From National Gardening - Nasturtiums: So questions you must answer to help pin down the problem- Do you have bad aphids? They drain the plants juices and can cause yellowing. Have you been giving them lots of water? They need it, lack of water can cause yellowing. Have you or a nearby neighbour used any herbicides (including lawn care products such as 'weed and feed')? Nasturtiums are very sensitive to chamicals and this can cause yellowing of the leaves. One last thing that can cause yellowing is lack of, or excessive nutrients. Try to remember if you've been fertilizing them (you don't need to and this could be the problem), or if an extra rich compost was added. Too much nitrogen can make the leaves yellow. Hope you can track the problem down!