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Have you noticed any other signs of concern (i.e. insects)? It could be aphids affecting your plum tree resulting in the leaf curl. Neem oil should help with this and is an effective fungicide as well in case anything else pops up. Here is more information: https://www.gardeningknowhow.com/edible/fruits/plum/control-leaf-curl-plum-aphids.htm

One possibility is leaf curl plum aphids: https://www.gardeningknowhow.com/edible/fruits/plum/control-leaf-curl-plum-aphids.htm
However, there are also a number of fungal and bacterial infections that can cause leaf curl or little holes. If you take some samples of leaves and branches to the local Extension Service, they should be able to diagnose and prescribe treatment. This link will help you locate one: https://www.gardeningknowhow.com/extension-search/

There are several possible culprits from the ranks of bacteria and bugs. The first line of defense in good cultivation practices, so the tree is healthy and able to repel invaders. This article has a number of ideas: https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-dropping-leaves.htm
The only way to know for sure is to take some leaf and twig samples to the Extension Service for analysis and recommendation on treatment. This link will help you locate the nearest one; https://www.gardeningknowhow.com/extension-search/

It sounds like it may be a pollination issue. The tree will abort the fruit if not properly pollinated. Depending on what kind of insecticide you used, you may have made the tree inhospitable to pollinators, so they could not do their job. This article will help with attracting more pollinators to the tree: https://www.gardeningknowhow.com/garden-how-to/beneficial/creating-a-pollinator-garden.htm

There are a number of things that might cause the symptoms you describe: too little water, problems with the soil, winter damage, aphids, and disease. Here is an article on basic plum growing: https://www.gardeningknowhow.com/edible/fruits/plum/growing-plum-trees.htm
and one on common plum diseases: https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-diseases.htm
To find out what the problem is, you should have a professional analyze the leaves and soil.

Sounds like it could be plum spot disease. This article has information on plum diseases: https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-diseases.htm
To be sure what the problem is, you should have the leaves analyzed by a horticultural expert. This link will help you locate one:http://mastergardeners.org.uk/get-involved/find-your-nearest-master-gardener/