New approach to measuring powdery mildew

A free app to help identify and measure the presence of powdery mildew on grapes and grapevines should be available to the Australian wine community by next vintage.

Researchers in a University of Adelaide-led project have begun second-phase testing and are seeking volunteers to put the new software through its paces this vintage and suggest improvements.

The initiative, which is supported by a number of wine companies, is part of a broader AGWA-funded project looking to establish objective measures for assessing powdery mildew.

“An app is something people have been thinking about for a while now and the industry reference group has been very supportive,” said Professor Eileen Scott, Deputy Head of the University’s School of Agriculture, Food and Wine.

“While our primary focus is on developing objective molecular and spectroscopic measures for powdery mildew, we all acknowledge that assessing disease visually in the field, and sometimes also at the winery, is going to be with us for some time.

“The aim is to create a simple tool that will allow assessors to capture information more accurately and efficiently in the vineyard and improve their disease recognition skills.”

The development model can be used on an iPhone or iPad, and a version for Android devices will be created.

The information component of the app is reasonably straightforward, although there are software smarts behind it. Assessors simply input the patch details and their estimate of the percentage of powdery mildew present on each bunch being assessed.

The screen displays the number of bunches assessed, the last five assessments, and constantly updated figures for the total incidence (number of bunches deemed affected) and severity (calculated from the average of the scores for the amount of the surface of the bunch affected).

All results are stored and can be emailed from the device as CSV or XML files.

Just as important are the training components. An image bank containing a mix of photos and computer-generated images helps users familiarise themselves with various disease patterns and severities, while a self-testing tool marks their assessment of a range of provided scenarios and indicates whether their tendency is to over- or under-estimate infection severity.

Powdery mildew is hard to assess because it appears indiscriminately and the fungus can be hard to see in less than perfect light, or it can be confused with dust or spray residue. Scarring on a grape may be caused by powdery mildew or by simple physical damage. And while rain may wash away the spores, the damaging hyphae are left on the surface.

“Powdery mildew also can mean different things at different times of the season,” Professor Scott said.

“If a grower sees it on leaves or very young berries early in the season, there’s the opportunity to adjust their spray program to eradicate the infection and prevent further damage, but close to harvest it can mean a downgrading of fruit quality.”

Professor Scott said most wineries had thresholds of tolerance for powdery mildew (usually 3–5% of the bunch area affected) and it was important for growers to produce grapes that met a winery’s requirements.

The other side of the project is using techniques such as mid- and near-infrared spectroscopy to determine exactly how much powdery mildew is present on grapes.

Professor Scott said the disease’s impact on wine was caused by a combination of the physical presence of the fungus on a grape’s surface and the impact of the infection on the quality of grape itself, but the contribution of each was not yet clear. A lot also depended on the winemaking style and how the grapes were handled.

“There isn’t really a specific marker for powdery mildew yet. In earlier research, we found that affected grapes have increased content of phenolics, hydroxycinnamates and flavonoids, but these components can also be elevated in response to other diseases, insect attacks and physical damage.

“We do know that the grapes change biochemically, but we don’t yet know how much of a difference there is between having one heavily diseased bunch and all of the rest being healthy, and having a light infection spread across a lot of bunches. We don’t know how that affects wine quality.”

If you are interesting in being part of the new trial, please contact Professor Scott at eileen.scott@adelaide.edu.au.

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