Cannabis powdery mildew was previously associated with at least two parasitic fungi Podosphaera macularis (formerly Sphaerotheca macularis), Leveillula taurica (6) and recently Golovinomyces cichoracearum was demonstrated to cause PM in cannabis (7).

The effect of PM is often devastating, resulting in a high percentage of unmarketable products and heavy yield reduction. PM is of special concern in the industry due to the general susceptibility of clonal material, the pathogenicity of the fungus, large seasonal spore loads, and lack of effective and approved mitigation strategies.

Only little scientific information is available about the genetic diversity of PM pathogens and genetic resistance mechanisms to these pathogens in cannabis. However in other crops, using genetic disease resistance (either naturally occurred or synthetically mutated alleles) to control PM disease incidence, is regarded as the most effective approach and has been successfully applied for a wide range of crops such as wheat, barley, tomato and pepper (10),(5). The use of crops that are genetically resistant to pathogens not only decreases the cultivation costs and chemical residues but also reduces the environmental risks caused by chemical control of plant diseases.

In other species, resistance to PM is conferred by dominant defense mechanisms that directly target the pathogen and may be encoded by a single major gene. In other cases, PM resistance was found to be determined by one or several dominantly inherited single loci. The most impressive resistance is conferred by the recessive Mildew resistance Locus O (mlo) (8). Naturally occurring or experimentally induced mutations in this locus can lead to the loss of function of the coded transmembrane protein MLO, resulting in strong broad-spectrum immunity to the fungus. The MLO gene family is encoded by a variable number of loci from which only one locus—or, in some cases, multiple loci—needs to be mutated to achieve complete immunity, meaning several MLO deletions are likely to increase resistance. The simplicity of the MLO-associated resistance has attracted multiple experimental mutagenic and gene editing projects that are currently being implemented and may confer efficient, broad-spectrum immunity/resistance to PM in cannabis.