Effect of pseudolaric acid B on biochemical and physiologic characteristics in Colletotrichum gloeosporioides.

In our previous study on natural products with fungicidal activity, pseudolaric acid B (PAB) isolated from Pseudolarix amabilis was examined to inhibit significantly mango anthracnose (Colletotrichum gloeosporioides) in vivo and in vitro. In the current study, sensitivity of 17 plant pathogenic fungi to PAB was determined. Mycelial growth rate results showed that PAB possessed strong antifungal activities to eleven fungi with median effective concentration (EC50) values ranging from 0.087 to 1.927µg/mL. EC50 of PAB against spore germination was greater than that of mycelium growth inhibition, which suggest that PAB could execute antifungal activity through mycelial growth inhibition. Further action mechanism of PAB against C. gloeosporioides was investigated, in which PAB treatment inhibited mycelia dry weight, decreased the mycelia reducing sugar and soluble protein. Furthermore, PAB induced an increase in membrane permeability, inhibited the biosynthesis of ergosterol, caused the extreme alteration in ultrastructure as indicated by the thickened cell wall and increased vesicles. These results will increase our understanding of action mechanism of PAB against plant pathogenic fungi.

Antifungal activity of compounds extracted from Cortex Pseudolaricis against Colletotrichum gloeosporioides.

Cortex Pseudolaricis is the root bark of Pseudolarix amabilis Rehder, found only in China, and has been widely used in folk antifungal remedies in traditional Chinese medicine. In order to find the natural antifungal agents against mango anthracnose, eight compounds, namely pseudolaric acid A (1), ethyl pseudolaric acid B (2), pseudolaric acid B (3), pseudolaric acid B-O-ß-d-glucoside (4), piperonylic acid (5), propionic acid (6), 3-hydroxy-4-methoxybenzoic acid (7), and 4-(3-formyl-5-methoxyphenyl) butanoic acid (8) were isolated from the ethanol extracts of Cortex Pseudolaricis by bioassay-guided fractionation and evaluated for in vitro antifungal activity against Colletotrichum gloeosporioides Penz. Results demonstrated that all of the eight compounds inhibited the mycelial growth of C. gloeosporioides at 5 µg/mL. Among them, pseudolaric acid B and pseudolaric acid A showed the strongest inhibition with the EC50 values of 1.07 and 1.62 µg/mL, respectively. Accordingly, both Pseudolaric acid B and Pseudolaric acid A highly inhibited spore germination and germ tube elongation of C. gloeosporioides. Dipping 100 µg/mL pseudolaric acid B treatment exhibited more effective suppression on postharvest anthracnose in mango fruit when compared to the same concentration of carbendazim. Scanning electron microscopy observations revealed that pseudolaric acid B caused alterations in the hyphal morphology of C. gloeosporioides, including distortion, swelling, and collapse. Pseudolaric acid B caused the mycelial apexes to show an abnormal growth in dimensions with multiple ramifications in subapical expanded areas with irregular shape. These findings warrant further investigation into optimization of pseudolaric acid B to explore a potential antifungal agent for crop protection.