Answer: What you describe sounds like sooty mold, the end results of sucking insects such as aphids. Unfortunately, nasturtiums are magnets for aphids. They feed by sucking the fluids from the tissues of the plants. As they feed they excrete a sticky substance called honeydew. This fluid falls onto foliage and it has a high enough sugar content to become moldy. The black coating on the surfaces of the leaves is the moldy honeydew. It can be wiped or rinsed off and the leaf is not damaged by it. To prevent this from happening again you'll need to control the aphids. Insecticidal soap works well - be sure to treat the undersides of all the leaves. You might also try spraying with a strong stream of water. This will at least dislodge the pests. It might also help if you have lots of flowering plants in your garden to help encourage ladybugs which are an aphids worst enemy.
Best wishes with your nasturtiums!
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