Plum/apple care

Depends which fungus you're trying to prevent. Not all sprays work against all diseases and not all sprays are used in the same way. 'Leaf curl' is usually a problem of peaches, but on other trees leaves often curl due to aphid attack.
I posted a topic a while ago that if you care about bees (who are the insects doing the pollinating so you get fruit) you ought to think twice before killing aphids:
http://www.growfruitandveg.co.uk/gra...ids_79652.html

Poor crops could be due to problems unrelated to fungal diseases.

Identify the exact cause of your problems before you nuke everything in sight. Some pictures would be helpful.

Personally, I grow varieties that are strong enough to tolerate aphid/insect damage and disease-resistant enough that they don't need spraying. Common varieties often need spraying. Certain varieties of fruit are not suitable for growing in certain areas due to soil, climate or rainfall.