Today I received my November 2012 issue of Arboriculture and Urban Forestry. This is one of the few peer-reviewed journals that generally has information of immediate value to gardeners and landscape professionals as well as academics. This issue contains an article entitled “Evaluation of biostimulants to control Guignardia leaf blotch (Guignardia aesculi) of horsechestnut and black spot (Diplocarpon rosae) of roses.” (And before you ask, no, I can’t attach the article or link to it. You’ll need to read it in the journal itself or wait for a year when the organization makes it available to everyone.)
Anyway, this study looked at eight different self-identified biostimulants, including Superthirve (which every gardener must have heard of by now). In addition to Superthrive, the other products tested were Maxicrop Original, Resistim, Bioplex, Fulcrum CRV, Redicrop, Crop Set, and Systhane. Purported active ingredients within this group include seaweed extract, molasses, vitamin B, and Lactobacillus fermentation product.
And the $64,000 question – did they work? Here’s the authors’ summary: “Irrespective of pathogen or concentration applied, none of the biostimulants used in this investigation provided a significant degree of Guignardia leaf blotch or black spot control compared to water-treated controls.” In other words, you can expect the same results by spraying your black spot-infested roses with water compared to any of these biostimulant products.
The authors end their article with a caveat sure to warm the cockles of every Garden Professor’s heart: “Results of this study indicate that where independent scientific data are not available to support the pathogen control claims of the manufacturer, then using an unevaluated biostimulant for this purpose is not recommended.”
(I’m glad this article is finally out. I was one of the peer reviewers for it, and I’ve been wanting to share the results on the blog ever since I read it.)