Pear tree disease

Hi, I am new to the site and wondered if anyone can help me with some advice on treatment of my pear trees. They have what seems to be 2 host diseases, ( I googled the pics) orange/redish spots with a black centre and a canker/fungi type of thing on the underside of the leaves. I am fairly novice in the garden but these trees were in the garden of the house we bought 3 yrs ago. They were so healthy initially but now are clearly not doing well and have hardly any fruit. My google search says Venturia Pirin and Gymnosporangium sabinae ????? Any ideas on any treatment and if they can be treated effectively at all. Im worried as to wether it may spread to my apple and plum trees? The trees are huge and must have been here for quite some years. Any help would be greatly appreciated, thankyou.