I don't know Harrow Sweet, but it sounds like the same problem I have.
It seems to affect just the twigs, but look for early leaf drop, these turn brown and crispy.
I sprayed both my trees with Lime sulphur, but ran out on the second tree which now suffers with scab. This year it will get two treatments, fall and spring.

Rob,

Can't answer your questions but my 2nd year Harrow Sweet has fruited for the first time this year too.

This summer was hot and humid, it had something that looked like fire blight. I cut of several inches of infected branches. Then, there was pear blister mites on many leaves. I sprayed with wettable sulfur. As of now, the fruit look fine, a little russeted, nothing else (fingers crossed). Maybe, the sulfur helped with scab.

Then, I just went out to check HS again today. The tip of the main branch has wilted!!! Don't know it's a beginning sign of another round of fire blight or the weather. It's cool (50's -60's) for a week and today was up to 90.

What get me nervous is the fact that my nearby Blake's Price shoot is fine, nothing wilted. Both trees planted at the same time and have about the same amount of growth. In fact, BP got more fire blight and pear blister mites this summer. (it's supposed to be more disease resistant!!).

Fire blight should be no mystery, leaves suddenly brown with a slightly bleeding canker at base of dead wood.

Harrow Sweet tends not to get scab or psyla here- one of my healthy type pears. I have much more trouble with Seckel, Bosc and a couple of others. The book says psyla can lead to fab. leaf spot which I really don't know the difference from scab. Either can create the symptoms you describe. You can see psyla with a strong mag glass at the point leaves come out of growing tips. Find a picture on the internet.