First report of internal black rot on Carica papaya fruit caused by Fusarium sulawesiense in China.

Abstract

Papaya (Carica papaya L.) is a tropical fruit consumed worldwide due to its nutritional, medicinal and pharmacological properties. In China, papaya was widely planted in Guangdong, Guangxi, Hainan, Yunnan, Fujian and Taiwan provinces. From September to December in 2015-2020, fruit with internal black rot disease was observed in papaya plantation in Xuwen, Guangdong province (N20°20'9"; E110°14'45"), approximately 5% fruits on about 85% trees were infected every year. The infected fruits showed the symptom of 'false-ripening' and the pericarp color changed from green to yellow earlier than that of normal fruits. In the cavity of diseased fruits, the sarcocarp black rotted and conspicuous mycelia were observed. Mycelia and infected tissues from symptomatic fruits were picked up, placed on potato dextrose agar (PDA) with 50mg/L ampicillin and incubated at 25± 2 ℃ in the dark. The fungus was purified by spore dilution plate method. Fast-growing colonies with dense, floccose, cottony mycelium were initially white gradually becoming buff brown. Macroconidia were falcate, 3-5 septa with foot-shaped cell and 10.35-41.50 (av. 25.41±6.82) ×1.90-5.95 (av. 3.67±0.85) µm (n>140) in size after 7 days of incubation on carnation leaf agar (CLA). There were scarce microconidia. Chlamydospores were intercalary, solitary or in chains, globose or irregular, hyaline to light brown. The morphological characteristics of the fungus were similar to that of Fusarium sulawesiense (Maryani et al. 2019). The internal transcribed spacer region (ITS) (KU881904 and KY436233), translation elongation factor 1-alpha (tef1) (KU894408 and KY436232), and RNA polymerase second largest subunit (rpb2) (KU894409 and KY436231) were sequenced from two isolates to cofirm species identification. Blast analysis in the FUSARIUM-ID and the NCBI databases revealed above 99 to 100% identity match with the F. sulawesiense strains NRRL34056, NRRL34059, NRRL34004 and NRRL43730 (Xia et al 2019). Maximum likelihood (ML) analysis and Bayesian inference (BI) based on the concatenated sequences using RAxML v.1.0.0 and MrBayes v. 3.2.1 software revealed that the isolates were resolved in the same clade with the F. sulawesiense strains. Thus, the fungus was identified as F. sulawesiense based on morphological characteristics and molecular criteria. To confirm pathogenicity, five healthy fruits were injected with 200 μl of spore suspension (approximately 104 spores/ml) in the field and laboratory, and isovolumetric sterile water served as control. Each fruit was sealed with a plastic bag and kept at natural temperature (about 25-30 ℃). All the inoculated fruits developed typical symptoms after 30 days in the field and 15 days in the laboratory, whereas no symptoms were observed on the control fruits. F. sulawesiense was reisolated from inoculated fruits, but not from non-inoculated fruits. F. sulawesiense displayed a broad host which included Oryza sativa, Musa nana, Citrus reticulata, and Colocasia esculenta etc. in China (Wang et al. 2019). To our knowledge, this is the first report of F. sulawesiense causing internal black rot on papaya fruit. This work is important for papaya growers to prevent this disease in time.