Over three years ago this area was growing an 20" diameter Silver Maple. The tree was removed and soon afterwards compost and sand were roto-tilled into the existing clay soil around the stump. Still no clue? Right.
Well after the tree was cut down, the stump was treated with Picloram (Tordon RTU) which is a herbicide used to treat cut stumps. If treated within 30 minutes the cambium layer picks up the herbicide and relocates it into the root system, effectively killing the roots and preventing sprouts. Because of Picloram's solubility it trans-locates readily through the root system. However it is also very soluble in the soil. Three years later the herbicide is still creating damage in broadleaved plants (dicots). It does not effect grass plants(monocots), and it doesn't effect all broadleaved plants. It has affected milkweed, potatoes, tomatoes, and goldenrod. It caused this leaf mottling and prevents the plant from blooming.
Hopefully, this will be the last year I see this and I can begin planting more dicots in the future. There have been reports of the same type of damage from using lawn clippings or compost derived from lawn clippings or hay/straw treated with Aminopyralid herbicides which Picloram is one. Visit the Compost Gardener.com to glean some more information about this possible problem. Hopefully this is not a problem in your garden.