AbstractThe most recent outbreaks of boxwood blight caused byCalonectria pseudonaviculata, in Hyrcanian forests of Iran, have sparked the need for research to identify eco‐friendly fungicides for management purposes. Our study was conducted to test the efficacy of humic substances againstC. pseudonaviculataunder both in vitro and in vivo conditions. For this purpose, humic acids (HA) and fulvic acids (FA) extracted from tea waste compost (HAT & FAT), sawdust compost (HAW & FAW) and manure compost (HAM & FAM) were tested against hyphal growth and conidial germination ofC. pseudonaviculata. In addition, EC50and MIC50were determined for all humic substances tested. While none of the substances caused complete inhibition of the fungus, they showed a range of antifungal activities. Based on the high inhibitory efficacy and low related EC50and MIC50, HAT and FAT provided the highest efficacy against the pathogen. In the greenhouse, curative and protective effects of the humic treatments at 2,000 mg/L were also investigated against disease severity. Of the substances tested, FAW and FAM showed the weakest antifungal effects and HAT had the greatest curative and protective effects. These results indicate that HAT application can be used as an eco‐friendly control method for boxwood blight epidemics in the disease hotspots of Hyrcanian forests.