rain usually causes it ...so when you water, avoid using overhead watering with a sprinkler head ...and only water at ground level
try a 50/50 spray mix of Liquid Copper Fungicide + Daconil Fungicide - spray to run-off - leaves, under leaves and stems ...
be sure to remove all yellowing leaves and toss in bag to be hauled off - do not put yellowing leaves in mulch pile
if you use the 50/50 spray to run-off you wont have to replace your mulch -
