Bag test for downy mildew of grapes

Introduction

Downy mildew of grapevines, Plasmopara viticola, can cause serious crop loss if weather conditions are favourable. The presence of oil spots in the vines indicates that primary or secondary infection events have occurred. However, climatic conditions or fungicide application may have rendered the oil spots inactive.

The easiest way to determine whether oil spots are active, and therefore 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 down develops.

Figure 1 Grape leaf showing oil spots caused by downy mildew

The bag test creates conditions that resemble secondary infection and enables the mildew to sporulate. For this to occur the temperature must be above 13°C and relative humidity must be at least 98%.

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).

Figure 2a Place suspect material in a sealed plastic bag

Figure 2b Remove excess water

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.

Figure 3 Grape leaf in moistened bag filled with air

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.

Figure 4 Placing the bag in a cupboard overnight encourages formation of spores

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.

Figure 5 Grape leaf showing sporulation on the underside of the oil spot in the morning

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.

Figure 6 Sporulation of old oil spots showing area where downy mildew has killed the leaf tissue