Identification, pathogenic mechanism and control of Rhizopus oryzae causing postharvest fruit rot in pumpkin

Fruit rot, caused by a variety of pathogens, is one of the most severe postharvest diseases of pumpkin ( Cucurbita moschata ), seriously affects the storage life of postharvest pumpkin. Understanding pathogenic factors is the premise and basis for taking effective control measures. In 2022, an uncommon fruit rot was observed in the storage in Harbin, China. Five different fungal isolates were obtained from infected pumpkin fruits, and identified as Rhizopus oryzae based on morphological, molecular, and phylogenetic analysis. To our knowledge, this is the first report of R. oryzae causing fruit rot in pumpkin in China. Pathogenicity tests of the isolates revealed that most isolates were virulent pathogens. R. oryzae isolates causing pumpkin fruit rot greatly increased cutinase polygalaturonase, pectinesterase, pectin lyase, β-galactosidase, cellulase, and hemicellulose activities, and released their associated cell wall component products. The rapid disassembly of the host cells walls by these enzymes was concomitant with flesh softening and rapid growth of R. oryzae, and was probably the main determinant of this fungus as a pathogen. In addition, R. oryzae isolates infected muskmelon, strawberry, wild peach, apple, emerald melon, white pear and tomato fruits, but not cocozelle and cucumber fruits. R. oryzae isolates were the most sensitive to pyraclostrobin, moderately sensitive to physcion. Pyraclostrobin, the most effective fungicide evaluated, completely inhibited sporangiospore germination at ＜0.01 mg·L −1. Pyraclostrobin treatment of inoculated pumpkin fruit with 5.0 × 10 −6 mg·L −1 reduced subsequent disease by approximately 80%. Therefore, the use of intercropping host and non-host plants, and chemical fungicides can be considered to control the fruit rot in pumpkin. These results provide a basis for controlling R. oryzae in postharvest pumpkin fruit.