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.
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%.