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The potential of detached leaf assay in cannabis:

"Screening for disease control options can be time consuming and costly"

In greenhouse production, grey mold caused by Botrytis is a widespread and damaging disease affecting medicinal cannabis. "Fungicide options to control this disease are extremely limited due to the regulations surrounding fungicides and chemical residues as the product end users are medical patients. Screening for alternative disease control options, such as biological and organic products, can be time-consuming and costly." In a recent study, researchers optimized and validated 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 product efficacy.

The researchers explain that screening for host varietal resistance or susceptibility to pathogen infection and product efficacy using whole plants is time-consuming, requires segregated space within the glasshouse away from healthy plants, and often leads to plant death. "Alternatively, screening for varietal resistance/susceptibility and product efficacy using detached leaf assays has the potential to be a fast and reliable alternative to whole plant screening."

According to the researchers, there is a need to rapidly screen for varietal susceptibility as well as product effectiveness in reducing disease symptoms in this emerging industry that is dealing with fungal pathogens like Botrytis regularly. "The validation of a detached leaf assay method allows for the rapid screening of varieties or products with high replication and consistency, without the risk of contamination or loss of stock plants."

Study results
The researchers tested eight industrial hemp varieties for susceptibility to B. cinerea infection. Using detached leaves from a susceptible variety, they screened a variety of chemical or organic products for efficacy in controlling the lesion development caused by B. cinerea. "The application of the pathogen as a mycelium contained on an agar plug onto unwounded leaves was determined as the most effective method to obtain reproducible results in this study. These results correspond well with whole-plant assays."

According to the researchers, detached leaf assay could also help in the development of disease-resistant cultivars. "The detached leaf assay is a fast, cost-effective method to screen existing and new cultivars and crosses between varieties before growing them on a large scale within growing facilities. Susceptible varieties or crosses can be eliminated from the breeding program before spending time, space, and money on susceptible lines."

The study also screened a variety of chemical or organic active ingredients for efficacy against B. cinerea lesion development. "The most consistent reduction in lesion growth was obtained from the treatment that contained Tau-fluvalinate and Myclobutanil as active ingredients. Both ingredients can have mild to moderate toxic effects on humans or non-target organisms and, therefore, may only be appropriate for foliar control of B. cinerea and other pathogens well prior to inflorescence harvest," the researchers say.

The study showed that control of B. cinerea was also accomplished using softer chemical options, including a treatment that contained a number of potassium salts. "Products containing these active ingredients present a promising alternative for the control of bud rot in medical cannabis."

Read the complete study at mdpi.com