I got the following questions about this pest on May 20th:
“Is it too late to treat the pear tree for leaf blister mite at this stage of the season(?) – with dormant oil spray to save any of the abundance of tiny pears emerging? Or is there any other thing I can do to prevent any further damage to the fruit?”
My response: The standard response to your question is that it is too late to do anything for Pear Leaf Blister Mite for this year, particularly in home gardens. The reason for this is the mites are now in the blisters or pockets in the leaves and available organic and most conventional pesticides will not reach them. Dormant sprays (dormant oil + lime sulfur or other product) provide excellent control when applied in late October (the best time to spray for blister mites) or February, this is the wrong time of year to try to control it. Don’t use dormant oil at dormant rates when the trees during the summer as it will damage your trees likely resulting in loss of leaves and your fruit.
Blisters take a couple of weeks to show up after the first leaves appear on pear trees. Some of the mites in the blisters may migrate to the blossoms and small fruit resulting in damage to the fruit. Since the migrating mites are not protected inside blisters they are easy to kill to protect the fruit. Wettable sulfur (or dust) and lime sulfur can be used at this time of year at label rates to control these mites. These two products are acceptable for commercial organic fruit production. They both have miticidal and fungicidal properties and are often used for control of spider mites, apple scab and pear scab during the spring and summer. One of the issues with using either of these forms of sulfur during the summer is they are very good miticides, they kill both the bad mites and the good mites (predators). Watch out for spider mite flare ups after you use them.
Washington state has a great web page about this pest – http://treefruit.wsu.edu/crop-protection/opm/blister-mites/ . Note, most of the information in this page is aimed at commercial orchardists