 Try to water only the soil without getting the leaves wet and try to avoid splashing the dirt from the ground up onto the leaves. This is where mulching really helps. Mulching with even such grass clippings that haven't had weed killer applied. That will help prevent the soil-borne fungal diseases from getting up onto the leaves. You can also apply fungicide. Look at the garden centers for an all-purpose vegetable-type fungicide and you can even apply that with a watering can.