Fungicide options to control diseases in hemp are limited due to the lack of off-label permits available, which require controlled residue tests to prove safety, efficacy and absence of phytotoxicity. New crop varieties may not reveal themselves as susceptible to a particular pathogen until grown in a particular location, climatic zone or under a specific combination of these factors. Performing the required screening for disease susceptibility and control options can be time-consuming and costly.
Here, we optimise and validate a detached leaf assay as a quick and non-destructive method to evaluate interactions between plants and pathogens, allowing the assessment of potential pathogens' infectivity and control product efficacy.
Grey mould caused by Botrytis cinerea Pers. is one of the most widespread and damaging diseases affecting industrial hemp and medicinal Cannabis crops. We tested eight industrial hemp varieties for susceptibility to B. cinerea infection. Using detached leaves from a susceptible variety, we screened a variety of chemical or organic products for efficacy in controlling the lesion development caused by B. cinerea. A consistent reduction in lesion growth was observed using treatments containing Tau-fluvalinate and Myclobutanil, as well as the softer chemical alternatives containing potassium salts. The performance of treatments was pH-dependent, emphasizing the importance of applying them at optimal pH levels to maximise their effectiveness. The detached leaf assay differentiated varietal susceptibility and was an effective method for screening treatment options for diseases caused by Botrytis. The results from the detached leaf assays gave comparable results to responses tested on whole plants.
Our method presents a fast and effective method to identify varietal susceptibility/ resistance and efficacy of control methods. This method could be adapted to screen for additional pathogens of interest in industrial hemp.