i am infected. pachysandra, leaf spot then root rot. a stand of (beautiful) Solomon's Seal Polygonatum commutatum

browned and died within a few weeks. astible dead as a doornail. and now i see my perennial geraniums dying off. while this is one area about 12x12 ft....across the lawn the pchysandra and perennial begonia are being decimated. spent several days trying to look web sites. have seen pictures and descriptions of diseases/pests. but cannot really figure out what i have...or what to do . have pulled out dead and thrown into trash. have looked at anti-fungals, etc. but am disheartened by this mess. could i have brought this in with bagged mulch from H.D.? what to do before i pave over everything with concrete.