Hi- this is black knot disease, a common plum tree fungal disease. The fungus ejects spores during rainy periods; some spores landed on your plum and within hours infected it. If not removed, black knot will continue to infect your tree and will spread up the same branch as it grows and also adjacent branches until the tree appears loaded with black warty growths. The growths can girdle the stems (cut off nutrient flow and make the branches wilt and die) and can eventually kill the whole tree.

The knots/swellings need to cut removed/cut out. The timing is when the tree is leafless and dormant. With pruners, cut 3-4 inches below that knots to where tissue is healthy. You will want to sterilize pruners between cuts. Do not allow any cut knots or fungus infected branches to remain on the ground or the tree will be re-infected. Dispose immediately in bagged trash. If a tree is severely infected, it is best to remove it.
A fungicide like lime-sulfur is recommended when you are dealing with a moderate infection. This would be a spray applied at bud break and every 2 weeks until the terminal branch growth stops.

There are no varieties of plums that are completely resistant to black knot --only those that are moderately resistant. A commonly planted plum, Stanley, is highly susceptible. A list of those with some resistance to black knot from 2 sources:
Shiro, Castleton, Santa Rosa, Methley, Early Italian, Fellenberg, Seneca, Damson, Blufree, NY9, Au Rosa and President , Formosa, Milton, Brodshaw, and Fellenberg.
Jon

The fruit is safe to eat.
Jon