Since I was the originator of the previous long discussion,I thought that I would start again. Thanks to you all for the comments and advice,which is mostly contradictory but very interesting.Just a few more comments that struck me in the discussion.
1)Wood chip mulch(unrotted) would certainly rob nitrogen from the top soil layers.This is a well known fact to gardeners,who cannot get anything growing on wood chip added soil,but can as soon as nitrochalk is added;the effect is sometimes almost immediate. 2)My soil is dry limestone on a S-facing slope,and can suffer from very long periods with little rain.The subsoil is stony clay which holds moisture.I also have a hard cider orchard,and it takes about 10 years or more for the apple tree roots to start moving into the lower soil.My grapes are grown in an area with typically 25" rain annually,and also in a microclimate where the rain bypasses us down the valley.Surprisingly I am able to grow good potatoes,onions,tomatoes on this soil,but the top few inches often becomes very dry.Hence I thought that the mulch (garden compost) would keep the upper soil a little moister. 3)The quote from Cox,which I have,that mulching encourages some late growth,and may hinder ripening is interesting. 4)My vines this year are very healthy,and possibly with a darker green leaf this year.This maybe an indication of too much nitrogen . I did add some small amount of general fertiliser to the mulch,which I also do not usually do. 5) The bottom line is that my vines and grapes look extremely healthy this year,with a great crop on.They are ripening slowly and withou any sign of disease moving in yet. Another thing I did this year is to put several wasp jars(containing beer) around the vines-I must have killed a couple of thousand wasps over a few weeks.I am sure that in previous years wasps begin to attack the grapes in early August-they nibble a grape or two which rot,and this spreads to the rest of the bunch.A similar thing happens regularly with plums. 6)I have good site for grapes,but every year is not good as our temperatures can be rather low sometimes.But this year is excellent,particularly in June when I had great flower and fruitset.Since then the vines have been flourishing.So,I am not sure what to do next year.I have done so many different things,I am not sure which has been the most important and in particular whether to mulch or not.I will possibly give a final sulphur spray after harvest as Paul suggests.Since most of the vine is cut away in winter pruning-in my case in November-isnt better to spray then when there is not much wood to spray,but then it may not be as effective.
Thanks again,and I would certainly welcome more discussion.I am learning a lot,despite becoming a little confused.
Best regards Michael