Agrochemical Updates, eBulletin

Managing disease in wet conditions

12 November 2021

Most grapegrowing regions across Australia have experienced some rainfall in the past week. The Bureau of Meteorology has maintained the ENSO (El Nino-Southern Oscillation) Outlook at La Niña ALERT. The chance of a La Niña forming in the coming months is around 70%, or roughly three times the normal likelihood of an event forming in any year. For grapegrowers, La Niña events typically mean increased rainfall and cooler daytime temperatures. A cooler than normal summer is forecast for some parts of eastern Australia, but for other parts a hotter or average summer is forecast. Refer to the Bureau’s latest climate outlook video.

Control of fungal disease is much more challenging in wet seasons, which means it is even more important to get the most out of every spray. The 4Ts of effective spray application are a good reminder of how to do this.

TIMING: Time spray applications to coincide with the most vulnerable life cycle stages of the target disease or when the disease is most destructive. Grapevine flowers are very susceptible to Botrytis and should be protected when conditions favour the disease. Downy mildew and powdery mildew are a threat to any unprotected green tissue, which means that rapid leaf expansion can dilute the protective cover last applied. It may be necessary to shorten spray intervals at times of rapid growth to ensure grapevine tissue is adequately protected. Heavy rainfall or multiple wet days can restrict vineyard access. Consider spraying ahead of wet weather to make sure you can get protective sprays on as required. Spraying in windy conditions is not recommended as it can affect spray coverage and cause off-target impacts.

TARGET: Focus your efforts on making sure you are delivering sprays to the target. It is not easy to get good coverage of all plant tissue, but it is the best chance you have of managing diseases when pressure is high. In low pressure years, this is less important because the climatic conditions (temperature, UV, low humidity) are helping to manage diseases. Sunscreen products can assist in assessing spray coverage on all tissue including hard to reach targets such as inflorescences. Mix up a partial tank, spray a part row, allow a few minutes to dry, assess coverage, adjust sprayer as needed to improve coverage (see TECHNIQUE) and repeat.

TREATMENT: The type and amount of chemical (the treatment) must be appropriate for the pest target. Follow the chemical label instructions with regards to rates and be aware of CropLife resistance management strategies. If you have relied heavily on a few chemical groups in the past, choose a new chemical group. Make sure you are using the appropriate concentration factor if you are doing concentrate spraying – underdosing will result in failure to control and may increase the risk of fungicide resistance.

TECHNIQUE: Good technique involves sprayer adjustment, calibration and evaluation of spray performance using a method to test coverage such as wettable paper or the addition of a sunscreen product (as described in TARGET above). Consider adjustments to:

  • nozzle type, operating pressure, distance from the canopy and direction
  • air volume, speed, and direction
  • water volume applied to the canopy
  • ground speed of the sprayer.

Additional resources on spray application

For more information or assistance, contact the AWRI helpdesk on or 08 8 313 6600.