Leaf miners are generally noticed affecting the leaves of the host, showing long lesions of a width of slightly more than one centimeter. These lesions may be colored brown, tan, or white. The cause of disease is a species of moth or butterfly specific to the host species. Two specific variations have so far been observed:
Variation One: Host species is Plantago major, commonly known as broadleaf plantain. The lesions are a tan-brown and black spots about a milimeter thick by around five milimeters long can be seen at the center of the lesions. These lesions generally start somewhere in the perimeter of the leaf and eventually converge with the midvein after some length. The cause is a brownish nocturnal moth, which can be seen at lights during the night, with an approximately 1" wing span and red eyes. (See thread "Pesty Nocturnal Moth" in 'Insects' forum for photo).
Variation Two: Host species is Viola papilionacea, commonly known as wild violet. The lesions are white and tiny caterpillars about one millimeter in width and five or more millimeters in length can be seen with the naked eye. If cut in two, a green hemolymph fluid will ooze out of the sectioned caterpillar's viscera. The lesions generally originate towards the leaf's perimeter, especially below the third pair of veins. The cause is a moth about 3/4" in wingspan with black eyes and clubbed antennae that can be observed at lights during the night. (See my photo album titled "Case Histories of Afflictions of Forbs and Grasses" for photo).
Poplars often develop a foliage affliction that is manifest as a whitening of the perimeters (especially below the third vein pair) of the leaflet. The whitening occurs in a mottled pattern, and is quite contagious via leaf and sap contact. Most curiously, this affliction never appears manifestly on the seed leaves. The already-developed discoloration will appear dulled after a direct application of the garlic mustard root extract to the affected areas; nevertheless, the affliction will continue to spread.