why do pear tree leaves turn black

I'm thinking back on my history with Comice--there was one lone tree in a one acre block of Bartletts. It always looked unhealthy and vey rarely gave much fruit (but, as Hugh Williams said If you only got one good pear a year it would be worth farming it, that's how good it tastes Comice Burn but I could not help but pipe in.

There is a cadre of Comice growers up in Oregon whose fruit when it shows up in California is beautiful (but of course does not taste as good as my normally russety, blotchy, lumpy Comice).

I have not made contact with these folks. They were not organic so maybe (likely) used chemical fungicides. The plot thickens and the mystery deepens.
The bacteria Erwinia amylovora can infect a pear tree with potentially fatal effects.

This bacteria causes the condition known as fire blight, which makes the leaves of the tree appear to have been burnt or scorched. Telltale symptoms of fire blight include blackened, deformed leaves that remain attached to the tree, fruit deformation and oozing from the bark.

Insecticides and fungicides are ineffective against Erwinia amylovora -- if your pear tree is infected with this bacteria, all you can do is prune away all infected areas and hope for the best.

You should prune 12 inches below the lowest visible infection on the tree. Clean your pruning tools thoroughly before using them on another pear tree, as the bacteria can spread to the healthy tree on the pruning shears.