Are those tunnels in those leaves?
Your plants were beautiful…then all of a sudden they look like something is tunneling through the leaves, leaving behind ugly trails of damage. Once you see these “tunnels”, you can be pretty sure you have leafminers. Leafminers are the larval stage of flies: The larva feeds between the upper and lower surfaces of leaves, leaving hollowed out tunnels in the foliage.
The adult insects that lay the eggs in the leaves can be tricky to spot because they resemble common houseflies. They are often gray or black in color with yellow stripes. Leafminers can attack many species of trees, shrubs, perennials, annuals and edible crops. They can be particularly destructive in the vegetable garden and have a special affinity for spinach. In addition to being unsightly, the damage they cause can often become entry points for fungal and bacterial diseases and severely reduce yields in vegetable crops.