first begomovirus infected sample was collected from sweet potato plant in 2003 and since then there has been gradual increase in the begomoviral epidemics specially in tomato and sweet potato crops. So far, 48 begomovirus sequences originating from various plant species have been submitted in public sequence data base from different parts of the country. The rapid emergence of begomoviral epidemics might be with some of the factors like evolution of new variants of the viruses, appearance of efficient vectors, changing cropping systems, introduction of susceptible plant varieties, increase in global trade in agricultural products, intercontinental transportation networks, and changes in global climatic conditions. Another concern might be the emergence of a begomovirus complex and satellite DNA molecules. Thorough understanding of the pathosystems is needed for the designing of effective managements. Efforts should also be made towards the integration of the resistant genes for the development of transgenic plants specially tomato and sweet potato as they have been found to be widely infected in South Korea. There should be efficient surveillance for emergence or incursions of other begomoviruses and biotypes of whitefly. This review discusses the general characteristics of begomoviruses, transmission by their vector B. tabaci with an especial emphasis on the occurrence and distribution of begomoviruses in South Korea, and control measures that must be addressed in order to develop more sustainable management strategies.

During investigating of shiitake mushroom competitors, 289 isolates of Trichoderma spp. were collected from shiitake mushroom farms in different districts and the Forest Mushroom Research Center of Korea, among which 29 representative strains were selected. Based on the DNA sequences of the rpb2 and tef1 genes and the ITS rDNA, and their morphological characteristics, they were identified as T. atroviride, T. citrinoviride, T. harzianum, T. longibrachiatum, and two undescribed species, Trichoderma spp. 1 and 2, which are considered to be the candidate of new species. Competition tests between Lentinula edodes (Sanjo302) and the Trichoderma species indicated that the six species of Trichoderma were significantly different from each other in terms of their ability to invade the mycelial blocks of shiitake. In both of dual cultures on potato dextrose agar and sawdust media, Trichoderma spp. 1 and 2 strongly invaded the mycelial blocks of shiitake. Our results suggest that the two Trichoderma species may cause potentially serious economic losses in shiitake cultivation of Korea.

A field trial was conducted in 2009/2010 and 2010/2011 growing seasons at Giza Agricultural Research Station to examine correlations between some biochemical componets and powdery mildews ($PM_s$) resistance in flax cultivars. Nine flax cultivars could be divided into five distinct groups, i.e., highly susceptible (Cortland and C.I. 2008), moderately susceptible (Giza 7, and Marshall), moderately resistant (Cass), resistant (Koto, Dakota and Wilden), and highly resistant (Ottowa 770B). The cultivars showed considerable variation in PM severity ranged from 8.05 on Ottowa 770B to 97.02% on Cortland. Total soluble proteins, total phenols, antioxidant enzymes (peroxidase and polyphenoloxidase), ascorbic acid, tocopherol, and malondialdehyde (MDA), were determined in uninfected leaves of the tested cultivars. Pearson's correlation coefficient was calculated to measure the degree of association between PM severity and each component. All components showed significant (P < 0.05) or highly significant (P < 0.01) negative correlation with PM severity except MDA, which showed positive correlation (P < 0.01). Linear regression analysis was used to evaluate the causal relationship between the biochemical components (independent variables) and PM severity (dependent variable). Coefficient of determination ($R^2$) values of the generated models ranged from 48.76 to 77.15%. Tocopherol, MDA, and proteins were the most important contributors to the total variation in PM severity as the $R^2$ values of their models were 71.78, 75.28, and 77.15%, respectively. The results of the present study suggest that tocopherol, MDA, and proteins in uninfected leaves can be used as biochemical markers to predict PM resistance in flax.

Stem rots caused by Rhizoctonia solani and Sclerotinia sclerotiorum have been known as devastating diseases in balloon flower plants. Antifungal activities of four fungicides, azoxystrobin, polyoxin B, trifloxystrobin and validamycin A were evaluated in vitro, showing effective suppression with mycelial growth of the fungal isolates on PDA media. Efficacies of the four fungicides were also demonstrated in stem tissues of balloon flower plants against R. solani and S. sclerotiorum. A commercially available Bacillus subtilis strain Y1336 was tested in terms of antagonistic biological control of stem rot disease of balloon flower plants. The bacterial strain revealed its antifungal activities against R. solani and S. sclerotiorum demonstrated by dual culture tests using paper discs and two plant pathogenic fungi on PDA media, as well as by plant inoculation assay, indicating that this antagonistic bacterial strain can be incorporated into disease management program for balloon flower stem rot diseases together with the four chemical fungicides.

