The epidemiology and management of Botrytis cinerea causing bud rot on greenhouse cultivated cannabis (Cannabis sativa L.)

Botrytis cinerea Pers. causes bud rot on Cannabis sativa L. (Cannabis) inflorescences, significantly reducing yield and quality. We investigated the timing of B. cinerea spore infection during the 49-day flowering period (FP) and how environmental conditions and host genotype influence disease development. Artificial spore inoculations made at 14, 21 or 28 days of the FP resulted in the highest disease development compared to inoculations made at 7 or 35 days. Visible mycelial growth within inflorescences was observed at 33–41 days, regardless of inoculation time. The disease severity under greenhouse conditions from natural inoculum was highest during summer and fall seasons (June to November), which resulted in bud rot incidence of 1–13%, depending on the genotype; lower disease incidence occurred at other times. The highest disease was observed during September and October, which corresponded to average daily outdoor absolute humidity of 14 g m−3 and temperatures of 20–22°C. Notably, humidity and temperatures within 49-day-old inflorescences were higher by 15.4% and 2.5°C, respectively, compared to ambient conditions. These findings suggest that bud rot development is strongly influenced by environmental conditions within and outside the greenhouse, which can impact spore germination and subsequent infection. Among management practices evaluated, enhanced air circulation around inflorescences reduced bud rot incidence by 66–92%. Additionally, applications of Rootshield HC (Trichoderma harzianum, 10 g L−1) on days 14, 21 and 28 of the FP reduced disease by 47–91%. Treatments with Double Nickel (Bacillus amyloliquefaciens), LifeGard (Bacillus mycoides), Prestop (Gliocladium catenulatum) and Regalia Maxx (Reynoutria sachalinensis) provided varying levels of disease reduction.