Two diseases of Rhubarb that could cause these spots are: Ascochyta leaf spot and Ramularia leaf spot.  If the stalks are also becoming infected i'ts more likely the disease is Ramularia.  Ascochyta does not infect the stalks. Rhubarb that is in poorly drained soil is more susceptible to disease problems.  Good sanitation by removing the infected leaves as soon as the disease symptoms appear will help keep the disease in check.  Do a thorough cleanup of all the leaves after they've frozen this fall. Copper based fungicides are possible solutions for keeping the disease under control during the growing season.