Copper sulphate reduces Pythium root rot and powdery mildew development on greenhouse-grown cannabis ( Cannabis sativa L. marijuana) plants

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Copper sulphate has a long history of use in agriculture to manage fungal, bacterial and oomycete pathogens. Several concentrations of copper sulphate pentahydrate (CSP) were tested for inhibition of mycelial growth of five pathogens affecting cannabis (Cannabis sativa L.) in vitro. At 0.05 mL L−1 (50 ppm) CSP, growth in potato dextrose broth was completely inhibited for Alternaria alternata, Penicillium olsonii, and Pythium myriotylum; growth of Fusarium oxysporum and Botrytis cinerea was reduced by 30% and 80%, respectively. Colony growth of P. myriotylum was reduced at 0.001 mL L−1 (10 ppm) CSP. When added to hydroponic nutrient solution, CSP at 2.5 ppm significantly reduced the development of Pythium root rot caused by P. myriotylum and oospore development in root tissues was also reduced. Plant height, root length, and root and shoot fresh weights were increased following CSP treatment. Four applications of 100 ppm CSP to powdery mildew-infected plants at weekly intervals reduced disease development by 70%. To assess the bioaccumulation of copper levels in cannabis plants, flowering plants were treated under greenhouse conditions with 4 weekly applications of 100 mL L−1 CSP. Copper levels in fresh leaves and inflorescence tissues were 158–244 µg g−1 and 60–97 µg g−1, respectively, over four crop cycles compared to 6–10 µg g−1 in the controls. In dried inflorescences from these plants, copper levels were 31–45 µg g−1 compared to 4–7 µg g−1 in the controls. Such levels may be noncompliant with current regulations. Treatment of roots with low levels of CSP (2.5 ppm) provides an optional approach to managing Pythium root rot.