This section is from the book "The Gardener V1", 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.
A low temperature, and a stagnant over-moist atmosphere when the Grapes are in bloom. The roots being in a cold, heavy, and wet soil, causing all the young roots to rot in winter, and, under such circumstances, when Vines are forced into bloom early and before young rootlets are again produced. General debility, especially in the case of Muscats. These are some of the causes which produce the results you are experiencing. When your Grapes are in bloom air freely; and when the pollen is dry give the rods a tap with the hand to help to distribute the pollen. Some growers syringe them when in bloom, and find it a good way to get Muscats and other shy-setting sorts to set. Our own practice is to give a little extra heat and air, and keep the house moderately dry; and we always find the treatment successful.