HD/Lowes will have a product. On the road this week, but I think what I have in my garage is a Spectricide-brand spray. It is pricey. (By my recollection, only sold in the single-use pre-loaded sprayer that you hook up to your hose. YMMV)
Usually caused by prolonged dampness on the blades. Is it mainly in shady areas? Do you water in the evening? If you have an irrigation system, what time does it come on?