A term referring to two dangerous plant diseases caused by fungi. Until the second half of the 19th century, they were unknown in Europe and were only introduced via infected vine stock from North America. Shortly afterwards, the phylloxera and, as a further plant disease, black rot were also introduced from North America, which is why these pests were referred to as the ‘four great plagues’.

The four great plagues: 1 = Powdery mildew (Oidium), 2 = Downy mildew (Peronospora), 3 = phylloxera, 4 = Black rot

The two species of mildews feed on the living cells of the infested host and are therefore classified as biotrophic parasites. There are plant-specific species affecting apples, peas, cucumbers, roses, spinach and grapevines. These strictly host-specific fungi can live exclusively on their host or hosts. Although the different disease symptoms are distinct, the two fungi are often confused due to their similar names.

Control

The two fungi thrive in different weather conditions. Accordingly, powdery mildew is referred to as a ‘fair-weather fungus’ and downy mildew as a ‘bad-weather fungus ’. Botrytis (grey mould) also tends to occur in cool, damp weather and is therefore also classified as a ‘bad-weather fungus’. Nevertheless, there are some commonalities regarding prevention. Plants should not be planted too densely so that the leaves can dry out quickly (dew could also promote the growth of powdery mildew). A sunny spot is preferable, as sunlight boosts the plants’ vitality whilst also inhibiting fungal growth.

Conventional control involves the use of sulphur (powdery mildew) and copper sulphate or Bordeaux mixture (downy mildew). In addition, specialised fungicides or plant fortifiers are used. This is often necessary several times during the growing season. For environmental reasons, there is now an increasing trend towards using newly bred, fungus-resistant PIWI varieties with high tolerance or resistance. Some species of ladybird feed exclusively on mildew. However, this is of no significance in terms of control.

Early warning systems

Rose bushes are often planted at the edges of vineyards or at the end of each row of vines to act as an early warning system. They act as ‘guardians’ or indicator plants because they are affected by both types of mildew earlier than the vines, thereby alerting the winegrower to the infestation in good time so that preventative measures can be taken. They also provide shelter for beneficial insects. This practice is common in the Graves wine-growing region of France.

Powdery mildew (Oidium)

The pathogen responsible for the disease belongs to the Ascomycota phylum; its botanical name is ‘Erysiphe necator var. necator’ or ‘Uncinula necator var. necator’. The fungus is also known as ‘Oidium’ or ‘Oidium tuckeri’, named after the gardener William Tucker, who first discovered it in England in 1845. However, it had already been identified and described in North America as early as 1834. It was probably introduced to Europe...