Last summer, I was called out to view what appeared to be a herbicide drift incidence in a commercial tomato field. The leaves of affected plants were distorted with cupping and twisting that is characteristic of the growth regulator herbicides such as 2,4-D and dicamba (Figure 1). Cattails and mustard weeds growing in an adjacent drainage ditch appeared as though they had been recently sprayed with a herbicide, however, the symptoms looked to be more like that from glyphosate. (Figure 2). If these tomatoes were the unlucky recipient of drift from a Roundup application, they certainly were not expressing typical symptoms (Figure 3). Additionally, there had been no recent sprays of any herbicide made to the ditch, at least according to the farm foreman, PCA, and the irrigation company. And there were definitely no applications of 2,4-D, this being much too late in the season for using such a herbicide.

Further complicating matters were that the symptoms did not fit the typical pattern of herbicide drift. Symptomatic plants did not gradually attenuate the further away they were from the presumed source, which in this case was the drainage ditch. Rather, crop injury was in angular and well-defined sections, with less than 5 feet between healthy and sick plants (Figure 4). From 100 feet above the field, the damage would have probably looked like somebody had drawn a crazy jagged line along a portion of the western edge of this field. But the length of the jags did not match any cultivation equipment or spray boom swath length. In other words, there was no obvious pattern of direct herbicide application from a 20 -ft tool bar or a 45-ft boom sprayer, for example.

During our conversation, we moved from herbicide drift to possible pre-emergent herbicide residual in the soil. Soil samples could also be submitted for direct analysis, but this is expensive, especially when the active ingredient is not known. Besides, the field had been in tomatoes the year before, so why would just certain parts be affected?

I left the field with the issue unresolved, but I did suggest that everyone double check their spray logs just to be sure. I thought there was a chance that some left over 2,4-D from a February or March application to wheat was still sloshing around in the spray tank, and that same sprayer had been used to clean up the side of the drainage ditch. But on a hunch, I also grabbed some soil to do a bioassay with some tomato transplants I happened to have. Perhaps I could duplicate the symptoms.

In 2014, veteran farm advisor Richard Smith in Monterey County made a report in this UC Weed Blog about tomato plants from small growers and gardeners with deformed leaves similar as to what occurred in this field (Herbicide Carry-Over in Compost? https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=14175 ). Based on a bioassay, his conclusion was likely contamination of compost made with municipal yard waste that contained clopyralid that had been applied to turf. Clopyralid has both crop and non-crop uses in agriculture, and goes by the tradename Stinger and Transline. Another product with a similar mode of action and slow degradation half-life is Milestone (aminopyralid).

As it turned out, I didn't need to do the assay. The grower decided to do his own bioassay on a much larger scale: he replanted a large portion of the field. In two weeks, all the plants had exactly the same symptoms as before (Figure 5).

The second planting confirmed that the source of the herbicide damage originated in the soil, and not a result of spraying the drainage ditch. But did not answer the question how it got there in the first place, and why to only a portion of the field.

Both clopyralid and aminopyralid are labeled for use in California for non-crop areas, including barrow ditches and roadsides, for both pre- and post-emergent weed control. Both also have 18-month plantback restrictions for many crops, including tomatoes. These are very strong herbicides that offer extended weed control, but can harm sensitive crops even if inadvertently moved onto land by runoff, wind, or farm machinery.

Once we knew that drift wasn't the explanation, the solution pretty much clicked into place. Turns out, the previous fall the field road had been scraped, and the soil moved onto the tomato field. That roadside had been treated with Transline. That explained the jagged, zig-zag pattern of dead plants on the side of the field next to the road that matched no spray boom. The road ran between the drainage ditch and the field, and the grower had scraped a bunch of the dirt to fill some low spots in the field.

This situation would seem to be a very rare occurrence, but just 1 week later I was in a fresh market tomato field with similar symptoms (Figure 6), except they were much milder, and the grower attributed the reason to a little drift from spraying the ditch with Rely (glufosinate). This was not Rely drift, but more likely a case of clopyralid movement with dust from the field road. No crop losses on this one, but it does illustrate how these long-residual herbicides can move into places by unexpected ways.

 

Scott Stoddard is a UC Cooperative Extension Vegetable Crops and Soil Farm Advisor in Merced and Madera counties.