This is not a straightforward answer but the truth is fireblight is active anytime the tree grows. You will hear people say the rule is 65 degrees and 65% humidity the fireblight bacteria is active which is more the guidelines but not exactly the rules. Fireblight is never active when the tree tissue is not growing and that’s the only time your 100% safe. You can prune in dormancy without fear of fireblight but use good cleaning techniques on your saw blade. If the cells are rapidly multiplying e.g… blooming or new leaf shoots are forming the fireblight bacteria multiplys rapidly. Thats why the rainy season during a warm spring is so dangerous because trees grow like crazy which is perfect breeding ground for fireblight bacteria. To make matters worse some people fertilize which causes rapid growth like throwing gasoline on a fire that really provokes fireblight to multiply. If you have a spreading agent e.g. cicada, grasshoppers, birds etc. A orchardist like us grafting from tree to tree with fireblight juice all over their hands cutting non dormant trees then we have big trouble. I paid dearly over the 17 year cicada 17 year cicada's woke up hungry . I’m no longer naive about insects and fireblight and though its been years ago i remember it well Late season Fireblight
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