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Botrytis attacks the unprepared

Man holding grapes
Landmark Great Southern WA agronomist, Steve Poole, said the use of Switch as a central part of the botrytis spray program has delivered very good prevention against botrytis attack in a difficult season.

Southern WA regional roundup, 2015/16
By Steve Poole, B.Sc (Hort)
Horticulture and Viticulture Agronomist at Landmark, Great Southern WA.

The 2015/2016 season has, for many Western Australian wine-grape producers, been one where valuable lessons were learnt, as it has certainly been a season with its own special challenges.

For much of WA’s Great Southern, vintage is well underway. Most of the white varieties have been picked and the reds will soon follow.

For the Southwest of WA, it’s been a wet summer. Three significant rain events that, on reflection, couldn’t have been timed any worse. Along with the minor rain events in between, it created the ideal conditions for botrytis (B. cinerea) to wreak havoc. The first critical rain event fell during (or close to) flowering for many growers and the last was just prior to harvest for the early varieties such as Pinot Noir and Chardonnay. On the other hand, there were a few fortunate growers that had fruit ready to harvest before the rain, making some winemakers very excited.

Given the conditions we’ve just experienced, botrytis bunch rot control was essential. Often known as grey mould, it can devastate unprotected grape crops with yield losses, reduced quality and off flavours in the wine. Bad infestations can even cause a complete crop write-off.

Botrytis needs warm, moist, humid conditions to proliferate. In a wet season, with vigorous canopy growth, good fruit set and big tight bunches it’s always going to be a hard disease to stop. 

Botrytis will often infect grape berries during flowering. At cap fall, the scaring can be a point where the botrytis enters the berries and often made worse during wet weather. This infection (called latent infection) can lay dormant during berry development, waiting for sugars to increase at ripening. Further wet weather near harvest, compounded with the risk of berry splitting, can allow the perfect storm for botrytis growth and sporulation and the resultant bunch rot.

Good canopy management, balanced nutrition and a preventative fungicide spray program are essential to control botrytis, particularly in tough seasons like this. During 2015/16, SWITCH® proved its value as a very effective fungicide. It contains two active ingredients from two different chemical groups, providing very good preventative control of botrytis. Switch is best applied either at 80% cap-fall to protect the berries over the critical flowering period, or at peppercorn berry stage (growth stage EL 29) to control latent infection and protect bunches through the main period of bunch fill.