Was it hot last year? LOL! Hot humid weather stimulates the spread of fungal diseases. Some ryegrass types are particularly sensitive. Red thread, gray leaf spot, and brown patch can be serious. Avoid nighttime irrigation. Irrigate between noon and 4 so that the grass leaf blade dries out immediately. Switch to irrigation no more than three days per week--so that the average humidity and leaf wetness are at a minimum.
Low nitrogen makes red thread worse...so possibly the organic contained less nitrogen or released it more slowly.
If disease becomes chronic every year...try to replace the area with bluegrass sod. Or perhaps tall fescue. Or perhaps overseed with a highly disease-resistant perennial rye. Make sure it is one of the top 10 in Rutgers NTEP tests. Their new variety called "Wicked" is excellent. Perhaps combine it with "Cutter II" which has very good red thread resistance.