Some varieties of mustard naturally have a purplish tinge to the leaves, so our guess would be that the tissue immediately surrounding the holes in the leaves starts to die and lose its green chlorophyll, leaving the dark color around the holes. On the other hand, a fungal disease called alternaria leaf spot may cause purplish-black lesions in leaves. Search google for an image of alternaria leaf spot and see if it matches what you’re seeing. If alternaria leaf spot is the culprit, remove heavily affected leaves and avoid planting mustard in the same spot next year.
Some varieties of mustard naturally have a purplish tinge to the leaves, so our guess would be that the tissue immediately surrounding the holes in the leaves starts to die and lose its green chlorophyll, leaving the dark color around the holes. On the other hand, a fungal disease called alternaria leaf spot may cause purplish-black lesions in leaves. Search google for an image of alternaria leaf spot and see if it matches what you’re seeing. If alternaria leaf spot is the culprit, remove heavily affected leaves and avoid planting mustard in the same spot next year.