Monitoring the spread of viruses after vineyard replanting with heat-treated clones of Vitis vinifera ‘Nebbiolo’

Two vineyards planted in the Langhe (Piemonte, Italy) with clonal vines of Vitis vinifera cv Nebbiolo originating from heat-treated mother plants, were monitored for the occurrence and spread of viral infections. More than ten years after the establishment of one of the vineyards, where rows of vines infected with Grapevine virus A (GVA) and Grapevine leafroll-associated virus 1 (GLRaV-1) or GVA and Grapevine leafroll-associated virus 3 (GLRaV-3) were interplanted with rows of healthy plants, viruses had spread to 18.5% of the healthy vines. Spreading was slow and limited to small groups of vines, the size of which increased slowly, thus providing indirect evidence of transmission by vectors. Mealybugs collected in the vineyard were classified as Heliococcus bohemicus. RT-PCR analysis of mealybugs revealed the presence of at least one virus in more than 45% of the tested insect batches, indicating that H. bohemicus can acquire viruses during feeding on infected plants, thus acting as a potential vector. No natural spread of Grapevine fanleaf virus (GFLV) was detected in a second vineyard which had been fumigated before re-planting because of the heavy incidence of fanleaf disease in the preceding uprooted vineyard, and where Xiphinema index, the GFLV vector, was found only occasionally.