@ abc123, hello!
The number of ("how many") applications depends on the calendar dates conditions are favorable for the particular diseases you are targeting. Where I am, biggest concerns are brown patch, take all, etc. AND OTHERS all (or at least most) of which are normally avoidable through good cultivation practices. Challenge for me is, where I am at, overnight high heat temps combined with high humidity from end-of-May through October results in "it don't matter" how good one's cultivation practices are -- if folks in my locale don't apply fungicides preventatively, they're getting fungal disease outbreaks, simple as that. Anyway, I say all this as, you're so much farther north than I am, not only are your "high-overnight-heat-and-humidity" dates shorter than mine, you are likely dealing with targeting diseases I am not the least concerned about (snow mold, etc.).
The label on each product will guide you on not only how many days to wait in between applications but, also how many ounces (or fractions of ounces) you want to apply, per 1,000 square feet. Note that the amount of product you apply per 1K sf is different than the amount of water you need to do so. Water is only the carrier; I may use 2 gals of water to apply a 1/2 ounce of fungicide to 1k sf or I may use 4gals of water to apply 1/2 oz of the same product to 1k sf of lawn. What's important is that I got 1/2 oz of product down on that 1k sf of grass.
Similarly, "how often you cycle through different ones" depends upon the specific products you're using but, a universl good rule of thumb (there are some exceptions) is to not use the same product more than 2x in a row. So, for my lawn, where I am at, I may apply Product 'A' from Group 3 on May 1 and then again (21 days later, based on what the label tells me) on May 21. BUT! THEN, 21 days after that (June 12) I apply Product 'B' from Group 11 and then again (14 days later, based on what the label tells me) on June 26.
After that, I go back to Product 'A' and the cycle starts over, make sense?
Step # 1 for me is always determining just how many gal.s of water does it take for me to apply anything to 1,000 square feet of my lawn, anyway?! First, I measured a 10' x 100' section of driveway and using my backpack sprayer I determined it took 2gals of plain water to uniformly cover that marked off 1k sf of concrete. Next, knowing 1,000 sq feet = roughly a 31' x 31' square, I counted how many steps it takes me to walked off the edges of that area and I determined it takes 13 of my normal pace steps to / equals 31-feet and so, then I proceeded to walk off a 13-step by 13-step "block" of lawn and, sure enough, it took me 2 gals of water to uniformly spray that area.
Step # 2 in looking for the literature (homework reading) is determining what diseases you're up against (you may not even be up against any for all I know and I have relatives who live all over the US who contact me because they see others - "in so-and-so forum, on the internet" - mentioning certain pests and diseases and they are concerned they should be applying certain things when it turns out, quite fortunately for them, those weeds, pests or diseases aren't even a concern to them, where they're at.
Step # 3 is (more often than not) finding more than one fungicide -- and each from a different "mode of action" Group --with the reason being fungi, bacteria and viruses nearly all possess the wonderful quality of being able to DEVELOP RESISTANCE to molecular structures effective at disrupting different cycles of their development, if used over and over again. So, use any any product from one MOA or FRAC Group 2x in a row BUT THEN SWITCH to a product from a DIFFERENT MOA or FRAC Group.
Step # 4 (for me) is cost-comparison shopping. Because mfr 'a' may only sell their product in gallons while mfr 'b' only sells theirs in quarts, I always break the price down "per ounce"
I am more right-brained than left so, instead of "spreadsheets" I keep an annual "yarden diary" or "log" (or, "journal," if you will) that helps me keep track of "when and what and how worked" vs what did not. That said, I can think of some forums "Around The Internet" where I KNOW I would REGRET doing this but, I get the vibe this ain't one of them so, in the hope it is of any assist to you, here are my "fungicide application notes" from 2017:
First, I measured a 10' x 100' section of driveway and it took 2gals of water to uniformly cover the concrete. Next, we found 13 of my steps measures out to / equals 31-feet and we know 31' x 31' = roughly 1,000 sq ft so, then I proceeded to walk off a 13-step by 13-step "block" of lawn and then another for two total and after which I would mix and spray 4 gals of water with 2.0 oz of Eagle mixed into the tank.
2017-04-27: First application spray of Eagle fungicide on turf Notes
Five (5) spray tankfuls = 20.0 gals of water and 10 oz of Eagle. Based on my "calibrations" above, I figured 1oz of Eagle for every 2 gal.s water. Problem is, I wound up using a lot less than 2 gals to do each 1,000 sq ft of yard - closer to 3,000 sq ft for every 2 gal.s of Eagle mix.
2017-05-16: Second application spray of Eagle fungicide on turf Notes
Applied five (5) full spray tankfuls, each tank four (4) gals with 2 oz of Eagle fungicide = 20.0 gals of water and 10 oz of Eagle. Start in front yard on top of hill and work down til empty; 2nd tank full up at street and work down to top 1/3 to ½ of meadow; 3rd tank full finish meadow; 4th tank full backyard and 5th tank full go back to where left off in meadow and then also where missed at bottom of hill in front . . . (fingers crossed!!!!)
2017-06-07: First Application Aramda (Followup note: See 6-7-2017 notes for 1st app. Of Armada)
Applied five (5) full spray tankfuls, each tank four (4) gals with 1 oz of Armada fungicide (= 20.0 gals of water and 5 oz of Armada fungicide, total). Start in front yard on top of hill and work down til empty; 2nd tank full entire backyard; 3rd tank full start up at street and work down to to swamp portion of meadow and then also where missed at bottom of hill in front . . . (this ½ recommended preventative spray … fingers crossed!!!!)
