| Summary: | Jackfruit (Artocarpus heterophyllus Lam.) is a popular tropical fruit cultivated
in Malaysia and is classified under the family of Moraceae. Unfortunately,
jackfruit has been found susceptible to the damage bronzing disease since
2008, which can be visualised from the rust coloured specks or streaks
appearing on the pulps, rags and seed membrane of the fruit. This
predicament has extended to causing the bulbs bear an unpleasant taste
and appears asymptomatic from the outer fruit surface. Many possibilities on
the potential causative factors have been brought up by several research
agencies. Not at least, an important report on similar disease symptoms was
described and concluded that the bacteria, namely Pantoea stewartii to have
caused the bronzing disease in the Philippines recently. At present, research
approaches to identify the causal agents and the effect of the disease on
jackfruit are still scanty. Thus, the purpose of this study was to identify the
biotic causal agent of the jackfruit bronzing disease from samples collected
in Peninsular Malaysia using morphological, biochemical and molecular
identification, (ii) to conduct pathogenicity test of the suspected causal
agents on the susceptible variety ‘Tekam Yellow’ (J33) and (iii) to elucidate
the infection process of jackfruit bronzing by histological examination of
infected tissue using scanning electron microscope (SEM). For disease
incidence (DI) survey, a total of 59 jackfruits were sampled randomly from
five states in Peninsular Malayisa i.e. Perak, N. Sembilan, Pahang, Johor
and Selangor in this study. The result showed that the disease incident (DI)
of fruit bronzing on jackfruit was high, ranging from at least 20% to 70%. For
isolation and identification, a total of 15 samples of fruit with bronzing
symptoms were collected from 8 locations in three states in Malaysia. Total
of 120 bacteria colonies with dark yellow pigment were observed during
isolation plating of all samples. Morphologically, this bacterium is Gram negative, oxidase negative, appeared as a non-capsulated straight rod with
an average of 0.35 (width) x 1.31 (length) μm, facultative anaerobic short-rod.
BIOLOG GEN III has identified 26 isolates as Pantoea sp. while the
molecular data, produced through DNA sequence analysis on the two
selected 16S rRNA gene universal primer (B27F and U1492R) using Basic
Local Alignment Search Tool (BLAST) of which all the 15 isolates as P.
stewartii and 1 isolates as P. dispersa which showed 99-100% similarity.
However, from established phylogenetic tree fifteen isolates were grouped in
the same cluster with P. stewartii and distinct from P. ananatis and P.
dispersa. The result has further clarified that the isolate that caused fruit
bronzing is P. stewartii. From pathogenicity test, the two selected P. stewartii
isolates UPM 001 and P. stewartii isolates UPM 002 were confirmed to
produce the same type of symptom as observed in the field samples. The
pathogens were successfully re-isolated from the bronzing symptom of
inoculated parts like rags and bulbs, thus fulfilling Koch’s postulates and
confirmed that P. stewartii as a jackfruit pathogen in Malaysia. Besides,
natural opening such as stomata was foreseen to be the possible pathway
taken by P. stewartii for the intrusion into the fruit.
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