Your sour cherry tree may be suffering from  both black knot disease and brown rot.  However, our early spring and very wet weather may be contributing to the damaged cherries, so it's possible that's the issue and not the brown rot.  The black webbing on the branches 1-2 months ago is a strong indicator that black knot is affecting your tree.  It's ,a common fungal disease that affects cherry trees and  causes hard, swollen black galls to form on branches.

Brown rot causes discoloration and wrinkling of the fruit skin.  To deal with the black knot, prune out all infected branches, cutting at least 3 to 4 in iches below the visible knots.  Destroy all pruned clippings and consider applying a fungicide in early spring.  Brown rot is treated by removing all damaged fruit and pruning out infected branches.  Improving air circulation through proper pruning and avoiding overhead irrigation can help reduce humidity and the spread of both diseases. 

For detailed information on black knot disease, see:https://extension.umn.edu/plant-diseases/black-knot

Hi,

It's best to remove all damaged fruit, including whatever has fallen on the ground.  Don't put it in the compost, as the fungus may spread and damage other trees.  The powdery substance may be powdery mildew, due to all of the rain we've been getting.  Remove infected leaves and prune for good air circulation.  You could try a copper-based or clay-based fungicide and consider disease-resistant cultivars for future purchases.  It's generally best to prune a sour cherry tree In the winter, but galls do not need to be pruned. For more information on this cultivar, see

https://trees.umn.edu/north-star-sour-cherry-prunus-north-star

Thank you for your help.