Dr Joanna Gambetta uses the analogy of Aussie beach culture to explain her Wine Australia Incubator Initiative research into sunburn damage in Chardonnay grapes.
‘Consider a situation with two people heading into summer. The first person covers themself up completely over the summer and avoids the sun and tanning. While another person waits until February, when the sun is at its hottest – and their skin is at its most fragile because it hasn’t yet been exposed to the sun – and then bakes for hours and gets terribly sunburnt.’
The project – developed in collaboration with Wine Australia’s Regional Program cluster in Greater NSW – is evaluating three different timings of defoliation in Chardonnay vines grown around Orange, NSW: a control group in which no leaves are removed; a second group where leaf removal is carried out at the end of flowering; and a third group of vines where leaf removal is applied at véraison (mid-January), when the sun is usually at its fiercest and the grapes are most vulnerable.
Dr Gambetta, a Post Doctoral Research Fellow at the National Wine and Grape Industry Centre (NWGIC) in NSW, said it’s a similar experience for grape berries over the harsh Australian summer.
‘Approximately 5 to 15 per cent of berries in Australia in any given season are affected by sunburn as a result of high-light in combination with high ambient temperatures and ultraviolet (UV) radiation’, said Dr Gambetta.
The result? Browning, cracking, or even complete berry shrivelling.
‘Symptoms can appear quickly, depending on the conditions. For example, on sensitive, fully mature Chardonnay grapes, symptoms can appear within five minutes once surface temperature on the berries reaches a temperature of 40–43° C’, Dr Gambetta said.
She said if these temperatures persisted for several days, growers could expect ‘maximum damage’.
Dr Gambetta said sunburn in grape berries could be exacerbated by a number of factors, including grape variety, phenological stage, water stress and, especially, canopy management practices.
‘For example, incorrect timing of leaf removal can dramatically increase the number of sunburnt berries.’
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Dr Gambetta’s research is focused on trying to determine the optimum timing for leaf removal to balance disease reduction and sunburn, as well as determining how altitude affects the degree of sunburn. She said her research was important, because sunburn reduced both the visual and aromatic quality of grapes, particularly in white varieties such as Chardonnay.
‘Depending on the severity of sunburn, grapes can be downgraded from A to C grade, causing significant economic losses for grapegrowers and wineries. Berry shrivel can also result in up to 10 per cent reduction in production volume – and is likely to get worse given the increase in frequency, intensity and duration of heatwaves occurring every year.’
Dr Gambetta said the research was still in its early stages.
‘We have installed equipment for the first part of the project, and removed leaves from the early removal treatment. At véraison, we will proceed to remove leaves from the late removal treatment.’
Dr Gambetta said the research was important as it could provide future guidelines around the timing of vine leaf removal to reduce sunburn incidence.
The NWGIC is an alliance between Charles Sturt University (CSU), the NSW Department of Primary Industries and the NSW Wine Industry Association.
You can follow Dr Gambetta’s research on Instagram @dr_joannag.