Last year, my peach tree had leaf problems and I sprayed copper and it worked. This year, the plum tree and its leaves have been twisted and there is only one plum on the tree. Is this due to the weather?
Unfortunately, over the past two springs, many parts of NZ have experienced heavy and continuous rainfall with a disastrous effect on the pollination of plum trees.
Bees find it exceptionally difficult to work in these conditions as wet pollen is very hard to distribute from tree to tree. The end result is an apparent lack of pollination and therefore little or no fruit set.
Unfortunately, there is little you can do - even hand pollinating is difficult in wet conditions as the wet pollen will not adhere to a small paint brush.
Hopefully in the next few years the spring weather will improve, allowing for pollination of your plum tree.
The disfigurement of your tree is in no way related to the poor cropping. It is quite normal for some leaves on all plum trees to be misshapen and/or partially eaten by a number of insects, especially bronze beetles. You could spray with a good quality horticultural oil to keep pests at bay.
To keep your fruit trees healthy, mulch around the tree with Daltons Mulch & Grow from early November. Apply Garden Time™ Fruit & Citrus Fertiliser out to the tree drip line in early spring when plants are coming into growth. Feed at six weekly intervals until mid-December, recommencing in mid-February and feeding through until autumn.