Thanks for writing. It could be a couple of things, including disease. To help prevent disease, avoid watering your plants in late afternoon or evening [morning is best] and encourage good air flow. If you fertilize your plants, cut back on fertilizer as it promotes the kind of soft, lush growth that diseases and insects love. Also be sure there's good air flow around your plants --- that may mean pruning or training them so you don't have on mass of foliage.
If it's happening to just a couple of leaves, you can often pull off those leaves; it may stop the disease from spreading to the rest of the plant. Throw the leaves in the trash and always be sure to pick up dead fallen leaves around your plants as diseases can proliferate there.
---Justin, Senior Garden Editor, BHG.com