When you say it's diseased do you know what the disease is ? If you had an animal like that you wouldn't keep it in case whatever it was got passed on to the rest of your flock/herd. So if the disease can be successfully treated - fine, otherwise it's firewood. Spits a lot though even when dry. Then it's time for a new tree, best planted in a different place so it can't catch what the old one had. A bit of research may lead you to a locally successful variety, we have several Scottish ones able to stand our weather (yes even in coastal Caithness !). And if your current one only fruits every two years, you won't mind waiting and not letting the new one fruit in its first year so it can really get established.