Note: 349 March 2009 How to bag test for downy mildew of grapes Andrew Taylor, Plant Pathologist, Bunbury Figure 1 Oil spots caused by downy mildew of grapes Downy mildew of grapevines, Plasmopara viticola, can cause serious crop loss if weather conditions are favourable for its development. The presence of oil spots in the vines indicates that primary or secondary infection events have occurred. However, weather conditions or fungicide application may have rendered the oil spots inactive. It is also possible that the symptoms are not those of downy mildew infection. The easiest way to determine whether oil spots are active and downy mildew a threat to the vineyard is to use a simple bag test. In a bag test, the suspect tissue is placed in a moist plastic bag and left overnight in a warm dark position to see whether ‘white down’ develops. to occur the temperature must be above 13 °C and relative humidity must be at least 98 per cent. Bag test procedure Pour clean water into a sealable plastic bag, shake it and then empty the bag so that only a small amount of water droplets remain (Figure 2A and 2B). ‘White down’ is the dense, raised, white cottony growth that develops on the underside of the yellow oil spots after suitable warm humid nights. The bag test also creates the conditions that are necessary for secondary infection and enables the mildew to sporulate into the ‘white down’. For sporulation 2a 2b Figures 2a and 2b show how to prepare the resealable plastic bag for the bag test. Important Disclaimer The Chief Executive Officer of the Department of Agriculture and Food and the State of Western Australia accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it. For more information visit our website www.agric.wa.gov.au 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. Do not place multiple leaves or bunches in the bag as this can reduce the effectiveness of the test. Generally, better sporulation occurs when the leaf is facing downwards. Figure 5 Sporulation seen on the underside of the oil spot in the morning after the bag test. Figure 3 Sample in the moistened bag filled with air Caution: Leaves or bunches with what appears to be ‘white down’ on the underside of the leaf may not develop more ‘white down’ even when the bag test is conducted. In this case repeat the test using fresh leaves with oil spots. Place the bag 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. Leave the bag overnight. Figure 6 Sporulation of old oil spots. Note the dead area where downy mildew killed the leaf tissue. 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 then the oil spot is either dead or is not that of downy mildew. Regular monitoring of vines and bag testing of leaves and bunches should continue throughout the season regardless of the bag test result. Old oil spots or those that appear to have been killed with 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. Figure 4 Placing the bag in the cupboard overnight encourages sporulation. 092396-03/09-ID10030 Copyright © Western Australian Agriculture Authority, 2009 Refer to Bulletin 4708 ‘Downy mildew in vineyards’ for further information. ISSN 0726-934X
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