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Powdery Mildew
Powdery mildew is caused by the fungus Uncinula necator and its effects are visible in the leaves, new shoots and mainly in the bunches. The cloudy weather and high relative humidity are conditions favourable to the development of powdery mildew.
In winter, powdery mildew takes refuge inside the buds, in the form of mycelium, or in the leaves and rods found in the soil, in the form of cleistothecia. In spring, the mycelium in the buds germinates and releases conidia. Cleistothecia burst and release asci and ascospores. The conidia and ascospores reach the grapevine’s receptive organs through the action of the wind and germinate. The mycelium develops on the plant, forming conidiophores that release conidia. When these conidia fall on the plant they germinate, forming mycelial filament and new conidiophores, which release conidia and will produce new infections. These cycles are repeated throughout the entire vegetative period. All the grapevine’s green organs can be infected.
Leaves:
- Slight wrinkling at the edges
- Small yellow spots on the upper page and, on the corresponding area of the lower page, small scratches that correspond to dead cells
- Formation of conidiophores
Shoots right after budding:
- The ends of the branch and leaves become greyish and stiff, just like a hoisted flag
Inflorescences:
- Flower buds covered with a white dust, which can lead to abortion and flower drop
Bunch:
- The small berries become wizened and dry
- The skin hardens
- The affected cells of the bigger berries die. The epidermis can’t grow and the skin bursts
- The berry’s content is exposed
Shoots:
- Dark green diffuse spots that later become brownish
Cultural control:
One should make a careful selection of grape varieties (if possible, choose those more resistant to powdery mildew), rootstocks and vine training system. Moreover, pruning should allow aeration and exposure to light; if necessary, some interventions should be performed, such as defoliation and desuckering. In winter pruning, the rods with visible cleistothecia should be removed.
Chemical control:
- Contact fungicides: Inhibit the formation of conidia, as well as of haustoriums before contamination.
- Sulphur (in the wettable or powder formulations) and dinocap
- Systemics of the EBI type (ergosterol biosynthesis inhibitor) (spiroxamine, fenarimol, fenbuconazole, flusilazole, hexaconazole, myclobutanil, penconazole, propiconazole, tebuconazole and tetraconazole)
- Systemic strobirulins: azoxystrobin, kresoxim-methyl and trifloxystrobin
- Other: quinoxyfen
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