A. In your climate late summer and fall is a nice time to plant trees. Temperatures are in the upper 70’s during the day and dipping into the upper 40’s at night. So soil temperatures are probably in the low 60’s; warm enough for good root growth and establishment yet air temperatures are cool enough to slow the loss of water from the leaves.
Fireblight would infect trees in the spring and the symptoms usually appear in early summer. It is possible to transport this disease to a healthy tree through pruning a sick tree and then pruning a healthy tree without sanitizing the shears. But I doubt that is what is happening.
It is most likely how it was planted. Maybe the roots got too dry before it received water or it was planted with dry soil surrounding the roots. Always soak the soil around the roots with water from a hose a few times before turning it it over to an automatic irrigation system.
Some basic rules to follow:
1. Make sure the hole is at least three times the diameter of the container. If the soil is really bad, make it 5 times.
2. Make sure water can drain from the hole in half a day or less
3. Get the hole sopping wet before planting
4. Amend the soil put back in the hole with compost or other good soil amendment
5. Make sure this soil is wet immediately after you shovel it into the planting hole. Put water in the hole with a hose at the same time you are planting
5. Stake the tree after planting so roots don’t move during strong winds
6. Build a basin or moat around the tree to capture water from a hose
7. Water the plant twice, immediately after you plant it
8. Water it with a hose every other day for the first week to settle the soil around the roots.9. Water once a week after planting with enough water to thoroughly wet the soil around the roots to a depth of 18 to 24 inches.