Humans have been domesticating crops for thousands of years. In the process, we have selected for many desirable characteristics. However, in doing so, we have also decreased plants capacities to defend themselves from disease.
The mass domestication of cannabis and commercialization of THC has, unsurprisingly, also led to decreased capacity for cannabis to protect itself against pathogens and pests. Current estimates suggest that up to 20% of commercially produced cannabis infected with powdery mildew: a fungal disease that damages plants, inhibits photosynthesis, and leads to the development of visible mold on a crop.