Pathogenic Variability of the Jackfruit-Bronzing Bacterium Pantoea stewartii Subspecies stewartii Infection to Jackfruit Varieties and Its Pivotal Plant Hosts in Malaysia

Infection with Pantoea stewartii subspecies stewartii, which causes jackfruit-bronzing disease, is a huge problem faced by the jackfruit industry in Malaysia. This study was carried out to assess the disease ratings and aggressiveness performance of 28 Pantoea stewartii subspecies stewartii isolated from jackfruit-bronzing diseased jackfruits from four collection areas (Jenderam in Selangor State, Maran and Muadzam Shah in Pahang State, and Ipoh in Perak State) in Peninsular Malaysia, inoculated into jackfruit varieties (Tekam Yellow J33, Hong J34 and Subang Chap Boy J39), the sweetcorn variety, Mas Madu (two-week-old seedlings and nine-week-old seedlings), the cucumber variety, Rocky, and the pineapple variety, MD2. The results revealed that Pantoea stewartii subspecies stewartii produced symptoms upon all inoculations in the pathogenicity testing, thus fulfilling Koch's postulates, except in the case of J39 and sweetcorn (two-week-old seedlings). No disease symptoms (disease rating 0) were observed in J39 and sweetcorn (two-week-old seedlings) within 14 days post-inoculation (14 dpi). The disease progression (based on disease ratings) proved that the jackfruit variety J39 was the most resistant, whereas J33 and J34 were susceptible to jackfruit-bronzing disease. The disease ratings of the 14-dpi period revealed a variability of disease progression among the 28 bacterial isolates, where the isolate JEN-14 had the fastest and highest disease ratings when inoculated into J33, J34, nine-week-old sweetcorn seedlings, cucumber, and pineapple. Likewise, the AUDPC value, based on disease rating, across the 28 isolates indicated that JEN-14 is the most aggressive and significant of the isolates (J33, J34, nine-week-old sweetcorn seedlings, cucumber, and cucumber pineapple inoculation; p < 0.05). Even though isolates from Jenderam (other than JEN-14) and Maran had better disease ratings and the area under the disease progress curve (AUDPC) values than isolates from Muadzam Shah and Ipoh, no significant differences were found among the isolates (p < 0.05). Based on our findings, we identified the isolate JEN-14 as the best potential candidate to assist in jackfruit-bronzing disease resistance breeding in the future. Last but not least, the methods, disease ratings, and variations of the aggressiveness profiles among the isolates from this study may be beneficial and significant by providing disease-rating references and appropriate screening approaches when selecting the most appropriately aggressive isolates for evaluating the defense response in the disease resistance breeding program.