Transmission of the virus diseases most relevant to the German wine-growing regions is carried out by nematodes living in the soil. The most important viral disease is the fanleaf degeneration of the grapevine, triggered by the Grapevine Fanleaf Virus (GFLV) or other nematode-borne viruses. Since the 1990s, the chemical decontamination of the soil was banned as a sanitary measure to eliminate the virus carriers, viral diseases are already observed in young plants. In these cases, the economic damage is significant. The fanleaf degeneration is the most common, but not the only viral disease that causes damage in the German wine regions.
The grapevine leaf roll disease is caused by the types 1 and 3 of the Grapevine Leafroll associated virus (GLRaV). The leafroll disease is the most common virosis of grapevine worldwide. As potential vectors different types of scale insects and mealybugs are considered. However, the transmission of both diseases can also occur in the course of vine grafting. As there are no direct control measures against these viral diseases and there are no natural sources of resistance, only careful control of the propagation stocks and exclusive use of healthy plant material remains.
The situation is also problematic when rare clones or varieties are only present as virus-infected material (variety collections). Since virus-infected vines are not allowed to be placed on the market, the only way out is to eliminate the virus.
In vegetatively propagated crops, the production of virus-free clones, which serve as basis for maintenance breeding, is one of the most important virus control measures. In principle, various methods are available for this purpose: Chemotherapy, thermotherapy and somatic embryogenesis and regeneration.
In the case of chemotherapy and thermotherapy, in vitro cultures of shoot tips are created and treated either with antiviral agents or with a specific temperature and irradiation regime. Depending on the type of virus present, rapid growth and multiple transmission of the sprout tips can lead to its elimination from the tissue. With some viruses, however, these methods are not sufficient, so that one has to go the more complex way of somatic embryogenic cell cultures. With this dedifferentiation process and the regeneration of whole plants, the virus infection can be completely eliminated.
In-vitro vine plantlets in the process of virus-eradication by thermotherapy: The sterile plants are brought to rapid growth on a culture medium by means of an optimized light and temperature regime. After several passages, the plants are transferred to soil and, after cultivation in a greenhouse, immunologically tested for any remaining viral infections.