Investigating the recovery phenotype phenomenon in aster yellows infected grapevines
Burger, J T
University of Stellenbosch. Faculty of AgriSciences. Department of Genetics
Burger, J T
Maree, H J
Van der Vyver, A
The objectives for the first year of the project was to identify a suitable experimental vineyard, to visually assess the disease status of the entire vineyard and select test plants based on symptomatology, and to confirm the disease status of these plants using PCR. AY-specific amplicons generated from individual plants were to be subjected to multigene analysis, and polymorphisms so identified, confirmed by sequencing. Moreover, 20 AY-infected and 20 AY-negative plants were to be coppiced. Lastly, the virus status of the selected vineyard was to be determined. These actions would provide an experimental vineyard comprising 80 test plants for the duration of the resistance phenotype monitoring. Individual vines (Colombar/R99) were visually assessed in February 2016, and 40 healthy and approx. 60 symptomatic plants selected and marked in the vineyard. These plants were subjected to PCR to determine the AY-disease status of all test plants during March and April. Multigene analysis of four different AY genes were performed from May to August. Selected plants in the vineyard were coppiced by cutting their stems above the graft union. Wounds were treated. PCR AY and virus diagnostic screens continued at the planned intervals, but were extended to a final screen in Feb 2018. A further one-year time extension to the project allowed for two more diagnostic screens to be executed during Nov 2018 and Feb 2019.
PCR diagnostic analysis of approx. 100 plants identified 40 AY-infected and 40 healthy plants. Two weeks before our planned coppicing exercise, the producer (without informing the VinPro consultant or us) started pulling out plants from every third row in the vineyard, in order to plant olives. Fortunately, this was done in a stepwise manner (i.e. first cutting plants above the graft union and then pulling these out). We were able to salvage most of our experimental plants, even though 32 (14 healthy and 18 infected) were “coppiced” by the producer. An additional eight plants were coppiced by us to bring the numbers to 20 + 20. Monitoring of the experimental plants over the two year period indicated a significant decrease in the number of plants infected with AY. However, this was the case for both coppiced and uncoppiced plants. Moreover, a similar remission in viral infection was observed in the vineyard over the test period. An 83.3% and a 64.7% decline in GLRaV-3 incidence was recorded in the AY-symptomatic coppiced and –uncoppiced sample groups, respectively. During the last two years of the project, severe drought conditions were experienced in the Western Cape, and to an even more intense level in the Vredendal region. Producers were severely restricted in their irrigation regimes. AY titres in all test plants (also uncoppiced) dropped significantly – eventually to a level where no plants tested positive in Feb 2018. Concomitantly, the very distinct AY symptoms also disappeared. A similar trend was observed for the viruses we screened for. After the drought was broken during the winter season of 2018, it was decided to request a one-year extension to the project, in order to obtain another season’s worth of post-drought diagnostic data. These results indeed demonstrated a gradual turn-around in the diagnostic results, for both the additional diagnostic screens that were conducted. We speculated that the uniform induction of a ‘recovery phenotype’ witnessed in the Colombar experimental vineyard was the result of the physiological stress of the severe drought, rather than the effect of coppicing. In order to ensure that the results were true and not ascribed to inaccurate detection assays, representative plant samples were sent to Italy, where our results were corroborated. AY symptoms also disappeared, and a concomitant reduction in virus titre was observed. This result was totally unexpected, but does support the hypothesis that a severe “shock” (biological, chemical, physical) can induce the recovery phenotype. Since the drought was broken during the winter of 2018, a logical next step was to extend the project for another season, and diagnostic data obtained in a further two diagnostic screens confirmed our prediction that AY titres would increase to detectable levels. The ‘recovery phenotype’ in response to biological, chemical or physical shock that has been reported in a number of phytoplasma diseases, including grapevine yellows disease, is real. Results from our experiments to induce this recovery phenotype by coppicing were inconclusive, because of the severe 2016 drought, which induced a reversible recovery phenotype in both coppiced and uncoppiced plants in our experimental vineyard. Whether the induction of the recovery phenotype by coppicing will also be reversible, remains to be proven, but if so, suggests that coppicing is at best a temporary relief from this debilitating disease.
Zambon, Y., Contaldo, N., Souza Richards, R., Bertaccini, A., Burger, J.T. (2015). Multigene characterization of aster yellows phytoplasmas infecting grapevine in South Africa. Phytopathogenic Mollicutes 5, S21-22.
Burger, J.T., Van der Vyver, A., Page, L.D. and Maree, H.J. Phytoplasma Recovery Phenotype: a case study for Aster Yellows in South Africa. 4th International Phytoplasmologist Working Group, Valencia, Spain – September, 2019.
Van der Vyver, A.E., Maree, H.J. and J.T. Burger, J.T. Investigating the ‘recovery phenotype’ phenomenon in Aster Yellows phytoplasma infected grapevine (Vitis vinifera). 50th Congress of the Southern African Society for Plant Pathology (SASPP) 15th-18th January 2017.