Rice blast disease caused by Magnaporthe grisea (Hebert) Barr [teleomorph] is one of the most devastating diseases in rice plantation areas. Silicon is considered as a useful element for a large variety of plants. Rice variety MR219 was grown in the glasshouse to investigate the function of silicon in conferring resistance against blast. Silica gel was applied to soil while sodium silicate was used as foliar spray at the rates of 0, 60, 120, 180 g/5 kg soil and 0, 1, 2, 3 ml/l respectively. The treatments were arranged in a completely randomized design. Disease severity and silicon content of leaves were compared between the non-amended controls and rice plants receiving the different rates and sources of silicon. Silicon at all rates of application significantly (${\alpha}$ = 0.05) reduced the severity of disease with highest reduction (75%) recorded in treatments receiving 120 g of silica gel. SEM/EDX observations demonstrated a significant difference in weight concentration of silicon in silica cells on the leaf epidermis between silicon treated (25.79%) and non treated plants (7.87%) indicating that Si-fertilization resulted in higher deposition of Si in silica cells in comparison with non-treated plants. Application of silicon also led to a significant increase in Si contents of leaves. Contrast procedures indicated higher efficiency of silica gel in comparison to sodium silicate in almost all parameters assessed. The results suggest that mitigated levels of disease were associated with silicification and fortification of leaf epidermal cells through silicon fertilization.

An infection risk model for Phytophthora blight on chili pepper was developed to estimate the first date of disease occurrence in the field. The model consisted of three parts including estimation of zoosporangium formation, soil water content, and amount of active inoculum in soil. Daily weather data on air temperature, relative humidity and rainfall, and the soil texture data of local areas were used to estimate infection risk level that was quantified as the accumulated amount of active inoculum during the prior three days. Based on the analysis on 190 sets of weather and disease data, it was found that the threshold infection risk of 224 could be an appropriate criterion for determining the primary infection date. The 95% confidence interval for the difference between the estimated date of primary infection and the observed date of first disease occurrence was $8{\pm}3$ days. In the model validation tests, the observed dates of first disease occurrence were within the 95% confidence intervals of the estimated dates in the five out of six cases. The sensitivity analyses suggested that the model was more responsive to temperature and soil texture than relative humidity, rainfall, and transplanting date. The infection risk model could be implemented in practice to control Phytophthora blight in chili pepper fields.

Sclerotinia sclerotiorum is a serious pathogen which causes yield loss in many dicotyledonous crops including potato. The objective of this study was to assess the potential of biofumigation using three Brassica crops including Brassica napus, B. juncea and B. campestris against potato stem rot caused by S. sclerotiorum by in vitro tests. Both macerated and irradiated dried tissues were able to reduce radial growth and sclerotia formation of five pathogen isolates on PDA, but macerated live tissues were more effective. Compared with other tested crops, B. juncea showed more inhibitory effect against the pathogen. The volatile compounds produced from macerated tissues were identified using a gas chromatograph-mass spectrometer. The main identified compounds were methyl, allyl and butyl isothiocyanates. Different concentrations of these compounds inhibited mycelial growth of the pathogen in vitro when applied as the vapor of pure chemicals. A negative relationship was observed between chemicals concentrations and growth inhibition percentage. In this study, it became clear that the tissues of local Brassica crops release glucosinolates and have a good potential to be used against the pathogen in field examinations.

Periwinkle seedlings (Cantharanthus roseus) were inoculated with jujube witches'- broom (JWB) phytoplasma via grafting to analyze the migration of JWB phytoplasmas within the host plant. The phytoplasmas were detected using nested polymerase chain reaction (PCR) and fluorescence microscopy. Fluorescence microscopy was a simple and easy method of detecting phytoplasmas; however, it was not sufficiently sensitive to detect very low phytoplasma concentrations. Therefore, the migration of JWB phytoplasma was investigated through PCR. The first migration of JWB phytoplasma from an infected tissue to healthy tissues occurred late. After grafting, the phytoplasmas moved from the inoculated twig (or scion) to the main stem, which took 28 days. Afterward, the phytoplasma migrated faster and took less than 4 days to spread into the roots from the main stem. All twigs were then successively colonized by the JWB phytoplasmas from the bottom to the top. JWB phytoplasma was detected via nested PCR in all parts of the periwinkle seedling 82 days after inoculation. Based on these results, the inoculated JWB phytoplasma appeared to migrate downward to the roots along the main stem during the early stages, and then continued to move upward, colonizing twigs along the way until they reached the apex.

