My plum tree has many fruits on it that are white enlarged and empty or hollow. The fruits are not ripened yet , we live in northern Alberta, the weather has been pretty normal for here.Clay soil, we have had enough rain and warm weather.
From your description your plum tree has what is called Plum pocket.
Plum pocket or bladder plum is an unusual disease that causes unripe plums to grow abnormally large within a month or two after bloom. Plum pocket is caused by the fungus Taphrina communis. The infection starts with a small blister on the fruit, which rapidly grows and soon covers the entire fruit. Infected fruit may grow to ten times their normal size, have a spongy texture, and become covered with velvety gray fungal spores. Eventually infected fruit dry out and turn black. If cut open, the center of these affected plums is empty. The fungus can cause the leaves to thicken and curl, but this is not common. Infected fruit usually fall to the ground, but some will stay on the tree through winter.
Plum pocket primarily infects wild plums and American type plums. In many trees only a small number of fruit are infected. The disease can be reduced by removing infected plums before they are covered with spores. If plum pocket is causing crop losses by killing leaves or destroying most of the fruit, fungicides applied before bloom will help control the disease. Plum pockets can be prevented with a single fungicide application just before bud break in early spring. Bordeaux mixture (copper-sulfate) or liquid lime sulfur can all be used to manage plum pockets.
You can read more on the use of Bordeaux mixture and Lime Sulfur from one of our earlier posts: Bordeaux Mixture or Lime Sulphur?