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A recipe for grape botrytis

discolored spots on leaf
Scott Mathew said an 80% capfall spray with Switch Fungicide could be considered as an insurance policy against the future unknown with botrytis.

Botrytis is often considered the most difficult fungal disease to control in vineyards. That’s partly because many fungicides registered for its control cannot be applied after flowering if the grapes are destined for export wine production.

Botrytis thrives in high humidity and still air. Good canopy management and viticultural practices such as leaf plucking are important to minimise these conditions within the fruit zone.

Whilst it can be a devastating disease, it is sometimes considered a weak pathogen, in as much as it needs some help to establish.

It often gains entry into young fruit through wilting blossom parts that remain attached to them, old blossom or trash that gets trapped within clusters after it falls from the old flowers, and scars left on the young berries by the fallen caps. It can also enter through wounds caused by mechanical injury, birds, insects, other fungal pathogens or berries split by rain.

A wet flowering period can lead to latent infections becoming established and how the disease progresses from there is really driven by the weather. If conditions remain dry leading up to harvest, infections most often remain latent and cause little damage.

If a wet flowering period is followed by a wet lead up to harvest, the conditions activate latent infections, assist with their spread and it becomes a perfect recipe for botrytis.

The challenge for growers in assessing the risk is that they often do not see significant disease levels until the season is quite advanced. That’s because this fungus does not grow well in berries until they start to ripen.

Serious botrytis losses are the result of rampant disease spread during the post veraison/pre-harvest period, after the berries begin to ripen and when they become highly susceptible to rot by the fungus.

“Latent infections established at bloom can be important, even if only a few of them become active and provide the initial ‘foot hold’ from which subsequent spread can occur during ripening,” said Scott Mathew, Syngenta’s Solutions Development Manager.

Scott said, “Controlling this early infection with a consistently effective product such as Switch Fungicide at 80% capfall, is critical in a year when a wet pre-harvest period favours both the increased activation of latent infections and their rapid spread.

“So in one sense, the 80% capfall spray could be considered as an insurance policy against the future unknown.

“If the flowering period has been dry with little or no latent infection periods then Switch can be used right up to growth stage E-L 29, when the berries are pepper-corn size (4 mm diameter).

“Switch has the benefits that it penetrates and protects the berries, has built in resistance management with two different modes of action and acts at four different stages of the disease development life cycle.”

Scott said viticulturists should note that if they have already used SWITCH at 80% capfall, they should not use Switch again, based on resistance management guidelines.

“When assessing the risk, growers should take into consideration the susceptibility of their cultivars, site conditions and pressure from other pests such as light brown apple moth.

“Past experience is a great asset and previously infected sites and sheltered vineyard areas such as hollows will be at greatest risk of developing the disease,” Scott said.