First started to treat SP in homeowner lawns in the mid 1980's in the Philly area and after several partially effective attempts I settled on a Fall/Spring regime extending over 2-3 years depending on how severe the initial problem was.
The initial schedule was a single Fall DMI app with a wetting agent in late Sept/early Oct - depending on temps - followed the next Spring by two more apps, one in April and a second in late May/early June. If that showed reduced symptoms, the second year was one Fall and one Spring app, with only one maintenance app in the Spring required in following years if symptom onset was well suppressed.
That said, at many of these sites the SP was often combined with Necrotic Ring Spot! With the most effect way of separating the two pathogens was to follow the SP schedule and change the A.I. package in the Fall and early Spring to include or replace the DMI with a NRS specific fungicide when the predominant early Spring symptoms was small slightly depressed areas in early Spring at green up and prior to the first mowing of the year. Also, grab sampling of both KBG and RG crowns frequently showed significant purple color and in the case of RGs often saw extended crowns with air roots growing from infected plants.
In the must severe cases annual Fall dethatching and two to three in season wetting agent apps combined with half rates of low N and high K fertilizers during the late June thru early August stress periods were often very helpful at minimizing and helping in recovery for infected turf areas.