A paper just published in Phytopathology by Pennerman, Dilla-Ermita and Henry (https://doi.org/10.1094/PHYTO-05-23-0154-R) reports that the host range of Macrophomina phaseolina has been exaggerated for decades. After a critical review of the literature (894 publications to be exact), the authors found that the claim that M. phaseolina causes disease in over 500 hosts was grossly exaggerated. It turns out that in all too many cases, proper demonstration of pathogenicity (what pathologists know as Koch’s postulates) was not reported. Instead, Pennerman et al’s critical review of the literature showed that the host range is closer to 100 hosts; 74 hosts confirmed by Koch’s postulates and 23 where all the steps from Koch’s postulates were completed, except for recovery of the pathogen from symptomatic tissues. That’s a big difference and it shows how claims can be made and accepted without rigorous examination in the scientific community. Kudos to the authors for setting the record straighter. I say “straigher” because the story is never over. It may be that M. phaseolina‘s host range is broader than we now understand, but that needs to be properly demonstrated before we make the claim.

Why should strawberry growers care? M. phaseolina has become a dominant pathogen in California. Our disease surveys (Steele et al. 2023 Watsonville-Salinas; Steele et al. 2023 Santa Maria) showed that M. phaseolina was at or near the top of all the root pathogens isolated from diseased plants in each growing district (Fig. 1). Our understanding of this pathogen needs to be accurate. With a host range of over 500, M. phaseolina would have one of the broadest host ranges of all plant pathogens. As a general rule, the broader the host range, the less specialized the pathogen is for its host. The evidence we have indicates that M. phaseolina affecting California strawberries is specific to strawberries (Burkhardt et al. 2019). That matters!