Potyviruses express their RNA genomes through the production of polyproteins that are processed in host cells by three virus-encoded proteases. Soybean plants produce large amounts of protease inhibitors during seed development and in response to wounding that could affect the activities of these proteases. The in vitro activities of two of the proteases of Soybean mosaic virus (SMV) and Tobacco vein mottling virus (TVMV) were compared in the rabbit reticulocyte lysate in vitro translation system using synthetic RNA transcripts. Transcripts produced from SMV and TVMV cDNAs that included the P1 and helper component-protease (HC-Pro) coding regions directed synthesis of protein products that were only partially processed. Unprocessed poly-proteins were not detected from transcripts that included all of the P1, HC-Pro, P3 and portions of the cylindrical inclusion protein coding regions of either virus. Addition of soybean trypsin inhibitor to in vitro translation reactions increased the accumulation of the unprocessed polyprotein from TVMV transcripts, but did not alter the patterns of proteins produced from SMV. These experiments suggest that SMV-and TVMV-encoded proteases are differentially sensitive to protease inhibitors.

Root colonization by the rhizobacterium Pseudomonas chlororaphis O6 in Arabidopsis thaliana Col-0 plants resulted in induced tolerance to drought and salinity caused by halide salt-generated ionic stress but not by osmotic stress caused by sorbitol. Stomatal apertures decreased following root colonization by P. chlororaphis O6 in both wild-type and ABA-insensitive Arabidopsis mutant plants. These results suggest that an ABA-independent stomatal closure mechanism in the guard cells of P. chlororaphis O6-colonized plants could be a key phenotype for induced systemic tolerance to drought and salt stress.

It has been known that streptomycin resistance in bacteria can occur as a results of chromosomal mutation or through gene acquisition or both. Chromosomal mutations for resistances are point mutations in the rpsL gene, which alter ribosomal protein S12. Acquired resistance has occurred when an $Sm^R$ plasmid carrying transposon Tn5393 with tandem strA-strB gene is transferred by conjugation. A total of 686 isolates of Xanthomonas smithii subsp. citri causal agent of citrus canker disease were collected from 26 citrus orchards in Jeju Island in 2003 and 2004 seasons. Forty-nine of 111 isolates from streptomycin non-sprayed orchards in 2003 season were resistant to streptomycin. Of 107 isolates from orchards sprayed one time with streptomycin, 58 isolates were resistant, and 166 of 221 isolates from orchards sprayed two times with streptomycin were resistant. In 12 orchards sprayed three or more times with streptomycin, 219 of 247 isolates were resistant to streptomycin. Twenty-five isolates of X. smithii subsp. citri were surveyed to identify the mechanisms of streptomycin resistance in this study. Twenty-one of these 25 isolates were resistant to streptomycin, and it was proven by PCR assay that 18 of the 21 streptomycin resistant isolates have the strB gene. In sixteen of the 21 streptomycin resistant isolates, it was occurred a point mutation altered codon lysine (AAG)-41 of rpsL gene to arginine (AGG). The streptomycin-sensitive isolates easily acquired the resistance by mixed culture with resistant isolates. The strB gene was amplified from the isolates that acquired the resistance by mixed culture, and one isolate of them was also point-mutated in codon 41 of rpsL gene to be resistant. In this study, most of the streptomycin-resistant isolates of X. smithii sub sp. citri in Jeju island expressed the resistance by both chromosomal point mutation and gene acquisition, and the resistance was easily acquired through conjugation by culture mixed with streptomycin resistant and sensitive strains.

Pyrenophora graminea, the causal agent of leaf stripe disease, is an economically important pathogen of barley found worldwide. It is critical to clearly define and standardize the leaf stripe assessment methods to avoid subjectivity and variability between assessors. Therefore, in this study, a comparison between the proportion of diseased plants (DP) and the proportion of diseased leaves (DL) per plant was investigated. Disease assessments were made visually at multiple sample sites in artificially and naturally inoculated research and production fields during four growing seasons. There were significant differences (P = 0.001) among cultivars in mean DP and DL averages, which are consistently higher in susceptible barley. However, DP values increased linearly as DL increased. The slopes and intercepts of the DP-DL relationship were consistent over the four growing seasons. This result might make a significant contribution for leaf stripe assessment in barley breeding programme.