Something is growing in five or six leaves of my young pear tree. On the upper surface they look like rusty red spots with black centers, about 1/2" long. The horror is on the back, where there's a hard swelling with tiny white tufts coming out of it. They're fuzzy and make my skin crawl. The tufts were white when I first picked the leaves, have gone brown since. Help me work out what this is? Thanks.