I am having an argument with a gardening friend concerning this problem. One thing we are agreed on is that it is not a virus. It occurs mainly early in the season and only the buds are affected. They just blacken and don't open. She maintains it is a bacterium and there is nothing you can do about it. Plants generally improve later. I think it is a pest as it does not get worse in hot humid weather, in fact tends to improve. It is possible of course we could both be right and the pest could be damaging the buds and introducing the bacteria. I've just never found the evil little critturs. They must be living inside the flowering stem... I don't care whose side you're on. I'd just like to meet a fellow sufferer....