Spatial analysis of epidemics of Grapevine leafroll associated virus-3

Several vineyards in Rias 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), bidimensional analysis, and disease gradients analysis were used as methods of study of spatial aspects of epidemics of GLRaV. At very low insect populations the mealybug Planococcus citri transmitted the GLRaV-3 from infected plants to healthy ones planted between them in an experimental plot at Beluso; an initial focus of leafroll-infected grapevines was detected 2 years after planting at the area where the vectors had been located infesting the old plants. Within 8 years some new foci appeared and coalesced, and the incidence reached > 80%. In three commercial plots where no vectors were observed, the spatial analysis of the diseased plants showed three different situations. In Meano, the study of the evolution of the spatial patterns of diseased plants between 1992 and 2005 suggested slow vectorial field transmission of GLRaV-3. In Goian the analysis for only 2 years suggested random distribution; therefore the viruses were arriving with the planting material, but the runs analysis of some lines suggested incipient spread of GLRaV-3. In Portomarin the incidence of both GLRaV-2 and 3 was low and their distribution was random, without any evidence of field spread. These examples of the study of the spatial analysis of leafroll-infected plants may be helpful to determine whether or not spread of the viruses is occurring, and the best control measures to take.