Potato virus Y (PVY) is becoming increasingly important in potato growing regions worldwide. The main reason for this is an increase in the incidence of infections with recombinant forms of PVY, such as PVY N Wi and PVY NTN . They are characterized by high virulence and low symptom expression, which is especially true of PVY N Wi. This makes it difficult to detect infected seed potato plants during certification. In Mecklenburg-Western Pomerania (North-East Germany) in 2008 an unusually high incidence of infection with PVY was recorded in fields where seed potatoes were being grown. In this study we examined, which strains of PVY caused these infections. Furthermore, we have developed a reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR) assay for direct tuber testing, which we compared to direct tuber testing by ELISA and growing-on tests. As a result, we recommend for direct tuber testing by RT-qPCR or ELISA. These methods are less space-and time-consuming and therefore less costly alternatives to conventional ELISA testing of eye cuttings from seed potatoes. Additionally, the RT-qPCR method has a high efficacy, so that even freshly harvested non-dormant tubers can be tested, which makes testing very fast and economical. This is of special interest in cases when tubers shall be exported to the other hemisphere of the world.

Maize streak virus (MSV) is the cause of one of the most devastating maize diseases in Africa. It is transmitted by leafhoppers of the genus Cicadulina. Due to the changing climate it is possible that species of this genus capable of transmitting MSV will become established and spread the virus in Europe. There is no data on the level of resistance of cultivars of European maize to MSV. The susceptibility of three maize and 15 sorghum cultivars, and Miscanthus × giganteus was investigated using agroinoculation with the virus. DAS-ELISA and a newly developed real time quantitative PCR was used to determine the concentration of virus. All three cultivars of maize were susceptible to MSV administered using agroinoculation, although there were significant differences in the levels of susceptibility. The 15 cultivars of sorghum and Miscanthus were resistant to MSV. Transmission tests using Cicadulina mbila as the vector confirmed the resistance of two of the cultivars of sorghum and Miscanthus. Agroinoculation can only be carried out under S2 biosafety conditions. Therefore, the persistence of agrobacteria in the plants was investigated. Five weeks after agroinoculation, the bacteria were no longer found in the above-ground parts of the plants, but still persisted in the roots of some plants. Transmission tests with an indigenous species of leafhopper, Psammotettix alienus, a vector of the related geminivirus Wheat dwarf virus, revealed that this species is not capable of transmitting MSV. Virus was found only in the body of these insects and not in their heads, which is necessary for persistent transmission through salivary glands.

Enhanced or reduced uptake of viruses by vectors and changes in resistance level can be sensitive indicators for metabolic changes in transgenic plants caused by the new trait and not observed by conventional methods. In addition, the plant transformation process itself can lead to such changes. To be able to investigate this hypothesis in cereals we decided to use two highly important insect-transmitted viruses infecting them - Barley yellow dwarf virus (BYDV) and Wheat dwarf virus (WDV). Corresponding molecular tools for their quantification in plants as well as virus vectors had to be developed. Both viruses cause similar symptoms: dwarfing, stunting, leaf discoloration leading to yield losses or death of plant. BYDV (Luteoviridae) is one of the most important cereal-infecting viruses worldwide causing substantial yield losses. In Germany, the strain PAV is widespread. It is transmitted by the aphids Rhopalosiphum padi and Sitobion avenae in a persistent manner. WDV (Geminiviridae) is found in Germany since the early 1990s and has gained importance over the last years. It is transmitted by the leafhopper Psammotettix alienus in a persistent manner. Real-time PCR is the state of the art method for specific detection of viruses even in minor quantities. It allows exact quantification of the virus content of plants and vectors and is hence suited to monitor changes of it. We developed qPCR assays for WDV and RT-qPCR assays for BYDV-PAV based on TaqMan probe technology as well as the DNA-binding dye SybrGreen. The qPCR assays proved to be more sensitive than DAS-ELISA. For example, WDV is detected even at dilutions of 1:10^8^, whereas the corresponding threshold for ELISA is about 1:10^4^. As nucleic acid extraction procedures proved to be time consuming and expensive, detection methods are adopted now for immunocapture procedures. These methods will be applied to the analysis of several transgenic wheat lines.

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