Bag test procedure
Pour clean water into a sealable plastic bag, shake it and then empty the bag so that only a few droplets remain (Figures 2a and 2b).
Place the suspect leaf or bunch inside the bag and seal it. Sealing maintains high humidity, which is the most important requirement for sporulation. Blowing air into the bag also increases humidity and helps prevent the lesion from touching the side, allowing the maximum number of spores to be produced.
Do not place more than one leaf or bunch in the bag as this can reduce the effectiveness of the test.
Generally, better sporulation occurs when the leaf is facing downwards.
Use fresh leaves with oil spots because leaves or bunches with what appears to be down on the underside of the leaf may not develop more down when the bag test is conducted.
Leave the bag overnight in a cupboard or drawer in complete darkness. Optimum temperature for sporulation of downy mildew is 20°C but anything above 13°C is suitable.
If infection is present, fresh white down will appear on the underside of the oil spot in the morning. This means downy mildew is present and active in your vineyard and control measures should be implemented.
If no white down is present, the oil spot is either dead or is not downy mildew. Continue regular monitoring of vines and bag testing of leaves and bunches throughout the season, regardless of the bag test result. Oil spots in the field can withstand the high summer temperatures experienced in WA.
Old oil spots, or those that appear to have been killed via fungicide sprays, dry out in the centre but can remain active on the outer edges. The bag test is a good way to determine whether the oil spot is dead or whether further fungicide applications are required.
The sample placed inside the bag can continue to produce spores in these conditions for up to a week but the number of spores produced will be fewer and fewer as time passes.