Begonia Leaf Damage

Here are a couple of possible explanations for your leaf damage. Cold water on the leaves can cause spots, especially if the leaves were warm when they were watered. It could also be thrips, a tiny bug that sucks the juices out of plants. They often leave black tar-like spots of excrement on the underside of the spots. Here's a link with information on detecting and treating for thrips - http://houseplants.about.com/od/pests/a/Thrips-Managing-Thrips-On-Houseplants-And-Greenhouses.htm Another possibility is a fungal leaf spot disease. The leaf to the left of the photo can't be seen clearly but it might have worse spots. Here's a link to more information about leaf spots - http://www.ipm.ucdavis.edu/PMG/GARDEN/PLANTS/DISEASES/leafspotdis.html