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Spatial Analyses of Viral Epidemics in Australian Hop Gardens: Implications for Mechanisms of Spread

Plant-to-plant spread of Hop mosaic virus (HpMV), Hop latent virus (HpLV), and Prunus necrotic ringspot virus (PNRSV) were monitored in two commercial hop gardens of cv. Victoria in Australia. At site 1, the cultural practice of mowing to remove excess basal growth operated exclusively along rows, while at site 2, mowing occurred both along and across rows. Other cultural practices such as stringing, bine-training, spraying, and harvesting were directed along rows at both sites. Spatial aggregation within and across rows was assessed by ordinary runs analysis, and by radial correlation analysis using the program 2DCORR. The dominant spread mechanism of carlaviruses differed between sites. At site 1, along-row aggregation of both HpLV and HpMV in 1997 reflected the importance of basal-growth intertwining for virus transmission, either due to mechanical transmission or movement of apterous or alatae vectors along rows. At site 2, the random incidence of HpLV and HpMV reflected spread by alatae vectors. The spread of PNRSV at both sites was associated with mowing direction. This study reports the first use of a modified version of radial correlation analysis.