When the crop is in, and winter is knocking on the door, rake up the fallen leaves and twigs from under your fruit trees. Then BURN them or take them to the DUMP. Do not compost them. More fungal infections overwinter on fallen fruit leaves than anywhere else.
When you have done that - clear a circle, down to bare earth around each tree with a radius of 30 cms (45 is better) and keep it clear if you can. It deters crawly pests, stops weed competition and makes mowing without bashing the trunks of your fruit trees much easier.
Two easy tips here.
Always prune at the right time of year. The general rule is to prune in Winter on a mild day when your fruit trees are dormant. The exception is for all "stone fruit" such as cherries, plums, apricots etc. These are prone to Silver Leaf disease and should be pruned in summer when the sap is rising.
Second, always use clean tools (wiped with a disinfectant such as Dettol between each cut) that are as sharp as possible to make the cleanest pruning cut. And then seal the wound with a pruning compound such as Medo or Prune 'n Seal.
It is so tempting to let your fruit trees crop their socks off. Don't succumb to temptation.
In the year after planting, by all means let your trees flower, but remove ALL the fruitlets that form after flowering as soon as you see them. Do not let your trees fruit within 12 months of planting as their energies need to be diverted into establishing a large root system to support future harvests. Not doing this can stress your tree and in extreme circumstances can kill it.
In subsequent years, in June you will see a lot of baby fruit fall off the tree. This is called the June Drop and is nature's way of making sure the tree does not stress itself by over-fruiting. When the June drop is finished, do a bit more thinning - we have specific instructions for different types of fruit tree (and different varieties of fruit where necessary) elsewhere on this site, but in general reduce what the June drop has left by a further 25-30%, leaving fruit evenly spaced along the branch with room to grow.
The single most common "ailment" in fruit trees is underwatering. It takes a lot of water to make an an apple or a plum.
All fruit trees need a steady, consistent even supply of moisture. Erratic watering (as in dry spells followed by wet spells) leads to excessive fruit drop, split fruit, misshapen fruit and so on. So, incorporate plenty of well rotted organic matter in the ground when planting. And then mulch well every year with more of the same. All organic matter helps improve the structure of the soil making moisture more available to tree roots and acting as a reservoir in dry periods.