Isolation, identification, biological characteristics and inhibitors screening of pathogen causing ring rot disease of guava fruit in Fujian province

【Objective】Guava, acting as a characteristic subtropical fresh fruit in southern China, is rich in vitamin C, dietary fiber and other nutrients, which has the good nutritional value and medicinal effects. However, in the harvest season at high temperature and with high humidity, the postharvest guava fruit is prone to the fungal diseases, resulting in the disease infection and fruit quality degradation. The occurrence of ring spot disease is a serious postharvest disease for guava fruit, which has seriously restricted the development of guava industry in Fujian province. The symptom of ring spot disease of guava fruit generally occurs in one week of storage at normal temperature, and the disease incidence of guava fruit Xiguahong is higher (about 30%), which has seriously restricted the maintenance of guava fruit quality after harvest. Therefore, the aim of this study was to isolate and identify the pathogen causing ring spot disease of guava fruit cv. Xiguahong in Fujian province and its biological characteristics, to study the effects of the infection of the pathogen on the postharvest quality attributes of guava fruit, andto explore the effects of key inhibitors of ε- poly- L- lysine (ε- PL), SA (salicylic acid) and melatonin (MT) on the inhibition rate of pathogen in vitro, so as to provide the theoretical references for controlling the ring spot disease and stabilizing the quality attributes of postharvest guava fruit.【Methods】(1) The pathogen was isolated from the rotten guava fruit, and identified via using the morphological characteristics, like mycelial morphology and conidium morphology, molecular identification and phylogenetic tree analysis based on sequences of rDNA-ITS, TUB and TEF-1α. (2) The biological characteristics of mycelia growth, including the different conditions of carbon source, nitrogen source, pH value and temperature, of the pathogen causing ring spot disease of guava fruit were studied. (3) The Xiguahong guava fruit was treated in the following two methods after washing and disinfection: Guava fruit was inoculated by 20 microliters of pathogen spore suspension at the concentration of 1×105 spores·mL-1 and treated with sterile distilled water (control group), severally. After inoculation, the treated guavas were stored at 28 ℃ with 90% relative humidity for 6 days. During storage, the effects of pathogen causing ring spot disease on the quality attributes (e. g., lesion diameter, fruit firmness, cell membrane permeability or hue angle h value) of harvested guava fruit were measured. (4) The antifungal effects of different inhibitors containing ε-PL (0, 0.125, 0.250, 0.500, 1.000, 2.000 and 4.000 mg · mL- 1 ), SA (0, 0.500, 1.000, 1.500 and 2.000 mg·mL-1 ) and MT (0, 1.000, 2.000, 4.000 and 8.000 mg·mL-1 ) on the mycelia growth and inhibition rate of pathogen causing ring spot disease of guava fruit in vitro were evaluated, and thus the optimal inhibitor was screened out.【Results】(1) According to the morphological characteristics of mycelium and conidium, and the homology of more than 98% with similar strains based on phylogenetic tree by rDNA-ITS, TUB and TEF-1α sequences, the pathogen causing ring rot disease of harvested guava fruit in Fujian province was identified as Neopestalotiopsis clavispora. (2) The results of biological characteristics displayed that the glucose and D-fructose, peptone, 7, and 25 ℃ were the optimum conditions of carbon source, nitrogen source, pH value and temperature for mycelia growth of N. clavispora in guava fruit, separately. (3) Compared with non-N. clavispora-inoculated guava fruit, N. clavispora-inoculated guava fruit revealed the higher levels of lesion diameter and cell membrane permeability, but the lower values of fruit firmness and hue angle h. Specially, on the sixth day of storage, the lesion diameter and cell membrane permeability in N. clavispora- inoculated guava fruit were 29.41 times and 1.39 times more than those of non-N. clavispora-inoculated guava fruit, respectively, while the fruit firmness or hue angle h value of N. clavispora-inoculated guava fruit were 66.96% or 91.10% of non-N. clavispora-inoculated guava fruit, severally. (4) The treatments of appropriate concentrations of different inhibitors, such as ε-PL, SA and MT, showed the obvious inhibitory effects on the mycelia growth of N. clavispora in vitro, which could be used as the potential technologies for inhibiting N. clavispora-induced ring spot disease of postharvest guava fruit. Further comparison showed that the inhibitory effect of 4.000 mg ·mL-1 ε-PL was the best, which could basically inhibit the mycelia growth of N. clavispora of guava fruit.【Conclusion】N. clavispora was the pathogen causing the ring spot disease of harvested guava fruit in Fujian province. The biological characteristics of N. clavispora of guava fruit were also obtained. The infection of N. clavispora could accelerate the occurrence of fruit disease, and seriously reduce the quality properties in guava fruit after harvest during the entire storage. The key inhibitors such as ε-PL, SA and MT could be used as the potential technologies for inhibiting N. clavispora-induced ring spot disease of postharvest guava fruit. Therefore, these results might provide a scientific basis for controlling the postharvest decay and extending the storage time for guava fruit.