Control of phytoplasma vectors in organic viticulture

A comparative study is carried out in 2002 and 2003 in the Mosel viticultural area in order to evaluate the specific risk of infection by Bois noir (Vergilbungskrankheit, VK) in organic viticulture. Occurrence, abundance and infestation of the disease vector Hyalesthes obsoletus is monitored in organic vineyards and compared to data from conventional plots. The organic vineyards under investigation are characterized by a wide variety of herbaceous weed species. Immature H. obsoletus were found on the roots of known host plants like Convolvulus arvensis, Urtica dioica, and Ranunculus bulbosus. Calystegia sepium was identified as a new host plant for the vector. It is, like C. arvensis, a widespread and problematic weed in organic viticulture. Different isolates of the VK-phytoplasma were detected in host plants and vectors, but their biological and phytopathological significance is not yet clear. While no significant differences in the abundance of H. obsoletus could be observed between organic and conventional vineyards, the infestation of the populations was significantly higher in the latter. Disease incidence was generally lower in organic vineyards compared to adjacent conventional plots. Hibernating larvae of H. obsoletus move to a depth of approximately 18 cm during winter. Ploughing brought them up to the surface where they were killed by frost. The efficiency of this method needs to be evaluated during the flight of adult planthoppers in 2003.