The spatial distributions of grape plants with symptoms of grapevine leafroll in nine vineyards in the Galician wine-growing region Rías Baixas (northwestern Spain) were analyzed. In five, significant aggregations of diseased plants and/or disease gradients toward one of the borders of the plots constituted indirect evidence of vector transmission by a nonflying vector. From 1991 to 1995, vines from two vineyards were tested for grapevine leafroll associated virus 3 (GLRaV-3) by enzyme-linked immunosorbent assay. In one of these vineyards, which had been virus-free when planted, GLRaV-3 was detected 3 years after planting. In the second, the incidence of GLRaV-3 increased from 33 to 83% between 1991 and 1995. The mealybug Planococcus citri was associated with the infected plants in this vineyard. GLRaV-3 was present in P. citri, and under controlled conditions this mealybug transmitted GLRaV-3 to five of five previously healthy grape plants.

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Several vineyards in Rías Baixas and one in the Ribeira Sacra (Spain) were monitored and the spatial pattern of leafroll-diseased grapevine was analysed at several dates. Unidimensional aggregation analysis (ordinary runs)

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With the aim of investigating the dynamics of transmission of GLRaV-3, we report and analyse timecourses of infection incidence in various plots in one vineyard in Rı´as Baixas (Galicia, Spain). GLRaV-3 infection was close to 100% after 15 years monitoring the epidemic in several plots where P. citri was widely spread although with low density populations. In two plots, virus-free plants were planted close to infected ones and after 8-9 years more than 80% of the plants tested GLRaV-3 positive (average infection rates of 7.8-12.4% per year). The Gompertz model best fitted the epidemiological data.