Planococcus ficus and the spread of grapevine leafroll disease in vineyards: a 30-year-long case study in north-West Spain

The mealybug Planococcus ficus is one of the main vectors of Grapevine leafroll associated virus-3 (GLRaV-3), which was commonly detected in cv “Albariño” planting material before certified stock was available. Mealybug infestations were rare in vineyards in southern Galicia (NW Spain) during the 1990s (2.2% of the vineyards surveyed) and are still rare in inland zones. However, mealybug infestations have spread since 2000, with 15% of surveyed vineyards infested in 2004 and 80% of surveyed vineyards infested in 2016. The spatial and temporal distributions of plants infected with GLRaV-3 were quantified over a 30-year period in an experimental plot established in 1989. The disease progress curve (DPC) was linear for 25 years, with a slow constant rate of spread of less than one newly infected plant per year (0.6%). Since 1992, >82% of infected plants were located on the west side of the plot as were 84% of newly infected plants. Newly infected plants were in contact with infected plants, suggesting vector-mediated transmission, but no potential vectors were found. In 2013, a small mealybug infestation was detected and identified as Pl. ficus . Between 2014 and 2016, the infection rate increased to >21% per year, and in 2019 all plants tested positive for GLRaV-3. This is a valuable case study illustrating how changes to the vector fauna can increase the rate of spread of an economically important virus of grapevine.