By Thomas Ford
To narrow down the cause of the injury, I asked the grower if he had applied anything to the field that may have caused the symptoms he was observing. After a momentary pause, the grower told me that he had applied an herbicide cocktail (tank-mix) over the plastic. He indicated that he had used the same basic tank mix as he had in the past, except that he had added a third herbicide product to the mix this spring—hopefully, to get better grass control. The grower assumed that the herbicide cocktail he had applied over the plastic would wash off before planting. In this case, the last week’s rain washed the remaining chemical residues on the plastic into the soils surrounding the new transplants causing plant injury/death.
Herbicides, like all pesticides, can cause plant injury/death if misapplied. In this case, the grower went “off-label” to keep weed pressure low in his row middles. He assumed that any chemical applied to the plastic would runoff. Still, he did not consider the full ramifications of his actions if the herbicides applied contacted the soils surrounding the transplants. Two of the three herbicides used in this “cocktail” were seedling growth inhibitors. The first product contained pendimethalin, the active ingredient in Prowl. Pendimethalin is considered a root inhibitor, and exposure to this herbicide can cause impacted plants to develop thickened and stubby roots.
The second herbicide is classified as a shoot inhibitor, and it contains the active ingredient metolachlor, which is found in the product, Dual. Broadleaf plants exposed to sub-lethal dosages of metolachlor will display cupped or crinkled leaf tissue. The third herbicide in this pre-plant cocktail was simazine, a photosynthetic inhibitor. Photosynthetic inhibitors have activity on both monocot and dicots. Plants exposed to photosynthetic inhibitors like simazine will develop interveinal chlorosis first before developing chlorotic leaf margins, then necrosis and plant death.
Simazine is prohibited from being used around most vegetable crops (except sweet corn). Its label states there is a 12-month rotational restriction between its application and the planting of vegetable crops because of injury concerns. This grower’s off-label usage cost him his transplants, and if the grower follows the simazine label as required, he will not be able to re-plant anything else in these fields except sweet corn or field corn for at least another year.
Source : psu.edu