Bacteria and yeasts as potential biocontrol agents for the management of blue mould and green mould diseases of ‘Valencia’ oranges caused by Penicillium italicum and Penicillium digitatum.

Abstract

Citrus fruits can be infected by microorganisms that affect the quality of fruit. Penicillium moulds caused by Penicillium digitatum (green mould) and Penicillium italicum (blue moulds) are two of the most problematic plant pathogens that affect the Rutaceae. Penicillium digitatum is a mesophilic fungus that can produce a potential mycotoxin called citrinin that has the potential to be carcinogenic to animals and humans. Management strategies in place to control these pathogens such as synthetic fungicides provide control, however they have impacts on the environment which does not appeal to consumers. This study aimed to assess the efficacy of bacterial and yeast strains isolated from leaves and peels of lemon (Citrus lemon), oranges (Citrus sinensis) and limes (Citrus aurantiifolia) in controlling P. digitatum and P. italicum in laboratory and fruit storage trials. In the in vitro studies, 102 isolates were screened against P. digitatum and P. italicum using the dual culture assay on petri dishes. The best seven isolates were selected for secondary in vitro screening against both fungi. The best two isolates from the secondary screening against P. italicum were UK37 (96.89%) and SCO13 (94.23%) and for P. digitatum, UK37 (98.83%) and SCO13 (95.29%). Isolates UK37 and SCO13 were sent to Inqaba Biotech Industries (Pty) Ltd for molecular characterization and identification. Both isolates were identified as Bacillus amyloliquefaciens. Bacillus amyloliquefaciens strain SCO13 and B. amyloliquefaciens strain B UK37 were used in in vivo experiments to assess their efficacy in controlling P. digitatum and P. italicum on oranges ‘Valencia’ stored at 25 ℃ for ten days. Bacillus amyloliquefaciens strain SCO13 showed potential as a biological control agent against both P. digitatum and P. italicum, with a low disease incidence of 19.54% and 35.87% respectively. Two yeast strains of Meyerozyma guilliermondii that were previously isolated, were used in in vivo experiments to assess their efficacy in controlling P. digitatum and P. italicum on oranges ‘Valencia’ stored at 25 ℃ for ten days. Yeast treatments provided the least control of the Penicillium moulds on oranges. The SEM images of the mycelial growth of P. digitatum Meyerozyma spp. displayed antagonism by growing on P. digitatum hyphae and colonizing orange fruit surfaces. micrographs showed the bent hyphae and reduced number of conidia in vitro against B. amyloliquefaciens spp., with B. amyloliquefaciens B13 completely colonizing and growing over P. digitatum in vitro for seven days, incubated at 25℃.

In conclusion, B. amyloliquefaciens demonstrated potential in controlling P. digitatum and P. italicum infection on oranges. These Bacillus spp. have the potential in the integrated pest management of Penicillium spp.