Tomato yellow leaf curl virus (TYLCV) and its vector insect, the sweet potato whitefly Bemisia tabaci, are major threats to tomato and pepper production in all around world. Since the last three decades, both B. tabaci and TYLCV have been invaded into many countries via different routes. Our studies showed that various geminiviruses including TYLCV can be transmissible by seeds as well as whiteflies. Furthermore, commercially developed resistant tomato strains against TYLCV infection can serve as TYLCV reservoirs and potentially influence on TYLCV epidemics. Therefore, transmission pathways through both insect vectors and seeds should be concerned for suitable management of geminiviruses and whiteflies.

Geminiviruses are plant-infecting viruses with monopartite or bipartite single-stranded circular DNA genomes. They are known to be mediated by insects such as whiteflies, treehoppers, leafhoppers or aphids and can cause devastating plant diseases in a wide range of economically significant crops worldwide. In Korea, occurrence of geminiviruses were reported officially after the 2000s. Although Honeysuckle yellow vein virus (HYVV) and Sweet potato leaf curl virus (SPLCV) were identified from honeysuckle and sweet potato in 2004 and 2006 respectively, these viruses did not spread and cause much concern for geminiviruses. In 2008, Tomato yellow leaf curl virus (TYLCV) that has caused severe tomato production loss in many subtropical and tropical countries was first reported in tomato plants cultivated in Tongyeong. TYLCV rapidly spread through the country and has been continuously reported from tomato cultivating areas in Korea. In addition to TYLCV, Tobacco leaf curl virus and Sweet potato golden vein associated virus have occurred. In recent years, new geminiviruses including Papaya leaf curl Guangdong virus (PaLCGdV) and Euphorbia leaf curl virus (EuLCV) have been introduced. Newly emerging geminiviruses from tropical and subtropical countries are due to increased demand for various fruits and vegetables, and climate change. In addition, there are reports that some geminiviruses including TYLCV can be transmitted by infected seeds. Therefore it is important to study on epidemiology of virus introduction and spread among the countries and within country.

Tomato yellow leaf curl virus (TYLCV) causes a highly serious disease in tomatoes in many countries. The most important thing in management is the prevention of virus transmission. TYLCV had been known to be only transmitted by a vector, the sweetpotato whitelfy, Bemisia tabaci. However, we identified two other important routes of TYLCV transmission into tomatoes. It is seed-transmissible. Many seeds available in domestic and global markets were infected by TYLCV. It is also infective into various weeds and other horticultural crops and transmit virus through vector insects. Therefore, we need a new strategy for the virus and vector management.

Loop-mediated isothermal amplification (LAMP) is a rapid, specific, cost-effective detection method by amplifying nucleic acid under isothermal conditions. In this study, we used LAMP for detection of Hamiltonella defensa that lives as a facultive endosymbiont of whitefly ‘Bemisia tabaci’. We designed the Hamiltonella-specific primers by targeting 16S ribosomal RNA gene and validated the specificity of one primer set. To find the optimum temperature for our primer set, the LAMP reaction was held at the temperature, 60℃, 62℃ and 65℃. As a result, 62℃ was the optimum reation temperature for LAMP reaction. Specificity of primer set was tested by the reaction to both Trialeurodes vaporariorum and B. tabaci. After the whole procedure, the amplicons by LAMP were visualized by adding SYBR Green to the reaction tube.

Acquisition of plant viruses has various effects on physiological mechanisms in vector insects. Bemisia tabaci is the only known vector of Tomato yellow leaf curl virus (TYLCV), which is a serious virus affecting tomato cultivars. In this study, the lifespan of Q1 biotype was compared between non-viruliferous (NV) and TYLCV-viruliferous (V) whiteflies. Total lifespan from egg to adult death of NV whiteflies was 62.54 days but 10.64 days shorter in V whiteflies. We investigated the temperature susceptibility of B. tabaciby comparing mortalities as well as heat shock protein (hsp) mRNA levels between NV and V whiteflies. For this, NV and V whiteflies were exposed for either 1 or 3 h at 4, 25, and 35 °C. The mortality of V whiteflies was higher than NV ones following exposure at either 4 or 35 °C, but there was no significant difference at 25 °C. Analysis of the expression level of heat shock protein (hsp) genes using quantitative real-time PCR showed that both cold and heat shock treatments stimulated higher expression of hsps (hsp40, hsp70, and hsp90) at various rates in V whiteflies than NV ones, but there was no difference at 25 °C. All together, our results show that TYLCV acquisition accelerated the developmental rate and increased susceptibility to thermal stress in B. tabaci. Therefore, this modification may result in reduced vector longevity due to increased metabolic energy utilization. Our results provide insights into the complex interaction between vector fitness and thermal stress in relation to the acquisition and transmission of plant viruses.

