STUDIES ON GRAFT-TRANSMISSIBLE DISEASES OF THE  GRAPEVINE (VITIS VINIFERA L.)

Sudarsana Poojari

A graft-transmissible disease showing red veins, red blotches and reddening of leaves, designated as grapevine redleaf disease (GRD), was observed in wine grape (Vitis vinifera L.) cultivars Merlot and Cabernet Franc. Using next-generation sequencing (NGS), three distinct viruses (a geminivirus, Grapevine rupestris stem pitting-associated virus [GRSPaV] and Grapevine fanleaf virus [GFLV]) and four viroids (Hop stunt viroid , Grapevine yellow speckle viroid 1, Citrus exocortis viroid and Citrus exocortis Yucatan viroid) were identified in grapevines showing GRD symptoms and GRSPaV and all four viroids in non-symptomatic grapevines. Molecular and phylogenetic analyses indicated that the geminivirus, tentatively named as Grapevine redleaf-associated virus (GRLaV), represents an evolutionarily distinct member in the family Geminiviridae. GRLaV was transmitted by Virginia creeper leafhopper (Erythroneura ziczac Walsh) under greenhouse conditions. Fruit yield and berry quality parameters were significantly affected in Merlot and Cabernet Franc grapevines showing GRD symptoms compared to non-symptomatic vines. Biochemical analysis showed higher quantities of carbohydrates in symptomatic leaves suggesting their possible role in symptom expression. A SYBR Green-based reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR) combined with melting curve analysis (MCA) was developed for specific and sensitive detection of Grapevine leafroll-associated viruses (GLRaV-1, -2, -3, and -4), GRLaV, Grapevine virus A, Grapevine virus B, GFLV and Tomato ringspot virus in grapevines. The assay was also used successfully for detection of GLRaV-3 in grape mealybugs and GRLaV in Virginia creeper leafhoppers. This method has the potential for practical applications to study epidemiology of grapevine viruses. The complete genome of a naturally occurring GLRaV-2 strain causing asymptomatic infections in the wine grape cultivar Sangiovese (GLRaV-2-SG) was determined to be 16,474 nucleotide (nt) long. In pairwise comparisons, the genome of GLRaV-2-SG was closely related to GLRaV-2-OR1 from USA, and GLRaV-2-93/9555 from South Africa, and distantly related to GLRaV-2-BD from Italy and GLRaV-2-RG from USA. GLRaV-2-SG had little impact on fruit yield and berry soluble solids and pH, but significantly affected berry anthocyanins when compared to grapevines not infected with the virus. The occurrence of asymptomatic strains of GLRaV-2 highlights limitations to symptom-based disease surveys and underscores the need for accurate diagnosis of asymptomatic infections.

STUDIES ON GRAFT-TRANSMISSIBLE DISEASES OF THE GRAPEVINE (VITIS VINIFERA L.)

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