Buckwheat is very sensitive to herbicides, and none are registered for use.
Aster yellows makes the flowers sterile and green. In affected fields, it is rarely more than one plant in a thousand.
Powdery mildew is a seedborne disease that causes yellow-green blotches on the leaves. This disease grows inside the leaf, not on the surface. It is not a serious problem, and it is controlled by planting seeds from uninfested fields.
Penicillium stem rot is occasionally seen late in the season if the plant population is high and the stems are thin. It causes the plants to fall down. Immediate harvest is the only remedy.
See disease pictures (large file)
Insects
The insects that are seen (wireworms and aphids) usually do
little damage in summer-sown buckwheat. Buckwheat that
emerges in May, whether volunteers, or an early cover crop, are at some
risk of aphid infestation. Early aphids attract lady beetles that
protect later crops. Buckwheat attracts many other beneficial insects
once it begins to flower, so it may even reduce the insect pest
pressure nearby.