First report of Fusarium falciforme and Fusarium pernambucanum causing root and stem rot on papaya plants in Brazil.

Abstract

Papaya (Carica papaya L.) is one of the major fruit crops of northeast Brazil, with an average annual production of 571,693 tons (IBGE, 2022). In August 2023, papaya plants (hybrid Tainung 01) in the production stage showed dark brown symptoms on roots and stems, wilt progression, and collapse of the plants (disease incidence of 20 - 50 % in sampled fields). Samples were collected from six production farms located in Apodi, Baraúna, and Caraúbas municipalities in Rio Grande do Norte state and Aracati in Ceará state. Small fragments of symptomatic tissues were surface sterilized sequentially in ethanol 70 % (1 sec), sodium hypochlorite 2.5 % (60 secs), and sterilized water. The fragments were placed in potato dextrose agar (PDA), supplemented with 0.05 % tetracycline, and incubated at 28° C with 12 hours of photoperiod for five days. Monosporic cultures were obtained from 10 isolates of Fusarium, characterized by morphology. The translation elongation factor 1 alpha (EF-1α) and the second largest subunit of RNA polymerase (RPB2) genes were partially amplified by PCR and sequenced from all isolates - the sequences were deposited in GenBank (PP723773.1 - PP723792.1). Maximum-parsimony tree was built in the Software MEGA (Version 11.0.10) (Tamura et al. 2021) with the concatenated partial sequences. The species were morphologically characterized in PDA (10 days), Spezieller Nährstoffarmer agar (SNA) (10 days), and carnation leaf agar (CLA) (30 days) (Leslie & Summerell, 2006). Phylogenetic analysis revealed that seven isolates are most closely related to F. falciforme (99 % bootstrap), and three isolates are most closely related to F. pernambucanum (99 % bootstrap). The morphological characters of the isolates correlated with the original descriptions of each species (Leslie & Summerell, 2006). Pathogenicity tests were performed on 45-day-old papaya seedlings (hybrid Tainung 01) using the infested soil method (Lefèvre & Souza, 1993). Autoclaved substrate was infested with fragments of PDA from each isolate colony and incubated for seven days to create the inoculum. Then, 36 g L-1 of inoculum was added to each pot, in which a papaya seedling was planted, and grown for 60 days under greenhouse conditions (33°C ± 5°). The experiment was conducted twice, each time five plants were inoculated with each isolate, and five plants were left uninoculated (mock). Symptoms on stems appeared 30 days after inoculation, while on roots it took 60 days. F. falciforme and F. pernambucanum caused identical symptoms in the field and in our pathogenicity test. No symptoms were observed on plants from the mock treatment. The pathogens were re-isolated from the necrotic tissue and re-identified, morphologically and through Sanger sequencing as described above, to fulfill Koch's postulates. Correia et al. (2013) reported the Fusarium solani species complex (FSSC) as the etiologic agent of stem rot in C. papaya in Brazil; however, they weren't able to identify the isolates at the species level. In our studies, we identified F. falciforme (FSSC 3 + 4) and F. pernambucanum (Fusarium incarnatum-equiseti species complex 17) as the pathogens causing this disease in northeast Brazil. To the best of our knowledge, this is the first report of F. falciforme and F. pernambucanum causing root and stem rot on papaya plants in Brazil. This study will help papaya growers and plant pathologists to better understand the causal agents of this disease complex in Brazil to develop disease management strategies.