Control Effect of Pyridine-2,6-Dipicolinic Acid Treatment on Postharvest Black Spot of Pear Fruit and Possible Antifungal Mechanism

Black spot, caused by Alternaria alternata, is one of the major postharvest diseases of pear fruit, which can cause severe economic losses. The inhibitory effect of pyridine-2,6-dipicolinic acid (DPA) produced by Bacillus subtilis on the growth of A. alternata and black spot development was studied through in vitro and in vivo tests, and its possible mechanism of action was also evaluated. Results showed that DPA significantly inhibited the colony growth, spore germination and germ tube elongation of A. alternata. The colony diameter of A. alternata treated with 2.0 mmol/L DPA was only 51.76% as compared to the control group after incubation for nine days, and A. alternata growth was completely inhibited by 4.0 mmol/L DPA treatment. DPA treatment had a significant control effect on the development of black spot on pear fruit. The lesion diameter on Zaosu pear fruit treated with 20.0 mmol/L DPA was only 55.17% as compared to the control after 11 days of storage. Furthermore, it was shown that DPA treatment significantly increased the cell membrane permeability and mycelial conductivity of A. alternata, leading to protein and nucleic acid leakage. At the same time, DPA treatment reduced the ergosterol content and increased the malondialdehyde content of A. alternata. The above results indicate that DPA has strong antifungal activity through delaying A. altenata growth, destroying membrane integrity, increasing membrane permeability and promoting membrane lipid oxidation.