During the winter, diease like downy mildew will move in on weak plants and cause spotting (light green to reddish-brown), yellowing and defoliation. Downy mildew  is a minor disease usually, but it may cause extensive defoliation. Once the hotter days of late spring arrive, the disease tends to dissipate. A key point in control is removal of fallen infected leaves. Get those out of your landscape and do not mix them in with the compost! Make sure the sprinkler system is not directly hitting the foliage as the water will disperse the spores and the disease will consume more of your hedge.
If you feel you need to apply a fungicide, try products labeled for ornamentals, such as Mancozeb ,Chlorothalonil, Propineb,Cymoxanil,Fosetyl Al,Zineb,Maneb,metham sodium or phosphoric-acid based products. These products should be reapplied every 10 to 14 days as a preventive. A fairly recent group of fungicides products contain strobililurins and are available to professional landscape maintenance applicators. Fewer applications are needed with these compounds, but they are less priced now.