It is too late to do anything about “pear rust” once the month of May is over. It is only active in April-May time period. If you would like to control it next year, use myclobutanil (Immunox, Immunox Plus or Fertilome F-Stop Lawn and Garden Spray). There are other fungicides that will work but those with myclobutanil have an advantage. Most fungicides must be present on the foliage before the disease spore germinates or they are ineffective. Myclobutanil will kill the rust spore up to 4 days after it germinates. Big deal. What does that mean in the real world.
Normally, we would recommend that trees be sprayed every 7 to 10 days starting at the beginning of April until the end of May. However, since we have this 4-day kickback with myclobutanil, we can wait until we actually see evidence of spores being released before we spray. How do we do that? First of all remember that cedar-apple rust and cedar-hawthorn rust must go back and forth between junipers (cedars) and apples (or pears in this case). The spores from junipers can only infect apples or pears and those from apples or pears can only infect junipers. Therefore we look at the juniper to see when to spray either apples or pears.
When you see the orange globs (galls) on the junipers, you know you have 4 days to spray the apples and/or pears. These orange globs are actually cedar-apple rust but cedar hawthorn rust develops under the same conditions. We key off of cedar-apple rust because it is much more noticeable on the juniper. If you see cedar-apple rust, cedar-hawthorn rust is likely present. It is also important to note that the orange galls only develop during rainy weather.
So, this is what you do. During any rainy period during April and May, watch the cedars. When they bloom (orange galls appear) get ready to spray. You have 4 days to apply your myclobutanil fungicide. Once May is over, you are done. (Ward Upham)