The best line of defence 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.

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 – but not on the Seyval Blanc.

If you are unfortunate and get powdery mildew, you must act quickly and spray with ‘Fungus Fighter’ (active ingredient myclobutanil); this product has ‘off label’ approval for use on vines. If you do not treat powdery mildew it will wipe out your crop completely.
Bordeaux Mixture (which contains copper sulphate) is also used against powdery mildew and can be purchased at garden centres. Spraying with this is said to produce a check in growth which is not good in the UK where we need all the ripening time we can get! If used to treat powdery mildew it often leaves a visible blue copper sulphate stain on the grapes that can be difficult to wash off - it can persist after torrential rain! Rowe (see book list) recommends using it immediately after harvest; at this time, Rowe claims, it helps to prevent dead arm. The use of just a single application each year also reduces the risk of accumulating copper in the soil.

There is nothing available to the amateur for botrytis except good ventilation. There are reports that potassium metabisulphite (as used in wine making) has some benefit. I tried it last year and did not find it made a significant difference.

The important thing to realize is that it is too late to apply ‘preventative’ sprays after a disease has taken hold so it must be done regularly if at all.

Risk to wine yeast

If curative fungicides can be obtained and these are applied, they may be fatal to the wine yeast used for fermentation; in general it is a wise precaution to avoid fungicide sprays for 30 days before the expected date of harvest.

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