With all the rain we have been having lately, our thoughts turn to disease prevention.
There are a large number of fungal, bacterial and viral diseases which can attack grapevines.
We have to deal with three of the most common fungal diseases here.
Downy Mildew (Plasmopara viticola) is the most prevalent and is spread by rainfall.
It attacks leaves, shoots and berries and will quickly defoliate a vine leading to entire crop losses.
Primary infection takes place after 10:10:24 conditions occur ie. at least 10mm of rain at a temperature 10C or more, over 24 hours.
The fungus survives as spores for 3 to 5 years in old infected leaf material in the soil and, with rain, are splashed onto the foliage. If the spores remain wet long enough the disease begins to develope.
This shows up as "oil spots" on leaves (see top picture)
Spores form under the oil spot and show up as a "white down" (lower picture). If conditions are right, secondary infection occurs from these spores and the spread of the disease becomes quite rapid.
Downy mildew can be controlled by the spray application of various chemicals either pre infection or post infection. In a damp maritime climate like ours, we prefer the pre infection strategy. This involves spraying at least every two weeks from when the shoots are 10cm long or at shorter intervals if rain conditions prevail.
There are two groups of spray chemicals, those with single site activity which act on only one site within the fungus organism or those with multi site activity which act on more than one site within the fungus. Overuse of the former group can lead to small mutational changes in the fungus which in turn can lead to the fungus being resistant to that particular chemical.
We use the relatively safe and multi site copper oxychloride as our main preventative for downy.
In the unlikely event that infection does occur we use phosphorous acid, which is also multi site, as a curative.
Powdery mildew (Uncinula necator) is also common and attacks leaves, shoots and bunches. It is evidenced by an ash grey to white powdery growth on both the upper and lower surfaces of the leaves which eventually distort and shrivel.
The disease also attacks the bunches with the same ash grey/white powder showing up on the berries and stalks.
Crop losses can occur but more importantly, bunches with as little as 5% disease may be rejected by wineries as the disease causes off flavours in wine.
Powdery mildew spores hide in the buds of dormant vines. Mild cloudy weather and low light in the canopy encourage development of this disease. It does not need a lot of rain to spread.
There are no approved post infection sprays so a two weekly application of a protective spray from budburst is necessary. Again there is a multiple choice of chemicals from the two groups. We use the relatively safe and multi site active wettable sulphur.
Botrytis or grey mould (Botrytis cinerea) is a common bunch rot in regions with warm, wet conditions but it can also attack shoots and leaves.
Large crop losses can occur and infected grapes will cause off flavours in wine.
It should be mentioned here that not all botrytis infection is bad. Under some conditions, the fungus takes hold and dehydrates the bunches increasing the sugar content without causing rot. This enables very sweet dessert wines with their traditional marmalade favour caused by fungus enzymes to be made eg. Sauterne in France or in Australia, Noble One et al. The disease is then known as noble rot. Conditions on the south coast hardly ever allow for this and it is too much of a risk to hold off spraying just in case they might.
The disease hides in decaying plant debris such as dead canes and mummified fruit. Spores are spread by wind and find a place in the developing bunch flowers. Once the bunch has "closed" and wet weather and high humidity occurs the disease spreads rapidly. There are virtually no curative sprays and it is essential that a protective spray be applied at very definite times of bunch development ie. 80% capfall (towards the end of flowering) and again just before bunch closure (just before the berries have stopped growing and become 'squished' together in the bunch). We use chlorothalonil for this purpose, again, multi site active but a bit "nasty".
This chemical is also a protectant against downy mildew so we can replace the copper with it at these two spray times.
All the chemicals we use are compatible so it is possible to mix them in the one spray application.
Lucklily we dont see other diseases like Phomopsis or Black Spot in the area.
But the three we have are quite enough.