First Report of Dickeya fangzhongdai Causing Soft Rot in Bananas in Ecuador

Bananas are Ecuador's second largest non-oil export product, and the quality of its fruit has established a strong presence in international markets. One-third of the world's banana exports originate from Ecuador. The Ecuadorian banana market is diversified, exporting fruit to various countries worldwide, making it a vital socio-economic and food security support for the country. Between January 2022 and December 2023, symptomatic Cavendish banana plants cv. Williams were observed in commercial plantations in three cantons of the Los Ríos province (Buena Fe, Valencia, and Quevedo), with an incidence ranging from 2% to 8%. Initial symptoms appeared as a dark purple lesion on the pseudostem that extended to the entire internal tissue. In later stages, the internal tissue softened, leading to pseudostem rot with a foul odor. In each canton, 9 samples of symptomatic pseudostems were taken. For each sample, seven 2 cm² pieces of the lesion were surface-sterilized and macerated in 9 ml of sterile peptone water (0.1% w/v). The macerate was diluted twice in sterile peptone water, plated on nutrient agar, and incubated at 28°C for 24 hours. The most abundant colonies in all cultures were yellowish-white, with irregular edges, smooth surfaces, and a whitish elevated center. Three colonies were isolated per canton. The isolates were Gram-negative bacilli, oxidase and urease negative, catalase, indole, and citrate positive. One isolate from each canton was selected for molecular identification and pathogenicity tests (FP220002 - Buena Fe, FP221366 - Valencia, FP231258 - Quevedo). The 16S rRNA gene was amplified and sequenced using primers 27F and 1492R (Wilson et al., 1991), along with gyrB, atpD, rpoB, and infB, as described by Brady et al. (2008). The resulting sequences were deposited in GenBank with accession numbers PQ279912-PQ279912 and PQ304949-PQ304960. The three isolates had identical sequences for each gene. The percentage identity ranged from 98.5% for infB to 100% for 16S with the Dickeya fangzhongdai type strain PA1 (NZ_CP020872.1) (Zhang et al., 2018). A multilocus phylogenetic analysis conducted with RAxML revealed that all isolates formed a discrete group along with other D. fangzhongdai strains, with the closest strains being DSM 101947 and QZH3 from China (Tian et al., 2016). Pathogenicity tests were conducted by syringe infiltration with 1 ml of a 1 × 10^8 CFU/ml bacterial suspension. Each of the three characterized isolates was inoculated into the pseudostems of five healthy four-month-old banana plants of the Williams cultivar. Negative control plants were infiltrated with sterile distilled water. The plants were incubated at 25°C and 74% relative humidity. Black lesions began to appear 20 days after inoculation, and 5 weeks post-inoculation, the plants exhibited clear symptoms of pseudostem soft rot, including dark purple lesions and the foul odor associated with tissue decay. Bacteria that morphologically resembled the inoculated cultures were reisolated and molecularly identified as D. fangzhongdai with 16S sequencing. Control plants remained symptom-free. This is the first report of Dickeya fangzhongdai causing soft rot in bananas, a disease recently described in Ecuador with Pectobacterium brasiliense as causal agent (Toaza et al. 2024), and which is becoming increasingly prevalent in the country.