Oak tatters is a disorder that primarily affects white oaks, but also is observed on hackberry trees. Leaves of affected trees lose the majority of interveinal leaf tissue, resulting in a leaf ‘skeleton' (Figure 1). The phenomenon was first reported in the early 1980’s, and has been observed in many Midwestern states. The number of trees affected varies widely from year to year, with a much higher level of incidence in 2017 than normal. The disorder occurs in both rural and urban areas and may affect single trees and those in woodlands. Symptoms usually are distributed uniformly throughout the tree, rather than being more severe on one side as usually occurs with herbicide drift.
There has been relatively little research to identify the causal factor of oak tatters and the cause is still debated. The consensus among many in the forestry arena is that acetochlor and other group 15 herbicides (chloroacetamides) are responsible for the damage. Researchers at the University of Illinois sprayed several herbicides commonly used in corn production at 1, 10, and 25 percent of labeled rates on oak seedlings grown in the greenhouse (Samtani et al. 2008, 2010). Metolachlor (Dual), acetochlor (Harness), and dimethenamid-P (Outlook) caused loss of interveinal tissue (oak tatters) when applied while leaves were expanding. Several years ago I worked with Jesse Randall (ISU Natural Resource Ecology and Management) and was able to induce oak tatter symptoms with lower rates of acetochlor than used in the Illinois research.
Foresters with Iowa’s DNR conducted an observational study in 2007. A tree that had developed oak tatters in the past was monitored during the early growing season. Tree pollination bags were placed over several branches in late April and left for five days. The tree developed tatters except for the branches covered with bags. Concentrations of acetochlor in air and rainfall at the site were monitored from April 10 to May 18. Acetochlor concentrations peaked during the time when oak tatters appeared and concentrations of acetochlor were 3-10 times higher in leaves not protected by pollinator bags than covered ones.
While research and observations suggest that the group 15 herbicides may be responsible for oak tatters, I still question their role in the problem. Oak tatters was first reported in the 1980’s, whereas the group 15 herbicides have been widely used in corn and soybean production since the mid 1960’s. Alachlor (Lasso) was used on 1.3 million and 4.0 million acres of corn in Illinois in 1971 and 1978, respectively (Gianessi 1992). Similar use patterns would have occurred in Iowa at the time. Thus, these herbicides were widely used before oak tatters was first reported.
Since oak tatters appears in trees both adjacent and isolated from agricultural fields, it has been suggested that herbicide in rain or the atmosphere is responsible. The presence of herbicides in rainfall and air during peak application periods is widely documented – "It’s raining Lasso" was a headline of a Des Moines Register article in 1991. Since the vapor pressure of alachlor is approximately 1000 times higher than acetochlor and metolachlor, rainfall and air in the 1970’s should have had higher concentrations of group 15 herbicides than now. Thus, it is likely that oaks were exposed to higher concentrations of group 15 herbicides prior to oak tatters being reported as a problem.
Group 15 herbicides are primarily active on emerging seedlings, they generally have little effect on emerged plants. Several of the products are registered for postemergence applications in broadleaf crops, and I have never seen symptoms similar to oak tatters developing on the crops or other vegetation contacted by the spray. Pennant, a formulation of S-metolachlor registered for use in nurseries and landscapes, is approved for over-the-top applications on oaks. Research and a half-century experience in the field suggest that the group 15 herbicides should not produce the symptoms associated with oak tatters; however, scientists have been able to replicate the symptoms in the greenhouse with applications of the herbicides. This is the real mystery to me.
A conundrum is defined as a confusing or difficult problem, and identifying the cause of oak tatters remains a real conundrum. While I am not willing to say that acetochlor and other group 15 herbicides cannot be involved in the development of oak tatters, the limited research that has been conducted is insufficient to confidently imply they are the primary cause of oak tatters.
Literature cited
Gianessi, L.P. 1992. U.S. pesticide use trends:1996-1989. Resources for the future. Wash. D.C.
Samtani, J.B., J.B. Masiunas, and J.E. Appleby. 2008. Injury on white oak seedling from herbicide exposure simulating drift. HortSci. 43:2076-2080.
Samtani, J.B., J.B. Masiunas, and J.E. Appleby. 2010. White oak and northern red oak leaf injury from exposure to chloroacetanilide herbicides. HortSci. 45:696-700.
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