Downy Mildew: Worst in 20 years
December 11, 2010
Right now, chemical suppliers can hardly keep up with demand for fungicides to control what is said to be the largest downy mildew outbreak Australia has seen for 2 decades. According to ABC Radio, Riverland branch of chemical producer, Landmark, says it has sold up to 10 times the amount of downy mildew chemicals it usually would at this time of year. Good news for them, but not good news for wine drinkers.
Downy mildew is a fungus that can spread rapidly in vineyards in the wet weather we have had recently. Primary infection is said to occur through rainsplash from the ground, though this is hard to see happening when the ground is densely covered with grass and weeds, as is the case here. The oily spots that develop on the odd vine then spread with subsequent rain and can kill the grape flowers. I took this photo in a neighbouring vineyard a few days ago, and you can see the slimy brown remains of the bunch flowers. In the worst case, ALL the flowers die and you get no fruit. The good news is that once the berries have developed from the flowers they are much more resistant, and it doesnt affect quality at harvest, like other diseases (powdery mildew and botrytis) This photo was taken after the 100mm we received last Thursday 9 December 2010.
We cannot claim to have no mildew at Rosnay now, and would say that any grower claiming not to have some probably hasnt looked hard enough. When we first saw oil spots in our Shiraz we incubated them in a plastic bag overnight with some wet tissue paper in it, and the classic “down” of fluffy white fungus grew overnight. Since then we have been spraying copper and sulfur (both organically approved) every 2 weeks or so, or about 3 times in total. We avoid spraying with the tractor (as in this photo) unless the ground is firm and dry, to avoid pugging and compacting it. However, we have also been spraying with biodynamic silica and equisetum (horsetail) tea with the Suzuki ute which is relatively lightweight and fast (doing 4-5 rows at a time), meaning we can do it when the ground is a bit wet. Silica helps make the vines tissue more tough and turgid.
After looking at the neighbours place I raced to see if it was the same at Rosnay, and looked in the Shiraz, where we have historically had the most downy. To my amazement, I have actually found very little bunch infection, but of course plenty of leaf infection. The vines seemed to be standing up much better, and the canopy, being hand pruned, is much more open to air and light. As you can see from this photo, there is not much mildew, and the grapes are uninfected. Our view, as expained elwhere in this blog, is that organic farming makes the vines tougher and more resistant, whilst systemic chemicals (ie metalaxyl and phosphoric acid) create weakness in the vines tissues, especially for late season infection with botryis. Systemics also suffer from problems of resistance and residues in the finished wine.
The big test will happen when it warms up, and if we get a heat wave some bunches that appear to be fine may not actually be fine. If it dries out enough, the next organic weapon to try will be compost tea, which we have tried int he past without success, but which is said to work against downy. We are in uncharted waters, and will keep you posted. If you have any questions or suggestions feel free to comment!