Chrysanthemum stunt viroid (CSVd) is a serious pathogen affecting chrysanthemum that has caused significant economic losses to Chrysanthemum flower production worldwide. Control of CSVd disease is difficult due to its contagious nature and long latent period in the field. As chrysanthemum is most often produced by implanting seedlings, it is necessary to diagnose CSVd infection before cultivation. In this study, we screened CSVd infection in seedlings from 30 varieties including 5 domestic, 6 Japanese, and 19 European varieties. Molecular diagnosis of the combination of RT-PCR and nested PCR showed that CSVd was not detected by the first RT-PCR but detected by the second nested PCR analysis in 10 varieties, including 1 domestic, 2 Japanese, and 7 European varieties. Further comparison of 10 identified CSVd nucleotide sequences showed that those are highly conserved (99-100%) and the most similar to an isolate (AB006737) identified in Hokkaido, Japan. Our study suggests that the combination of RT-PCR and nested PCR analysis is successful for the CSVd diagnosis of seedlings and the molecular diagnosis is necessary to prevent the introduction and propagation of viroid disease into the fields.

The sweet potato whitefly, Bemisia tabaci (Gennadius, 1889) (Hemiptera: Aleyrodidae), is one of the most important agricultural pests by direct feeding and begomovirus transmission in the world. B. tabaci species complex consisting of about 30 species worldwidely has been mainly distinguished by molecular methods because of difficulties detecting morphological differences. In Korea, distribution of two exotic groups, the Mediterranean (Q biotype) and the Middle East-Asia Minor 1 (B biotype), were confirmed by comprehensive molecular methods in 2012. Of them, the Q biotype as a vector of Tomato yellow leaf curl virus (TYLCV) has caused significant crop yield losses. Tobacco leaf curl virus (TbLCV), one of four begomoviruses in Korea, was first reported in 2011. It damaged, although not frequent, to greenhouse tomatoes at Iksan, Gimje, Sunchang, and Jeju. The vector transmitting TbLCV, however, is not known yet. In this study, we sampled an unknown B. tabaci populations on leaves of the Japanese honeysuckle (Lonicera japonica) at Iksan in outdoor conditions. PCR analysis revealed that some populations were infected with TbLCV. We identified an unknown indigenous genetic group named as JpL and clarified its phylogenetic relationships based on nucleotide sequences of mitochondrial cytochrome oxidase I gene.

The sweetpotato whitefly, Bemisia tabaci, acts as a vector of more than 100 plant viruses. B. tabaci is known to harbor a primary endosymbiont (Portiera) and 6 secondary endosymbionts (Arsenophonus, Cardinium, Fritschea, Hamiltonella, Rickettsia and Wolbachia). These endosymbionts play important roles in the acquisition and transmission of plant viruses. Using PCR analysis, we identified endosymbiotic bacteria in various B. tabaci populations collected from different places of Korea. Distribution of endosymbionts was different according to the biotype of B. tabaci. Subsequently, their relative densities of endosymbionts were compared between TYLCV-viruliferous and non-viruliferous populations of the Q biotype using quantitative realtime PCR. We found that the densities of Portiera, Cardinium and Hamiltonella are higher in viruliferous than non-viruliferous whiteflies. Our results suggest the role of endosymbiont for the TYLCV transmission of whiteflies.

