This section is from the book "The Gardener V2", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
In his Treatise on the Grape Vine, Mr W. Thomson gives it as his opinion that rust on Grapes is caused by sulphur coming in contact with the fruit at a very early stage of its development. Rust has been ascribed to many causes, and the above statement was at first received with some little hesitation. We beg to state a few facts which show that rust is caused by sulphur. In an old house where the Vines had of late years been neglected - a prey to mildew and other ills - an effort was last winter made to get these Vines into a more satisfactory state. Vines, house, and everything, were thoroughly cleansed, and by-and-by the Vines were gently moved on by a little fire-heat. The house is heated by an old-fashioned brick flue, which gets rather hot at the end adjoining the furnace. The first Vine just over this hot flue broke first; its Grapes were in flower while the rest were only just showing. At this stage, the weather being dull and cold, mildew was suspected, and sulphur was used both on the foliage and on the flue. The Grapes on the early Vine are now brown with rust, while not a speck shows where the bunches were not developed at the time of its application; but those in bloom and coming in bloom have suffered much, and prove the opinion above quoted to be correct.
May 14, 1873.
[Those who doubt that rust is caused by sulphur-fumes should coat their pipes with sulphur, and heat them well when the Grapes are young, and they will doubt it no longer. - Ed].