The last decade has seen a substantial spike in studies on allelopathy and allelopathic interactions. This trend is particularly interesting as claims about allelopathy have historically been controversial, largely due to experimental difficulties in separating its effects from those of resource competition and other plausible alternative mechanisms. One of the paradigmatic examples of allelopathy is the case of black walnut toxicity (Juglans nigra) via juglone (5-hydroxy-1,4-naphthoquinone). Despite that it is often referenced as a familiar and thorough case study, the efficacy of juglone or walnut extracts as allelopathic compounds explaining documented ecological field patterns is still tenuous. We performed a literature review of this text book case study to illustrate some outstanding problems with studies of allelopathy beyond those of differentiating its effects from resource competition.  
Results/Conclusions

The history of black walnut allelopathy reveals numerous and seldom recognized “negative” results, as well as methodological issues including inadequate experimental controls and poor articulation between laboratory (e.g hydroponic and greenhouse) and field results. Our results categorize, quantify and summarize these problems. Furthermore, it is our contention that the recent spike in allelopathic studies, stems, at least in part, from technological advancements that allow for faster and cheaper identification of putative allelochmicals. However, we argue that, by itself, this increased ability to identify candidate allelochemicals (of which juglone was one of the first), does not necessarily succeeded in generating a strong argument for their efficacy in explaining ecological patterns.