Hello,

This could be pear trellis rust. This year we have had several homeowner questions on it. 

For positive ID, you may send samples to MSU Diagnostic lab. Instuctions, sample submission form and fee schedule is here---

https://pestid.msu.edu

You may also contact a certified arborist in your area to diagnose the tree and give you a specific tree care plan: ww.treesaregood.org

There are other rust diseases that can affect pear trees. You will want to get a specific diagnosis so as to time your preventative treatments correctly. Prevention of rust diseases is done by spray or injection treatments in early spring, and depends on the weather conditions from year to year. There are other things you can do to help manage rust diseases.

These articles have good pictures, ID and Management details---

http://www.ladybug.uconn.edu/FactSheets/pear-trellis-rust.php

The following articles have good information, and were written for tree care professionals, so they mention specific products- however this is not an endorsement by MSU or by Ask-an-Expert of these products or companies---

https://treedoctor.msu.edu/sites/default/files/docs/Aug16_TrellisMgmtUpdate.pdf

And, this update--

https://treedoctor.msu.edu/sites/default/files/docs/Sept15_TrellisRustUpdate.pdf

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