Control strategy against ripe rot on the kiwifruit caused by Botryosphaeria dothidea in storage

Abstract

Ripe rot on kiwifruit caused by Botryosphaeria dothidea has been severed recently in the kiwi orchards in Jeju. In this study, in order to protect the kiwifruit from the disease effectively, current pathogen infection frequency and disease infection activated period were investigated. To know infection frequency by ripe rot, 86 kiwi fruits were collected and identified. Also, the kiwifruits from two orchards were sealed on different 11 periods during cultivation and counted infected kiwi fruit after harvest during storage season for window infection. In the result of disease infection frequency check test, 76 % of kiwifruits shown the disease symptoms were identified as B. dothidea. In the window infection, most of kiwi fruits were infected during fruitset to late June period. Basis on these results, most of postharvest disease was caused by B. dothidea and it was suggested that efficient control treatment should be carried out during May to June.

The ascomycete fungus Botryosphaeria dothidea is the causal agent of postharvest rot in kiwifruit. Production and spreading of conidia, differentiation of infection hyphae and invasion of intact fruit surfaces are required to cause fruit rot disease. In several pathogenic fungi, the homolog of the Saccharomyces cerevisiae mitogen activated protein kinase (MAPK) gene Fus3 has been identified as a pathogenicity factor. To investigate the role of the Fus3 homolog in B. dothidea, the gene was deleted and the resulting ∆fus3 mutants were characterized. On artificial media, strong growth defects of ∆fus3 strains were observed and mycelia were non-pigmented, and mutants were unable to conidiate. Scanning electron microscopy showed that the wildtype strain differentiated thick hyphae on the surface of kiwifruits, and nose-down growth of hyphae indicated tight attachment to the fruit. By contrast, hyphae of ∆fus3 strains mutants were thinner and did not firmly attach to the fruit surface. Importantly, pathogenicity test revealed that ∆fus3 strains were unable to cause disease symptoms on intact kiwifruits. These results shown here identify Fus3 of B. dothidea as a novel pathogenicity factor of the ripe rot pathogen on kiwifruit.

Ripe rot caused by B. dothidea is one of the serious diseases in postharvest kiwifruits. In order to control the ripe rot on A. chinensis cultivar Gold3, several commercial fungicides were selected by antifungal test on an artificial medium. Furthermore, disease suppression by the selected fungicides was evaluated on the kiwifruits by inoculation with conidial suspension of B. dothidea. On the artificial medium Tebuconazole, Iprodione and Boscalid+Fludioxonil were shown the most effective antifungal activity. However, in the bio-test Pyraclostrobin+Boscalid and Iminoctadine-tris were the most effective agrichemicals on the kiwifruits. On the other hand, the infection structures of B. dothidea on kiwifruits treated with agrichemical Pyraclostrobin+Boscalid were observed with a fluorescent microscope. Most of conidia of the fungus were not germinated on the kiwifruit treated with the agrichemical whereas on the untreated fruit the fungal conidia were mostly germinated. Also, chemical residue of of Pyraclostrobin + Boscalid was measured lower than MRL standard in Korea until 1 month before harvest. Based on this observation it was suggested that the ripe rot may be suppressed through the inhibition of conidial germination on the kiwifruit treated with the agrichemical.