The clumps of black leaves on your pear tree are likely symptoms of fireblight infection. This bacterial disease attacks pears, apples, and closely related species. The disease is most likely to spread during wet conditions. Since the infections in your trees are minor, you can probably control it by pruning out the affected shoots. Prune several inches back into healthy wood, and dip the pruning shears in rubbing alcohol or a 10% bleach solution between each cut.