Analysis of vitivirus spread relative to grapevine leafroll spread in South African vineyards
ARC Plant Protection Research Institute
Objectives and Rationale
Determine whether the spread of Vitiviruses occurs under field conditions independently of spread by GLRaV-3 as they share the same vectors. To determine if the spread occurs at different spatial rates to that of GLRaV-3, and whether mild virus-like symptoms still found in vineyards following control of GLRaV-3 may be due to Vitiviruses
In this study we determined whether natural Vitivirus spread occurs independently of GLRaV-3 in vineyards as well at what spatial rates relative to GLRaV-3 spread. This was done by collecting vines from a clump of leafroll infections (as this indicates that secondary spread is occurring), as well as from surrounding vines and testing them for GLRaV-3 and Vitiviruses. We compare the spatial position of such infections. Secondly, we tested whether virus-like symptoms observed in leafroll eradicated vineyards are due to Vitiviruses, and if so, whether it is necessary, within the South African certification scheme to further tests for this virus in mother-blocks where GLRaV-3 is eliminated.
Leafroll infection foci were identified on 5 wine estates. From 12 such foci, 347 samples were collected. All samples were tested for GLRaV-3 by ELISA. Those testing negative were subjected to GLRAV-3 RT-PCR tests. RT-PCR was done for Viti- and Fovea Viruses on all collected samples and 302 samples sequenced in one direction and identified using BLASTn. GVA, GVB, GVE, GVF, and GVH were identified and occur both with and without GLRaV-3 in roughly similar amounts, as does the Foveavirus Grapevine rupestris stem pitting associated virus. This suggests that there is not a co-dependence of the Viti-Foveaviruses with GLRaV-3 for spread. Furthermore, the spatial rate of spread of Vitiviruses is greater than that of GLRaV-3. In addition, in leafroll managed vineyards about 5 of 20 vines with mild, virus-like, non-leafroll symptoms contained Vitiviruses. This may suggest that Vitiviruses are spreading undetected in leafroll managed vineyards.
Key Conclusion of Discussion
Spatial analysis of GLRaV-3 and Viti-Fovea Viruses found in leafroll foci and surrounding vines suggests that spread of Vitiviruses are independent of spread of leafroll and more rapid than the rate of spread of GLRaV-3, and that a greater, unnoticed, prevalence of these viruses occurs in South African vineyards.
Take Home message for Industry
Vitiviruses spread does not require the co-presence grapevine leafroll associated virus – 3. Vitiviruses may be prevalent and unnoticed in vineyards where leafroll disease has been controlled. This suggests that mealybug control should be maintained even when leafroll has been eliminated or near-eliminated. The effect of infection of the Vitiviruses on their own needs to be determined to decide whether to include them in the certification scheme.