Researchers Identify the Root Pathogens Affecting Hydroponically Grown Cannabis Plants
Cannabis and hemp are both members of the family Cannabis sativa L., but while the risks of pathogen infection have been well scrutinized for hemp, the same cannot be said for cannabis.
Hemp has been cultivated for thousands of years for use as a fiber in the textile industry. As a result of the crop’s industrialization, any threats or diseases that might affect hemp were researched extensively in the interest of maximizing usable crop yields. Conversely, the high levels of the intoxicating compound tetrahydrocannabinol (THC) in cannabis led to widespread cannabis prohibition which, until recently, prevented the same levels of research being done on matters concerning cannabis.
With growing numbers of states and countries choosing to legalize cannabis for medicinal and recreational use, so emerged a renewed interest in researching the type of diseases and pathogens that could affect cannabis cultivation.
Identification of pathogens
Dr. Zamir Punja and his research team at the Simon Fraser University in British Columbia have recently published the full results of their three-year-long study into root diseases affecting hydroponically grown cannabis plants.
By sampling plants that exhibited symptoms of root disease - such as stunted growth, brown root lesions, root rot, and leaf discoloration due to minor chlorosis - the research group was able to isolate and identify two genera of pathogens that had infected the plants.
The pathogens from the diseased plants were identified by plating affected root samples on a suitable agar-based medium and letting the fungal cultures develop. These were then broadly identified as being from the Fusarium genus and the Pythium genus and sent to the Agriculture and Food Laboratory in the University of Guelph Laboratory Services for polymerase chain reaction (PCR) analysis, in order to identify the exact species of each fungus that was present. The PCR testing indicated the presence of two species of Pythium (P. dissotocum and P. myriotylum) and two species of Fusarium (F. oxysporum and F. solani). At the tail-end of the study, isolates of Pythium aphanidermatum were also observed in a sub-sample of the diseased root cuttings.
Pythium is a parasitic root fungus which most commonly affects young plants or seedlings. Most Fusarium fungi are actually harmless soil microbes, however, the two species identified here have been known to cause root rot and wilting in plants. Both the Pythium and Fusarium species observed here have been detected in other crops, so it can be concluded that these root pathogens are not uniquely adapted to cannabis.
Confirmation of pathogenicity
Following the positive identification of the pathogens, it was important to confirm that these fungal species were in fact responsible for the observed root disease. Pathogenicity tests were carried out using groups of healthy cannabis roots, where one group served as a control and the other groups were inoculated with one of the fungal species.
The roots that were inoculated with P. dissotocum or P. myriotylum experienced visibly stunted growth, as well as a browning and rotting of the roots, in concordance with the originally observed diseased plants. P. aphanidermatum was observed to cause damping-off of the cannabis plants, which is in accordance with what has already been observed with P. aphaanidermatum and hemp. Roots inoculated with F. oxysporum and F. solani exhibited browning and root rot, with F. solani appearing to be the more aggressive of the two species.
As a result it could be safely concluded that these Pythium and Fusarium species are indeed active cannabis plant pathogens.
Risk mitigation for hydroponic cultivation systems
Hydroponic growth systems use a nutrient-rich mineral liquid instead of soil to grow crops. This has the advantage of bypassing any risk due to soil contamination or soil-based microbe infections, and also allows the grower to tailor the minerals the plants receive to ensure healthy growth.
“[Another] advantage of hydroponics is you can visualize the root, you can hold them up, lift them out of the bucket, or just look at them in the rockwool system,” explained lead researcher Dr. Punja in an interview with Cannabis Now. “Visually you can actually see the roots turning brown, and that wasn’t normal.”
While this is helpful for monitoring the plants, the recirculating solution also means that if there is a pathogen present in the system it can be very easily spread and infect a large number of plants. As the pathogens identified here were not found to be exclusive to cannabis, this means that it would be possible for cross-infection to occur between cannabis plants and other hosts, or vice versa.
Dr. Punja explains that this risk of cross-infection may become an emerging concern in countries which are expanding their cannabis markets.
“In Canada, a lot of the greenhouse growers who used to grow tomatoes and peppers are moving to cannabis. And despite how well they try and clean everything up, there is some stuff that’s either left over or moved over from tomatoes and gets on the cannabis.”
The main source of initial pathogen contact and spread in hydroponic systems comes from the use of a contaminated hydroponic mineral solution. Unsterilized or poorly sterilized tools, equipment, and tubing can also harbor the harmful pathogens and lead to system-wide contamination.
In light of these pathogens being discovered, hydroponic cultivators should be mindful to continuously monitor their plants for any sign of pathogenic disease and to make sure that all tools and equipment are properly disinfected before use.