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Professional plum growers all thin out the crops on the plum trees in their orchards. They do this for three reasons:
1. Their trees bear tastier plums if they have to bring fewer to ripeness
2. Plum trees are famous for their branches cracking under the weight of enormous crops of plums. Those cracks and splits provide perfect points of entry for Silver Leaf which can only penetrate a tree's defences through an open wound.
3. People often do not understand why some trees fruit biennially (every two years). It is simply because a tree that has produced a large crop is exhausted and does not have the resources to grow the fruiting wood necessary for it to crop the following year. Thinning prevents this happening.
It is best to thin plum trees gradually. Towards the end of May remove some of the fruitlets as they begin to form. Then, in July when the plums are still hard (and concentrating on fruit that is either damaged, bruised or diseased) thin out the plums so they can develop until they are fully ripe without touching one another. Ideally you should leave about 6-7cms (about 3") between plums and you will get a perfect crop.