I’ve been reading about dormant sprays for fruit trees, and it seems the two main options are a mix of dormant oil and lime sulphur, or Bordeaux mixture. I have apple, nectarine, and quince trees that are young and I didn’t have any disease problems with them last year. Which is best as a preventative spray? Are there pros and cons? I’m in London.
This is an interesting question. The Toronto Master Gardeners have recommended dormant oil with lime sulfur, as does the City of Toronto. The Ontario Ministry of Agriculture, Food and Rural Affairs recommends Bordeaux mixture as being particularly useful for apples. Some Canadian experts say that Bordeaux mixture is a better control than lime sulfur for some of the pests and diseases that afflict apples and peaches. Others say that lime sulfur is an important component of healthy soil, and so is the better choice as copper can damage beneficial soil microorganisms and beneficial insects, and is generally more toxic to plants than sulfur. It is also poisonous to animals and toxic to fish. Canada’s Mark Cullen weighs in here – he uses both: https://markcullen.com/preparing-for-a-new-year-of-fruit-trees/ .
The effectiveness of dormant spraying itself is an important consideration, maybe more important than the formulation you use: to be effective it has to be done correctly. Here is a useful post by a nurseryman based in British Columbia that provides some clear advice on how to apply dormant spray for the best results: https://www.gardeningbc.com/page/brianminter56.html.
A previous Toronto Master Gardeners post discusses spraying fruit trees more generally: https://www.torontomastergardeners.ca/askagardener/spraying/. Finally, the Ontario Ministry of Agriculture, Food and Rural Affairs website is full of good information on fruit trees, their pests, diseases and integrated pest management, some of it aimed at larger scale growers, but useful to homeowners as well: https://www.omafra.gov.on.ca/english/crops/hort/news/orchnews/2015/on-0415a2.htm
I hope this is helpful in making your decision. It may also be useful to check the City of London guidelines for organic gardening.