Molecular Epidemiology and Physiology of Shiraz Disease with an Emphasis on Grapevine Virus A.

Abstract

Shiraz disease (SD) is one of the most virulent viral diseases of several SD-sensitive grapevine varieties including Shiraz, Merlot, Malbec, Gamay, Ruby Cabernet and Sumoll in Australia. The viromes of a large number of grapevines with SD, leafroll disease (LRD) symptoms, and asymptomatic grapevines (ASY) were assessed for grapevine virus A (GVA) and other viruses using endpoint reverse transcription polymerase chain reaction (RT-PCR) and metagenomic high throughput sequencing (Meta-HTS) approaches. A minimum of one GVA isolate from phylogenetic group II (GVAII) was present in a SD grapevine. In contrast, phylogroups GVAI and GVAIII were not associated with SD, since they were also present in LRD and ASY grapevines. These results support previous studies both in Australia and South Africa that demonstrated that GVAII was strongly associated with SD symptoms. At least one grapevine leafroll-associated virus (GLRaV) species was also found in SD grapevines and might contribute to disease expression, however no association to a specific GLRaV species was found. Analysis of GVA quasispecies using the median-joining networks (MJNs) demonstrated the potential emergence of new GVAII variants related to one or a few dominant variants within a population. When MJNs were constructed using all samples from different vineyards, variant groups were revealed, some of which were linked to the actual geographical locations of grapevine populations, demonstrating potential spread within a vineyard, and some which linked different vineyards indicating a spread through planting material. No single GVAII variant was specifically linked to SD. A series of physiological parameters of SD, LRD and ASY grapevines were monitored in two South Australian vineyards, Willunga (WIL) and Langhorne Creek (LC), during 2018-19, and at WIL only in 2019-20, and 2020-21. Vine decline and dead arms symptoms were also observed at WIL that may be attributed to grapevine trunk diseases (GTDs). Plant area index (PAI) data in combination with a vine health score (VHS) system, which was developed for this study to measure the severity of decline and dead arms symptoms, showed that canopy vigor in SD/GTD affected grapevines was significantly reduced, as compared to LRD and ASY grapevines. PAI can be used to distinguish SD/GTD and ASY grapevines using kmeans clustering. Other parameters like leaf chlorophyll, photosynthesis, total non-structural carbohydrate in cane and trunk, leaf stomatal conductance and water potential were less affected or unaffected. Fresh berry mass (FBM), total soluble solids (TSS) and juice pH were also compared. According to the timewise berry development curves for the three growing seasons, berries of SD/GTD affected grapevines tend to have a lower TSS and pH but a higher FBM. This is likely due to delayed berry ripening which could be a consequence of delayed canopy growth associated with SD and/or GTDs. The yield of berries was significantly reduced compared to non-Shiraz disease (NSD) grapevines at the Willunga site which was related to the presence of the physiological disorder called bunch stem necrosis (BSN) which caused berries to drop before harvesting, especially the SD/GTD affected grapevines. Thus only 900g of berries from two SD/GTD affected grapevines could be used to make wines and asses impact of disease on wine quality. The preliminary wine data suggests that wine made from SD/GTD affected grapevines with GVAII had lower ethanol, less colour and tannins compared to wines from NSD grapevines. The wines from SD/GTD affected grapevines may have more vegetal characters and less flavour compounds that contribute to pleasant aromas than those from NSD grapevines. The differences between wines from SD/GTD and NSD grapevines need to be re-investigated to see if this could be due to a chemical metabolism resulting from the GVAII infection or a direct consequence of delayed berry ripening. This study provides comprehensive insights into SD in Australia, encompassing virological, molecular, physiological, and oenological aspects, which may provide valuable guidance for future investigation of this disease.