First Report of Bronzing Disease of Jackfruit ( Artocarpus heterophyllus ‘Qiongyin 8’) Caused by Pantoea ananatis in China

Jackfruit (Artocarpus heterophyllus Lam.) is an economically important tropical fruit crop [1], and cultivar ‘Qiongyin 8’ is commercially cultivated in Yunnan Province [2]. In June 2024, an internal bronzing syndrome occurred in a commercial orchard in Manlan, Jinghong City, Xishuangbanna Prefecture, Yunnan Province, China (21.898424°N, 100.959126°E), with incidence reaching 86%. Symptoms began as small rust-brown spots on the edible fleshy bulbs surrounding the seeds and the intervening fibrous tissues (“rags”), then enlarged and coalesced into irregular bronze to rust-brown areas. Symptoms remained internal; no soft rot, exudation, foul odor, or conspicuous rind lesions were observed. Ten symptomatic tissue samples, each comprising affected fleshy bulb tissue and adjacent fibrous tissue, were surface-disinfested in 3% H₂O₂ for 3 min, rinsed with sterile water, plated on nutrient agar (NA), and incubated at 28°C. During primary isolation, only one predominant colony morphotype was observed. A single target colony from each sample was subjected to 10 successive rounds of streak purification on NA. Three representative purified isolates were randomly selected for 16S rRNA gene sequencing using primers 27F (5′-AGAGTTTGATCMTGGCTCAG-3′) and 1492R (5′-TACGGYTACCTTGTTACGACTT-3′), with bidirectional Sanger sequencing. BLASTn analysis showed that all three sequences had their highest matches to Pantoea ananatis. Representative isolate BNX 26514 was further characterized using 16S rRNA, gyrB, and atpD [3]. The sequences (874, 864, and 542 bp) were deposited in GenBank as PZ404167, PZ412448, and PZ723123, respectively. BLASTn identities to P. ananatis reference sequences were 100, 100, and 99.82%, respectively, with 100% query coverage. On yeast extract-dextrose-calcium carbonate (YDC) agar, BNX 26514 produced pale-yellow, smooth colonies after 48 h at 28°C (Fig. 2A). Gram staining of a 48-h culture showed Gram-negative, short rod-shaped cells (Fig. 2B). Because 16S rRNA alone provides limited species-level resolution within Pantoea, species assignment of BNX 26514 was supported primarily by the housekeeping genes gyrB and atpD together with phylogenetic analysis. Maximum-likelihood analysis of concatenated partial gyrB and atpD sequences placed BNX 26514 within the P. ananatis clade containing strains LMG 2665, OUF2, and OUG1, with 100% bootstrap support (Fig. 3). For the initial pathogenicity test, a 24-h LB culture was adjusted to OD₆₀₀ = 0.15; viable plate counting verified approximately 1 × 10⁸ CFU mL⁻¹. Thirty approximately 3-month-old ‘Qiongyin 8’ fruits at Production Team 10 of Jinghong Farm, a site different from the naturally affected orchard, were inoculated while remaining on the trees. Five sites per fruit were inoculated by syringe injection with 1.0 mL bacterial suspension to a depth of approximately 1–1.5 cm into the space between a fruit bulb and adjacent fibrous tissue; five opposite sites on the same fruit received sterile water. Fruits were not bagged and remained under natural conditions (24–29°C, approximately 64% relative humidity, sunny and dry). After 7 days, typical internal bronzing developed at pathogen-inoculated sites, whereas water-inoculated sites remained symptomless. To address the independence of the controls, an additional validation was conducted in March 2026 in a jackfruit germplasm orchard of the Honghe Tropical Agriculture Institute of Yunnan. Six approximately 3-month-old ‘Qiongyin 8’ fruits remained attached to the trees and were maintained under natural light at approximately 25°C and 50% relative humidity; environmental variables were not continuously logged. Three fruits were inoculated at three sites per fruit by syringe injection with 1.0 mL BNX 26514 suspension (approximately 1 × 10⁸ CFU mL⁻¹) to a depth of approximately 1–1.5 cm, while three separate fruits received sterile water at three sites per fruit. All three BNX 26514-inoculated fruits showed internal bronzing by day 7 and remained symptomatic at the final day-15 assessment, whereas all three independent water-control fruits remained symptomless. Bacteria were reisolated from one symptomatic site on each of the three BNX 26514-inoculated fruits, purified, and subjected to 16S rRNA sequencing; all three reisolate sequences matched the 16S rRNA sequence of BNX 26514. Pantoea ananatis is a broad-host-range pathogen of maize, onion, rice, and other crops [4]. Compared with disease presentations on onion and maize, the jackfruit syndrome observed here was notable for predominantly internal bronzing without soft rot or conspicuous rind lesions. The internal localization may reflect colonization of inter-bulb tissues after entry through localized wounds or natural openings, but this remains untested. No obvious insect punctures, feeding injuries, mechanical wounds, or rind damage were observed on naturally affected fruit; thus, injection inoculation demonstrated pathogenicity but did not establish the natural infection court. Fruit injury/insect exclusion, sanitation, and wound-minimizing fruit handling merit evaluation; entry routes and epidemiological risk also require study. To our knowledge, this is the first report of bronzing disease of jackfruit caused by P. ananatis in China.