Rowan Tree - Does my tree have a disease?

Hi,
I have a beautiful Rowan Tree, which was planted about 4 years ago. She is very strong and growing well (now standing at about 15-20 ft). This year for the first time we have noticed on her leaves strange 'light coloured bumps' (see my picture). Is this some fungal infection or is it something which we shouldn't worry about? If it is a disease is there any treatment someone can suggest? Any advice would be appreciated.

 

Pear leaf blister mite. No cure, remove and burn affected leaves.