Tales of a Phytopath: Fusarium oxysporum f. sp. cannabis
The Fusarium oxysporum is a species complex that includes soilborne plant pathogens as well as many nonpathogenic strains. From a diagnostician’s point of view, the mere presence of Fusarium oxysporum in any given crop or tissue is not the final verdict, even if it is isolated from symptomatic plants. In fact, we frequently recover nonpathogenic F. oxysporum isolates from healthy or asymptomatic plants. The final diagnosis must be reached by confirming the pathogenicity of the isolated F. oxysporum on the host it was isolated from (a.k.a. pathogenicity test) or, if available, by using highly specific molecular tests. For instance, if an F. oxysporum isolate recovered from a wilted watermelon sample causes disease on a susceptible watermelon variety or cultivar upon re-inoculation (pathogenicity test), then the isolated F. oxysporum is indeed pathogenic, and in this case, it is called Fusarium oxysporum f. sp. niveum (FON).
You may wonder, what does “f. sp.” mean? Simply put, formae specialis or f. sp. is a subdivision used to further classify some plant pathogenic fungal species, such as F. oxysporum. The reason for this classification is that these fungi comprise multiple specialized strains (i.e., f. sp.) that can cause disease on a small number of plant species (generally one). To this day, over 100 formae speciales (f. sp.) have been confirmed and described in F. oxysporum. Fusarium oxysporum f. sp. cannabis (FOC) is one such strain (or a group of strains) that causes significant economic losses in Cannabis sativa. However, due to the current absence of reliable molecular tests, the pathogenicity test remains the only dependable method to distinguish it (i.e., FOC) from nonpathogenic F. oxysporum strains that may be recovered from hemp.
The following is an example of our approach to identifying FOC in hemp samples. In October 2024, we visited a hemp farm in Northern California. Most plants showed varying stages of root and crown rot and wilting symptoms (see the following photos). The collected root and crown samples were brought back to CSP Labs for analysis.
As expected, F. oxysporum was consistently isolated from all collected symptomatic crown tissues. Upon completing the pathogenicity test, we were able to reproduce the root and crown rotting symptoms on hemp plants inoculated with the recovered “suspect” isolates, confirming that they were indeed Fusarium oxysporum f. sp. cannabis (FOC). To demonstrate the importance of the pathogenicity test, we inoculated a portion of our test plants with a previously isolated nonpathogenic F. oxysporum strain from hemp.
Even though both isolates are morphologically indistinguishable, unlike the FOC isolates, the nonpathogenic isolate [from hemp] did not cause disease on inoculated hemp plants (see the following photos). This result underscores the importance of the pathogenicity test for any “suspect” F. oxysporum isolate before reaching the final diagnosis.
At CSP Labs, we utilize our expertise in classic and advanced/molecular phytopathology tools and techniques to provide our customers with the most reliable and biologically relevant information.
Nonpathogenic F. oxysporum Inoculation
Inoculated by nonpathogenic F. oxysporum isolated from healthy hemp plants showed none of the mentioned symptoms throughout the duration of the pathogenicity test. The general yellowing/chlorosis seen in the photo is caused by nutrition deficiency due to incubation in nutrition-poor medium.
Pathogenic F. oxysporum Inoculation
Inoculated with pathogenic F. oxysporum (i.e. Fusarium oxysporum f. sp. cannabis (FOC)) isolated from symptomatic samples. Inoculated hemp plants with this isolate showed severe stunting, wilting, root/crown rot, severe vascular discoloration, and eventually death.
