Mechanical thinning for Botrytis Control

By Tessa Nicholson,
Editor of Winepress

Three years ago research began into determining if mechanical thinning was a viable method of controlling yields. That research has broadened as scientists discover mechanical thinning may be a way of controlling botrytis.

The scourge of grape growers and winemakers, botrytis is a multi million dollar threat to the industry. An opportunistic disease which can hang around until the conditions are just right, it can decimate quality in just a few days.

Controlling it has been the subject of much research and product development. Now new research is indicating that mechanical thinning could help alleviate the risk. When the initial mechanical thinning trials began in Marlborough back in 2009, those involved were concerned that a side effect of beating the canopy to remove quantities of fruit, would in fact damage bunches in the canopy. That damage could lead directly to more incidence of disease in later weeks.

Instead what they found in those first trials was that the control, un-thinned block had the most disease, whereas the heavily machine thinned block had the least. The same results showed through in 2010, where there was a significantly lower disease level in the mechanically thinned.

Plant & Food Research scientist Dion Mundy says they wanted to find out why that was, so in 2011 he concentrated on monitoring the fruit from both an un-thinned block, alongside the fruit from a heavily mechanically thinned block.

“We wanted to know if was because we were knocking the trash out of the bunch (during the thinning), or was it because the bunch architecture was changing due to smaller berries within the bunch. Or were we inducing some resistance, like the berries gaining thicker skins and promoting elicitors in the berry?”


Following the mechanical thinning, 25 bunches were collected from both the control and thinned blocks. The trash was incubated to determine how much botrytis was within.

“In the un-thinned control we had 35 pieces of trash in the bunch. We had just under 21 pieces of trash in the heavily thinned. More importantly, six of those pieces of trash in the un-thinned control had botrytis on them. In the thinned, there were only three pieces with botrytis. So we are reducing the total number of botrytised pieces of trash in the bunch, when we heavily mechanically thin.”

The next step was to see what effect the thinning had on bunch weights. After scientifically weighing and measuring the separate bunches, it was obvious the un thinned were heavier.

“In the thinned we had a much more open bunch, which affects how quickly the bunch dries, it is more open to spray penetration and the air can move around.”

There was more that needed to be determined, such as was the thinning affecting the physical make-up of the berry, other than in size? Berries from each of the trial sites were density separated, washed, surface sterilised and laid out in trays. In one experiment, the berries were wounded, (pierced by a sharp sterile needle,) and then inoculated with botrytis.

In another, the berries were left intact, and again inoculated with botrytis. The results provided another surprise, Dion says.

“The berries that were wounded were 90% plus susceptible to Botrytis. It didn’t matter if they were thinned or un-thinned.

“However when left intact, the un-thinned berries were also 90% plus susceptible to botrytis. BUT, the un-wounded, thinned berries were only 75% susceptible.

“A significant number of berries that had been mechanically thinned, that were not damaged, were actually able to resist high concentrations of botrytis spores being placed directly on the berry surface. However when we wounded the berries, it took away that resistance. It infers that something has been physically or chemically changed in the process of mechanical thinning that makes those berries more resistant.”

(As a note, the trial also involved berries that were not wounded or subjected to botrytis spores, showing a latent level of botrytis less than two and a half per cent.)

Dion says the good news from the results so far is, they indicate the mechanical thinning does remove more trash from bunches, (less botrytis), is changing the bunch architecture (less botrytis) and is inducing some resistance within the berry itself, (again less botrytis.)

“Now we have to test all three in the future to see which one is the most important again disease.”

The mechanical thinning research has received further funding from Sustainable Farming Fund and New Zealand Winegrowers, which will allow further exploration over the next two years. The trial is being conducted in both Hawkes Bay and Marlborough, on a range of varieties, including Sauvignon Blanc, Pinot Gris, Pinot Noir, Merlot, and Riesling.

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Winepress magazine is published by Wine Marlborough. The glossy magazine reaches over 1000 grape growers, wineries, contractors and associate members in Marlborough, New Zealand and abroad. To subscribe to the magazine simply  email your contact details to Kate Cameron, including postal address.  Annual subscriptions are just $77+GST.

NB: Plant & Food Research is not affiliated with Winepress magazine.

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