Grapevine trunk disease in Missouri vineyards : prevalence and causality of fungal species by cultivar
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Grapevine Trunk Disease (GTD) is an emerging fungal disease complex in vineyards worldwide. In 2019, pruning wood samples from a research vineyard in New Franklin, MO were collected from nine cultivars of Vitis interspecific hybrid cultivars, in which GTD pathogens were found at various frequencies depending on the cultivar. DNA was extracted from 48 individual isolates and submitted for Sanger Sequencing. White grape cultivars (63 [percent]) had significantly higher percentage of GTD than red grape cultivars (27 [percent]) (p [less than] 0.001). Cultivar Aromella had the highest percentage of GTD pathogens (88 [percent]), while Prophecy had the lowest percentage of GTD pathogens in its sample vines (12 [percent]). The more prevalent GTD pathogens in the samples (Diplodia seriata, Neofusicoccum parvum, Neofusicoccum ribis, and Pestalotiopsis uvicola) were then subjected to Koch's Postulates, which is the plant pathology method for determining whether an organism causes a disease. Koch's Postulates were evaluated in a greenhouse by inoculating shoots of Vitis interspecific hybrid cultivar Vignoles, the most widely planted white grape cultivar in Missouri. Shoots were inoculated and placed in a greenhouse for ten days. All shoots inoculated with the fungal endophytes displayed symptoms of GTD, including brown tissue around the wound site, demonstrating that these regionally prevalent endophytes can rapidly cause symptoms consistent with GTD on young Vignoles vines. Examining the differing prevalence of these GTD-causing pathogens in various interspecific hybrid cultivars will aid in the development of cultivar-specific management strategies for GTD. In a separate experiment, a survey of the incidence and severity of symptomatic vines was conducted at a commercial vineyard in Missouri. This site contained a block of Vitis interspecific hybrid cultivar Vidal blanc being removed, with much of the vines showing symptoms of GTD in the vascular tissue of the trunks. This research sought to evaluate the disease severity from the trunks in the vines being rogued. Although there was no statistical significance between the severity ratings and fungal species recovered from Vitis interspecific hybrid cultivar Vidal blanc samples (p [equals] 0.7153), 90 [percent] of the randomly sampled vines showed visual GTD symptoms, including cankers and brown discoloration of wood. The experiments reveal that Vitis interspecific hybrid cultivars may have varying susceptibilities to GTD pathogens, with white cultivars having a higher incidence than the red cultivars studied. Additionally, young Vignoles vines can show symptoms of GTD in just 10 days after inoculation with four GTD pathogens. Both of these results have management implications, in that cultivars may need to be managed differently for GTD depending on whether they are a red or white grape cultivar, and sanitation practices may need to be initiated at the time of planting to prevent the development of GTD symptoms in young vines.
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