Armada is a PAIN IN THE *SS to deal with! First off, it's a powder so, had to add granules to a ½ gal of water in a clean milk jug with cap and then shake the hell out of it until dissolved and THEN TRANSFER that "milkshake" into the 4 gal sprayer tank; On top of that, it foams so, hard to get 4gals in sprayer tank without it foaming over … DEFINITELY EXPLORE GOING WITH EAGLE AND HERITAGE IN FUTURE *if* Heritage is liquid (not powder)!
2017-06-28: Second Application Aramda
Applied only four (4) full spray tankfuls; however; due to "foam" effect of the Armda "milkshakes" I was only able to get 3 to 3-1/2 to gal.s of water in each tank four AND, I "up'ed" the amount of Armada powder to 1-1/2 oz of Armada fungicide = 16.0 gals of water and 6 oz of Armada fungicide. Start in front yard on top of hill and work down til empty; 2nd tank full entire meadow and up at street; 3rd tank full backyard; 4th tank-full start uppermost portion of lower meadow and work down to to swamp portion.
Armada is a FOAMY PAIN IN THE *SS to deal with!
2017-07-14: Third application spray (2nd rotation) of Eagle fungicide on turf (wasn't scheduled til 7-18 but, have to travel those dates
2017-08-6: Fourth application spray (last ½ of 2nd rotation) of Eagle fungicide on turf.
FIRST TIME USE OF "Turf Mark" blue indicator; couple of notes: 1) definitely requires at least 1oz of marker per 1gal of liquid in tank but, residue tends to stay in tank so, may be able to get by with less in subsequent tank-fulls; 2) USE OF THE INDICATOR DEFINITELY WOUND UP WITH ME USING MORE MATERIALS AND, INSTEAD OF ONLY NEEDING 5 BACKPACK SPRAYER TANK-FULLS, THIS TIME, (w blue indicator) IT TOOK ME SIX!
Due to increased favorable conditions for disease outbreak (overnight high heat / humidity) I mixed 3oz of Eagle into each 4-gal sprayer tank-full so, this time, the entire yard took a total of = 24 gals of water and 18oz of Eagle!
2017-08-25: Third (1st half, 2nd rotation with) Application Armada water dispersible granules
(4-hours, incl. clean-up)
Due to increased favorable conditions for disease outbreak (overnight high heat / humidity) I applied SEVEN (7 !!!!) full spray tankfuls; however; due to "foam" effect of the Armada "milkshakes" I was only able to get close 3-1/2 + gal.s of water in each tank four AND, I "up'ed" the amount of Armada powder to 3.0 FULL OZ OF ARMADA IN EACH 3-1/2 TO 4 GAL SPRAYER TANK FULL!!!
Total application for today = 28gals water : 21 oz Armada; (7 tanks x 4 gals = 28 gals water with 7 x 3oz Armada granules=21oz Armada) Started (tank # 1) top of yard along road down to "neck" above meadow; Tank # 2 top 2/3 meadow; Tank # 3 remainder meadow (both sides drainage) down to swamp; Tank # 4 top 1/3 of front yard hill; Tank # 5 90% of backyard; Tank # 6 middle 1/3 front yard hill and swamp; Tank # 7 bottom 1/3 front yard hill and "alley" along sub-basement and caught remaining 10% backyard (very bottom of hill). . . monitor.
2017-09-14: Fourth (2nd half, 2nd rotation with) Application Aramda water dispersible granules
Applied SIX (6) full spray tankfuls; however; due to "foam" effect of the Armada "milkshakes" I was only able to get close 3-1/2 + gal.s of water in each tank four AND, I returned the amount of Armada powder to 2.0 FULL OZ OF ARMADA IN EACH 3-1/2 GAL SPRAYER TANK FULL.
Total application for today = 21gals water : 12 oz Armada; (6 tanks x 3-½ gals = 21 gals water with 6 x 2oz Armada granules=21oz Armada) Started (tank # 1) top of yard along road down to top third of meadow; Tank # 2 top remaining 2/3 meadow (both sides drainage) down to swamp; Tank # 3 top 1/2 of front yard hill; Tank # 4 90% of backyard; Tank # 5 remaining 10% backyard (very bottom of hill) and "alley" along sub-basement; Tank # 6 bottom 1/2 front yard hill and caught swamp.
LESSON LEARNED FOR 2017: END-OF-SUMMER IS WHEN YOU NEED TO GET SERIOUS ABOUT APPLYING FUNGICIDE, NOT "DROP OFF"!!!!!!!!!!
2017-10-10: Fifth 2017 application spray (first ½ of 3rd Eagle rotation for 2017) of Eagle fungicide on turf. (Even though previous notes say to use at least 1oz of marker per 1gal of liquid in tank I only used 1oz per 4-gal "full" tank-full … seemed to work okay …)
Wound up using five (5) four-gallon (4-gal) tankfuls and I mixed 2.5 to 3oz of Eagle into each 4-gal sprayer tank-full so, this time, the entire yard took a total of = 20 gals of water and approx. 15 oz of Eagle!
Using the above products, application amounts, and intervals 2017 WAS THE FIRST YEAR WE DID NOT LOSE LARGE PORTIONS OF OTHERWISE VIGOROUS LAWN AND TURF DUE TO FUNGAL OUTBREAKS, during the course of the hot and humid portions of our growing season.
Wrap-up: In the future I am going to use (and monitor closely) alternating between Propiconazole (Group 3) and Azoxystrobin (Group 11).
Hope this helps but, there's no short-cutting the reading you'll need to do, in order to target the diseases specific to your area.
Best o' Success!