Fungicidal activities of camptothecin semisynthetic derivatives against Colletotrichum gloeosporioides in vitro and in mango fruit

Increasing attention to the resistance of plant pathogenic fungi to fungicides and their residues impels the development of more efficient fungicides with novel mechanisms of action. Camptothecin (CPT-1) is a naturally occurring quinoline alkaloid with significant antineoplastic and pesticidal activities. To evaluate the anti-fungicidal activities of CPT-1 and its derivatives against postharvest mango anthracnose disease and their potential as a lead compounds for fungicide development, CPT-1 and its semisynthetic derivatives (CPT-2-15) in vitro and in vivo against Colletotrichum gloeosporioides were tested. Five of the agents, CPT-1, 16a-thiocamptothecin (CPT-2), 7-ethyl-camptothecin (CPT-6), 9-methoxycamptothecin (CPT-11) and 7-benzyl-chloro-camptothecin (CPT-15) at doses of 20 mg L-1 produced the effective mycelial growth inhibition of C. gloeosporioides. Among these, CPT-11 exhibited the strongest inhibition, with EC50 and EC90 values of 1.79 and 7.37 mg L-1, respectively. At a dose of 100 mg L-1, 10 of the tested derivatives inhibited the germination of C. gloeosporioides spores. In addition, CPT-1, - 2, -6, -11 and - 15 showed different abilities to inhibit appressorium formation. Dipping treatment with CPT-11 at 500 mg L-1 exhibited an equivalent efficiency in suppressing postharvest anthracnose in three different cultivated varieties of mango fruit when compared with the commercial fungicide carbendazim at the same concentration, but it was less effective than prochloraz. Scanning and transmission electron microscopy observations revealed that CPT-11 caused alterations in the hyphal morphology and ultrastructures of C. gloeosporioides, including swelling, abnormal branching, and the rupturing and thickening of cell walls. These findings indicated that CPT-11 could be a potential antifungal lead compound for controlling postharvest mango anthracnose disease through a different mode of action than camptothecin.