Downy Mildew

Yes, it's that time of the season again. Unpredictable weather, flowering time and stressed farmers. Last year was an absolute nightmare for the South African wine industry with regards to this disease. This year we are prepared, but it is still a very dangerous and active threat.

Downy Mildew, Plasmopara viticola, overwinters in leaf debris as oospores and are immune to any fungicides. As soon as the temperature rises in spring time, (above 10 degrees Celsius ) accompanied by a minimum of 10mm rain, these spores germinate. They are now called zoospores. These spores penetrate the cells on the leaves at an early stage (6-8 square cm). As soon as this infection has taken place, the first symptoms are visible.

Primary signs are the formation of yellow "oily" stains on the front surface of the leaf which are visible 6-13days after infection.

Once the zoospores are dispersed they swim to the stomata, enter by forming germ tubes and invade inner plant tissues.

Secondary infection is associated with the production of more spores, visible at the back of the leaf. The germination or these spores are dependent on the availability of moisture and humid conditions.

Weather predictions play an integral role in the control strategy of Downy Mildew. Spraying before rain gives substantially better control than spraying after the rain.

Warning systems are in place to warn the respective wine growing areas when favourable conditions will occur.



The damage to the grapes obviously doesn't lay in the effect it has on the leaves, but rather in the bunches. If Downy Mildew progresses to the bunches, it has devastating effects. The berries dries out completely and the whole bunch can die. This is why preventative control is very important.

To the left are close ups of the zoospores.








































Here are some photos of the damage Downy Mildew inflicted on a Cabernet Sauvignon block in the 2010 harvest.



Till next time