How to bag test for downy mildew of grapes Note: 349

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
Figures 2a and 2b show how to prepare the resealable plastic bag
for the bag test.
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whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.
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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
Copyright © Western Australian Agriculture Authority, 2009
Refer to Bulletin 4708 ‘Downy mildew in vineyards’ for
further information.
ISSN 0726-934X