Spring growth and wet conditions

Wet weather conditions have been experienced in some grapegrowing regions in the last week. This presents a timely reminder to growers to assess their overall plant protection program. Significant or prolonged rainfall events increase the risk to grapevines from pathogens such as downy mildew and Botrytis. Growers should be vigilant in monitoring the disease status of their vines and, if symptoms of disease are present, assess why the disease has become established.

Some factors to consider:

  • In warm weather, shoots and leaves are growing rapidly and a two-week interval between sprays can mean a lot of tissue is unprotected. If rainfall occurs near the end of a spray cycle, the risk of infection is high.
  • It’s important to think about how ‘rainfast’ the chemicals being used are. Products that are systemic need time on the leaf for uptake. Once in the leaf, further rain is not a concern but as the leaf expands the chemical concentration decreases. Products that provide a protective layer on the surface of the leaf will be degraded by sun, rain and wind. Leaf expansion will also reduce coverage.
  • Is good spray coverage being achieved? The only way to know is to check, using wettable paper, fluorescent dye or kaolinite clay. If coverage is poor, many factors can play a role including spray cart travel speed, spray rate, direction of spray, nozzle condition and volume/speed of air output. When testing spray coverage, changes should be made one at a time, with coverage being checked after each change.
  • Rainfall duration, leaf wetness and temperature are key drivers for disease (especially downy mildew), so it is a good idea to record this information and combine it with knowledge of the disease life-cycle to help make disease management decisions. Vineyard management factors that can be adjusted include spray timing, choice of agrochemicals and shoot or leaf removal to change canopy architecture.
  • If disease is present, despite optimum timing and coverage of the vineyard spray program, it is possible that a fungal population is present that is resistant to the agrochemical(s) used. The resistance management strategies outlined in the ‘Dog book’ should be followed.

Useful resources/links:

The AWRI helpdesk provides technical support and advice to Australian grapegrowers and winemakers. Contact the helpdesk on 08 8313 6600 or email helpdesk@awri.com.au.