The sweetpotato whitefly, Bemisia tabaci, is a vector of more than 100 plantdiseased viruses as well as a serious pest to various horticultural crops. Virus acquisition affects the vector’s development and reproduction, but its mechanism is largely unknown. Here we compared the temperature responses between non-viruliferous and TYLCV-viruliferous Q biotype of B. tabaci. When both non-viruliferous and viruliferous whiteflies were exposed for 1 and 3 h at 4, 25, and 35°C, the mortality rate of viruliferous whiteflies is higher than nonviruliferous after exposure at 4°C and 35°C, but no differences at 25°C between them. Analysis of the expression levels of heat shock protein (hsp) genes using the quantitative realtime PCR showed that viruliferous whiteflies has higher expression in hsp70, and hsp90 at both 4°C and 35°C, but no differences at 25°C. The results suggest that vector insects may not be durable to unfavorable temperature conditions when they acquisite plant viruses.

The oral toxicities of symbiotic bacteria Photorhabdus temperata ssp temperata (Ptt), mutually associated with entomopathogenic nematode Heterorhabditis megidis, and P. luminescens ssp. laumondii (TT01) with H. bacteriophora, were demonstrated to adults of the sweetpotato whitefly Bemisia tabaci. Sucrose solution (25%) containing bacteria-free supernatant of culture media of symbiotic bacteria was ingested into adult whiteflies within the glass tube. Whitefly mortalities were shown similar patterns against two bacterial media. Mortalities were significantly increased to 60-64% at 36 hours and almost 100% at 60 hours after treatments. In addition, We demonstrated the effect of oral ingestion of symbiont culture media on the gene expression of B. tabaci. Several genes fluctuated those expression levels. Our results suggest that oral ingestion of symbiont culture media of entomopathogenic nematodes significantly changed metabolic rates and highly lethal to whiteflies. The use of symbiotic bacteria of entomopathogenic nematodes provides a great potential as an alternative genetic resource of Bacillus thuringiensis, a major resource of microbial insecticide.

The sweetpotato whitefly, Bemisia tabaci, is a vector insect of more than 100 plant-diseased viruses as well as a serious pest of various horticultural crops. B. tabaci is a species-complex that consists of at least 24 biotypes, which show different biological characteristics including host range, fecundity, insecticide resistance and virus transmission. Here we identified biotype, endosymbiotic bacteria, and tomato yellow leaf curl virus (TYLCV) acquisition of various B. tabaci populations collected in Korea. In addition, we compared those profiles with B. tabaci collected from Bangladesh and Myanmar, and the greenhouse whitefly, Trialeurodes vaporariorum. PCR diagnosis of cytochrome oxidase I (COI) showed that all B. tabaci populations of Korea were Q-biotype and closely related with a subgroup I (MedBasin 1), which is indigenous to the Western Mediterranean area. Ribosomal DNA analysis of 5 endosymbionts showed that both Cardinium and Hamiltonella were detected in most tested populations while the presence of Arsenophonus, Fritschea and Wolbachia dependent on populations. Our results suggest that the acquisition of TYLCV do not related with the endosymbiont profile of B. tabaci.

The sweet potato whitefly Bemisia tabasi is one of the most important pests of various horticultural crops. In addition, B. tabaci is a vector of many plant-pathogenic viruses and cause a serious secondary damage to crop plants. Association of plant-pathogenic virus with vector insects is known to be effective on the transmission capacity, fecundity, longevity of vectors including whiteflies. However, the interactive mechanisms between virus and vector insects are still poorly understood. Recently, a serious damage caused by virus disease together with B. tabasi emergence was identified at tomato glasshouse in Tongyoung. We detected the signals of Tomato Yellow Leaf Curl Virus (TYLCV) in tomato leaves and vector whiteflies using PCR amplification and confirmed its presence by those sequence comparison. To determine the effects of TYLCV acquisition on physiological status of vector whiteflies, transcript levels of genes that associated with metamorphosis, metabolism, stress and immune processes were compared between TYLCVinfected whiteflies and non-infected ones. Generally, the transcript levels of virus-infected whiteflies were lower than those of non-infected ones. In addition, the associations of endosymbiont levels within whiteflies were discussed in aspect of the acquisition and transmission of TYLCV.