Thank you for contacting Ask Extension.  This is any of a number of leaf spot diseases, which are really just cosmetic and I would not recommend treatment this late in the season, other than good sanitation.  Remove and dispose of leaves after they drop in the fall and consider covering the soil below the tree with a layer of mulch to bury any fungal spores.  There's nothing definitive in the photo, but this looks like it may be pear rust, which is a fungal disease requiring an alternate host (like juniper). Since you are in Washtenaw, we do have a local diagnostics lab at the Washtenaw County Extension Office that evaluates samples on Tuesday mornings for a $7 fee.  You could drop off a sample of several leaves showing different progression of disease, some heavily infected, some showing lesser symptoms. If you use this service, please call ahead to be sure someone is in the office to receive your sample. Once we have a better idea of the specific disease, we could discuss treatment options.  But again, the diseases are generally cosmetic, and any treatment would be done next spring prior to prevent reoccurrence if desired.

More information: 

European pear rust

Pear rust

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