Fungus spores produced on residue from earlier crops, some weed species such as wild mustard, and leaves infected within the same season, are blown by wind or splashed by rain onto plant parts. Numerous infections originate from these spores and occur during wet periods in the fall or spring when temperatures are in the 55 to 65 degree F range. The key to production of blight-free greens is initiation of a control program before white spot and other fungus diseases become established. So it's best to wait about 3 years before a repeat planting of leafy greens in such areas. If diseases were widespread in your past leafy green planting, don't use seed collected from such crops for the next crop, since they're likely to be contaminated with the white spot fungus.
Fresh seed are always a good investment.
Also, make sure you use fungicide-treated seed. Treated seed should be planted promptly. Following stand establishment, a fungicide application program will be needed to protect greens from white spot and other fungus diseases. Fungicides will protect greens from diseases provided spray applications are made on a preventive basis. For best results, start a spray program within 2 to 3 weeks after seedlings have emerged and continue through the season.