Apple farmers and apple exporters suffer significant losses due to core rot, a major post-harvest disease of apples caused by certain fungal species. The symptoms of core rot include a mouldy core, dry core rot and wet core rot on the inside and around the core cavity of apples. Red Starking, in particular, is susceptible to core rot, with over 20% decay caused in single consignments leading to a rejection at export destinations.

And while it is difficult to identify the species responsible for core rot, a new study at Stellenbosch University (SU) found that molecular methods might just do the trick. The study was published recently in the European Journal of Plant Pathology.

“Currently, the identification of fungal species that cause core rot is labour intensive and requires trained personnel and laboratory facilities. Molecular methods are promising tools for species identification, which would enable targeted control strategies," says Dr Julia Meitz-Hopkins from the Fruit and Postharvest Pathology Research Programme in SU's Department of Plant Pathology. The study was conducted by Meitz-Hopkins, Elaine Basson and Dr Cheryl Lennox who is the leader of the Fruit and Postharvest Pathology Research group.

The main objective of their study was to identify the Alternaria and Penicillium species sampled from symptomatic core rot fruit and inoculum sources such as air, overwintered mummied fruit and mites on the fruit in different apple orchards by using molecular techniques.

As part of the research, core rot-infected apples were collected from two 'Starking' orchards in the Koue Bokkeveld region in the Western Cape during the pre- and postharvest seasons, while Alternaria and Penicillium fungi were isolated from potential inoculum sources, grown in a petri dish and identified.

Meitz-Hopkins points out that their study was the first to report the Alternaria eureka and Penicillium polonicum species as potential core rot pathogens. They also identified Penicillium ramulosum and Penicillium expansum as the most commonly occurring species associated with wet core symptoms.

“Dry core rot-causing Alternaria were identified morphologically in over 70% of infected apples during preharvest and 40% postharvest," says Meitz-Hopkins.

“Furthermore, Penicillium caused wet core rot in 64% of cases during preharvest and in 36% of infected apples during post-harvest."

The study also reported that Alternaria and Penicillium fungi are commonly associated with core rot and it's difficult to identify them by only looking through the microscope, since their form, structure and shape can be identical.

Meitz-Hopkins further states that “controlling postharvest pathogens is complicated by the fact that infection might occur at any time from the bloom or during fruit development, depending on the disease-causing fungi."

“Fungicides are usually registered for specific disease-causing pathogens, since they have varying control efficacy on different species."

“Another complication is that fungal spores, such as Alternaria, could be carried into the apple cavity by certain mites and therefore, the use of fungicide during the blooming period alone might not be effective to control all fungi that cause core rot."

Therefore species identification of the organisms that cause core rot is required, adds Meitz-Hopkins.

She says the use of molecular techniques to quickly and reliably identify disease-causing fungi would allow for the monitoring of quarantined micro-organisms.

Meitz-Hopkins adds that an integrated management strategy should be implemented where chemical control should be used in combination with sanitation practices, both in the orchard and in the pack house.

• Source: Basson, E; Lennox, C.L; Meitz-Hopkins, J.C. 2018. Morphological and molecular identification of fungi associated with South African apple core rot. European Journal of Plant Pathology.

FOR MEDIA ENQUIRIES ONLY

Dr Julia Meitz-Hopkins

Fruit and Pathology Research Programme

Department of Plant Pathology

Stellenbosch University

Tel: 021 808 9177

E-mail: juliam@sun.ac.za 

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Martin Viljoen

Manager: Media

Corporate Communication

Stellenbosch University

Tel: 021 808 4921

E-mail: viljoenm@sun.ac.za