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

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.

First Report of Muskmelon Fruit Rot Caused by Fusarium sulawesiense in China

Muskmelon (Cucumis melo L.) is a widely cultivated and economically important fruit crop worldwide. In June 2022, fruit rot symptoms were observed on ripening muskmelons (cv. Boyang) in Shouguang City (36.81°N 118.90°E) of China. To determine the causal agent, we surveyed 200 muskmelon plants in about 1000 m2 of planting area and collected diseased muskmelons. Approximately 20% of muskmelon fruits had symptoms, and yield loss averaged 20%. Water-soaked lesions were observed on the surface and the fruit rotted from inside. Lesions were covered with white mycelium. Rotted fruit were surface-disinfested with 1% NaOCl for 1 min, 75% ethanol for 30 s, and washed three times with sterile water. Pieces (1 cm3) were cut from the disinfested fruit, placed on potato dextrose agar (PDA), and incubated at 25°C for 1 week. Ten isolates with similar morphology were obtained and isolates SG66 and SG68 were selected for further characterization. Colonies maintained on PDA in the dark had an average radial growth rate of 10-12 mm/d at 25°C. Surface was white, velvety to felty mycelium. Reverse was white to pale wheat. Diffusible pigments were absent. On carnation leaf agar, sporodochia appeared as slimy dots, macroconidia were 3- to 5-septate, 20-35 × 3-5 μm, falcate, with a pronounced dorsiventral curvature, with blunt to papillate apical cell, and barely to distinctly notched basal cell. Microconidia and chlamydospores were not observed. These morphological characteristics were consistent with descriptions of Fusarium sp. DNA was extracted from isolates SG66 and SG68 using a CTAB method. Nucleotide sequences of the internal transcribed spacers (ITS) (White et al. 1990), calmodulin (CAM), RNA polymerase II second largest subunit (RPB2), and translation elongation factor 1-α gene (TEF1) (Xia et al. 2019) were amplified using generic primers, the products sequenced, and sequences deposited in GenBank (ITS: OP251362, OP251363; CAM: OP266024, OP266025; RPB2: OP266028, OP266029; TEF1: OP266026, OP266027). Isolates SG66 and SG68 clustered with Fusarium sulawesiense (85% bootstrap) (Maryani et al. 2019). The Fusarioid-ID database pairwise alignment of ITS (526 bp), CAM (534 bp), RPB2 (861 bp), and TEF1 (636 bp) sequences from isolate SG66 showed 99.6% (98.9% coverage), 100% (100% coverage), 100% (100% coverage) and 100% (98.4% coverage) similarity with the corresponding sequences (GQ505730, LS479422, LS479855 and GQ505641), respectively, of the reference strains of F. sulawesiense (InaCC F940 and NRRL 34059). To perform a pathogenicity test, 10 μl of conidial suspensions (1 × 106 conidia/ml) were injected into ten muskmelon fruit using a syringe, and ten control fruit were inoculated with 10 μl of sterile distilled water. The test was repeated three times. After 7 days at 25°C, the pulp of all inoculated muskmelons began to rot, and the lesion expanded from the inside to the fruit surface at the injection site and became covered with white mycelia. No symptoms developed on the control fruit. The fungus was successfully re-isolated from infected tissues and confirmed as F. sulawesiense by morphological and phylogenetic analyses. F. sulawesiense has previously been reported on yellow melon (Canary) in Brazil (Lima et al. 2021) and on a range of hosts, including Luffa aegyptiaca, in China (Wang et al. 2019). To our knowledge, this is the first report of muskmelon fruit rot caused by F. sulawesiense in China.

Susceptibility of Aphis craccivora (Hemiptera: Aphididae) to three entomopathogenic Fusarium species

The black aphid (Aphis craccivora) is an insect pest that can cause significant losses to different agricultural crops. Entomopathogenic fungi can be good options for controlling this insect. Fusarium species have shown promising results in the biological control of several agricultural pests, mainly of the order Hemiptera. This study investigated the susceptibility of A. craccivora to 27 Fusarium isolates, distributed among F. sulawesiense (4), F. pernambucanum (6) and F. caatingaense (17). The viability of the conidia of all isolates was assessed by measuring their germination rate. Pathogenicity tests were conducted at 10⁷ conidia/mL, and the best-performing isolate was further tested at different concentrations (10⁴ to 10⁸ conidia/mL). Data were analyzed using ANOVA, Tukey's test at 5 %, and R for calculating lethal times (LT50,90) and lethal concentrations (LC50,90). All isolates had viable conidia with germination rates between 92.67 % and 100 %. Mortality rates ranged from 17.22 % to 90.23 %. F. pernambucanum URM 7559 had the shortest lethal times (LT50 of 2.24 days and LT90 of 4.42 days), followed by F. sulawesiense URM 7555 (LT50 of 2.35 days and LT90 of 4.77 days) and F. caatingaense with LT50 of 3.93 days for URM 6784 and LT90 of 8.27 days for URM 6807. The three Fusarium species tested, especially F. pernambucanum, showed promise in the biological control of A. craccivora. Although the results are promising, additional studies are needed to evaluate the safety, field efficacy and environmental impacts of Fusarium use, focusing on the interaction with the agricultural ecosystem and the risks to non-target organisms.