First Report of Cucumber Fruit Rot Caused by Fusarium incarnatum in Mexico.

Abstract

Cucumber (Cucumis sativus L.) is an important vegetable crop for Mexico, which is the fifth highest producer of this crop worldwide. In November 2019, a fruit rot disease characterized by the presence of white mycelial growth at both ends of the fruit was observed with an incidence of approximately 30% in a greenhouse production area in Sinaloa State (geographical coordinates: 24°35'25'' N, 107°26'21'' O). Culture of small disinfected sections from cucumber lesion edges was carried out on PDA medium at 27°C for seven days. Colonies had profuse aerial mycelium that was initially white and became beige with age, and light orange sporodochia were eventually produced. Macroconidia had 3 to 5 septa and were slightly curved and tapered at the apex with a foot shape at one end and measured 25.4±2.5 × 2.8±0.3 µm. Microconidia were fusoid in form mostly with 2 to 4 septa and measured 15.7±2.04 × 2.4±0.25 μm (n= 100). The morphological characteristics of this fungal isolate were similar to those of Fusarium incarnatum (Leslie and Summerell, 2006). To confirm the species identification, the internal transcribed spacer (ITS) region, calmodulin (CAL), β-tubulin (B-tub) and translation elongation factor 1-α (TEF1-α) genes were amplified and sequenced from two representative isolates: FPM01 and FPM02. These sequences were submitted to GenBank with accession numbers MT387313 and MT387314 for the ITS region, MT410503 and MT410506 for CAL, MT410502 and MT410505 for B-tub and MT410504 and MT410507 for TEF1-α for the FPM01 and FPM02 isolates, respectively. BLASTn analysis of the sequences showed between 99.33 and 100% identity with F. incarnatum sequence accession numbers MH865893, LN901598, AB587036 and MK328877, which corresponded to strains CBS 130313, ITEM 6748, MAFF 236521 and FIAD-1, respectively. Further BLAST searches in the FUSARIUM-ID database indicated that these sequences had between 99.76 and 100% identity with sequences of the F. incarnatum-equiseti species complex (FIESC) (FD_01682, FD_01640 and FD_01643). To fulfill Koch's postulates, healthy cucumber fruits were disinfected with 1% NaOCl for one min and then washed with distilled water. Ten nonwounded cucumber fruits were spray inoculated with a conidial suspension (1 × 103 conidia/mL), and five fruits wounded with a sterilized needle were inoculated with mycelial discs (6 mm) at three sites each. Distilled water and PDA medium discs were used as controls. All fruits were placed into plastic bags with 80% relative humidity and incubated at 25°C for three days. At 24 hours after inoculation (hai), a soft rot symptom was observed at the fruit poles on non-wounded fruits and around the inoculated area of wounded fruits, and white mycelial growth was observed at 48 hai for both cases. All inoculated fruit showed similar symptoms to those observed initially in the field, and the control fruits remained healthy. The cultures obtained from tissues of inoculated fruits were similar to those from the initially obtained isolates, thus confirming pathogenicity. Recently, Fusarium incarnatum has been reported in China, causing fruit rot in muskmelon (Cao et al., 2019) and peach (Zhang et al., 2020) and crown rot and leaf spot in cucumber (Mao et al., 2020; Gao et al., 2020). To our knowledge, this is the first report of cucumber fruit rot caused by F. incarnatum in Mexico.