To spray or not to spray?

The best line of defense is to grow ‘disease resistant’ varieties. As no vine is 100% disease resistant it is a matter of degree. Seyval Blanc is pretty resistant to powdery mildew in my experience, but I have experienced botrytis and dead arm on it.

The next line is good housekeeping – remove all debris to discourage breeding grounds.

If you are lucky, these two will suffice; but you may wish to do more and spray and with some varieties such as Pinot Noir, it is essential.

An important factor is that the number of corrective sprays available to the amateur is now very limited. The main purpose of spraying therefore is to prevent the onset of disease rather than to cure it.

Powdery mildew is probably the most common disease. Fortunately, it can be discouraged by the application of sulphur which is still accessible by the amateur. Washing soda (sodium carbonate) - a component of Burgundy Mixture – is also said to keep powdery mildew at bay. I use 1% by weight for either product. For the amateur, sulphur puffers can be bought at most garden centres and the sulphur can be extracted and made wettable (and hence sprayable) by adding one or two drops of washing up liquid to a teaspoonful of sulphur and then stirring into a thick paste with the minimum of water; finally dilute to the correct strength. Washing soda is very soluble. I use them alternately about every ten days.

Powdery Mildew

The result of powdery mildew.

This bunch was infected by powdery mildew.

Spraying eventually checked the spread but bunches like this are useless.

Bordeaux Mixture is also used against powdery mildew and can be purchased at garden centres. Spraying with this is said to produce a check in growth (not good in the UK where we need all the ripening time we can get).  Rowe recommends using it immediately after harvest; at this time, Rowe claims, it is also effective against dead arm and a single application reduces the risk of accumulating copper in the soil.

‘Fungus Fighter’ is available at most garden centres and contains the active
ingredient  myclobutanil which is approved for use on vines. This can be applied as a curative - allow 14 days minimum to harvest if you use this (note that powdery mildew is prevalent early in the season and there should be no need to apply in the 30 days before harvest unless the disease has been allowed to become rampant). You can use it instead of washing soda, or use all three, each at 10 day intervals.

The important thing to realize is that it is too late to apply ‘preventative’ sprays after this disease has taken hold so it must be done regularly if at all and if present in the previous year is a good idea to spray Fungus Fighter very early in the season before the flowers show.
Botrytis is a major problem for amateur growers as there is nothing available in the garden centres to treat it. Good hygiene and ventilation are even more important to keep this at bay as it is the only ‘defense’. There are, however, reports that potassium metabisulphite (as used in wine making) has some benefit. I tried this once and did not find it much help but I may not have used a strong enough solution.

Botrytis becomes evident on the grapes in autumn and its spread is encouraged by warm, moist conditions - just what we expect in the season of ‘mists and mellow fruitfulness’.

Risk to wine yeast
If curative fungicides can be obtained and these are applied, they may well be fatal to the wine yeast used for fermentation, so spraying with fungicides should be restricted to the early part of the season – ideally not within a month of harvest.

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