Contact herbicides typically will cause chlorotic or necrotic spots. Multiple spots can result in deformed leaves as well as leaf curling. Systemic herbicides, those that are translocated in the plant, tend to cause a yellowing of the foliage. They may also cause necrotic spots and leaf curling.
Conditions for Disease Development
Generally, foliar damage occurs when herbicides are applied at excessive rates, at the wrong stage of plant growth or during unfavorable weather conditions. Injury often occurs from herbicide drift when crops or weeds adjacent to onions are sprayed. Damage from drift is usually most severe at the edge of a field closest to where a herbicide was applied, with injury decreasing with increasing distance from the source.
Use herbicides as directed and apply during appropriate weather conditions. Plants will frequently recover from foliar damage if it is not too extensive.