Postharvest mould of pome fruit stems and calyx sepals – an investigation into the incidence, cause and management

Pome fruits (apple and pears) are considered important deciduous fruits. Postharvest losses on pome fruits (>50% in susceptible varieties) is considered a major problem, contributing to the crop losses faced by many pome fruit producers. Recently the Western Cape apple and pear exporters observed these moulds on fruit stems and calyx sepals after long term storage and the transport to overseas markets. This problem ultimately contributes to lower quality of fruits by adding additional costs to the exporter. Currently there is less known on the occurrence of these mould on stems and calyx end in South Africa. Therefore, the objective of this study is to identify the major fungi present and provide effective solutions to the problem of mouldy stems and calyx sepals.

Commercial fruit with symptoms of mould on the stem and calyx was sampled from three packhouses within the Western Cape and the incidence of mould was recorded. The symptomatic apple fruit with mould on the stem and calyx was isolated on culture media and prepped for identification. Morphological grouping of isolates was done, followed by molecular identification through PCR using three major gene regions for the three main genera groups and other fungal groups identified. Gene regions used were Alta1, OPA1-3, EF1, RPB2 and ITS. Pathogenicity trials were done using two methods; i) wound with spore suspension and ii) spore suspension on the stem and calyx sepal leaves. This was done to evaluate if these genera groups were pathogenic on apple fruit. To determine the sensitivity of these three genera groups, amended growth media for registered fungicides fludioxonil, pyrimethanil and thiabendazole was prepared. An additional sensitivity of Alternaria spp. to preharvest fungicides actives difenoconazole and fluopyram were tested. A storage trial was done on untreated orchard-harvested fruit from cultivars of apple fruit (‘Cripps Pink’, ‘Early Red’, ‘Fuji’, ‘Golden Delicious’, ‘Sundowner’) by applying alternative packaging types (perforated bag and MAM sheets) used within industry and application types (thermofogging only for fuji) and the fruit treated with fludioxonil and pyrimethanil or with sanitisers (peracetic based and biocide) and stored for 6 months was done to evaluate if changing the current methods employed with alternative packaging influenced mould development.

Mould for both the stem and calyx was recorded for a total of five cultivars (African Red, Cripps Pink, Cripps Red, Granny Smith, Top Red). With ‘Granny Smith’, and ‘African Red’ having the highest incidence. PCR work identified three main genera groups of Alternaria, Cladosporium and Epicoccum with other fungal groups in low frequency. Alternaria spp. had a larger frequency compared to the other two main fungal groups. Koch’s postulates determined that all three groups were not pathogenic when inoculated onto a wound, however, mould was seen when inoculated on the stem and the calyx of the fruit. After 3 days after inoculation, mold was observed. Between the postharvest fungicides, isolates of Alternaria spp. deemed more sensitive to fludioxonil which was similar to the Epicoccum sp. isolates. Cladosporium sp. was less sensitive to fludioxonil. For pyrimethanil, EC₅₀ and EC95 could not be determined for all three genera and a reduction in percentage relative growth could only be determined at 1 mg/L. Relative growth (%) was less for Alternaria sp. and Epicoccum sp. at this concentration compared to Cladosporium sp. The EC₅₀ and EC₉₅ could not be determined for thiabendazole fungicide and the relative growth % was above 80% at all tested concentrations. Alternaria arborescens isolates, except two isolates, were highly sensitive to fluopyram (Luna^TM) fungicide at 100.0 mg/L, while difenoconazole (Score^TM) fungicide suppressed all isolates mycelial growth at 1.0 mg/L.

When comparing current to alternative packaging types and application methods, both had an influence on mould development during the storage period. This was seen whereby fruit packaged in a non-perforated poly bag had a high incidence of mould development across all the apple cultivars tested. The lowest was achieved for using untreated fruit with a single modular absorbent membrane (MAM) sheet and perforated bag. A combination of treatments such as chemical fungicide or sanitiser and placed in the perforated bag also showed no mould development.

From incidence reports, this mould is not cultivar dependent and affects a variety of apple cultivars. Symptoms are only subjected to the sensing parts such as the stem and calyces and fruit flesh is not affected during the storage period. However, there are more susceptible cultivars such as ‘’Granny Smith’, ‘African Red’ and ‘Cripps Pink’. Stem and calyx mould is not limited to one specific type of region as seen from incidence reports’ analyses identified 3 main genera groups belonging to Alternaria sp., Cladosporium sp. and Epicoccum sp. as the most prevalent fungal group in a suggestive complex of fungi causing these moulds. Amongst postharvest fungicides fludioxonil was more effective in vitro compared to pyrimethanil and thiabendazole. Preharvest fungicides showed difenoconazole more effective than fluopyram for A. arborescens. Furthermore, with the problem mostly seen on fruit after long periods of storage, from this study it can be suggested that cold-chain breaks play a role in possible moisture build-up also caused ideal environments for these fungal groups to colonise these fruit parts

This project showed that fungi causing superficial mould are present on fruit in the form of latent infections in the orchards and under the optimal environmental temperatures created in the postharvest phase (storage and packaging) allowed for mould symptom expression. The current method of packaging fruit in bags was shown to exacerbate the problem of mould development. The use of postharvest fungicide fludioxonil is effective against the identified fungal groups as well as using alternative packaging types such as a perforated bag and MAM sheet. Storage trials also showed that the early stage of storage and when fruit are packaged in bag plays a role in possible moisture build-up also caused by breaks in the cold-chain, which exacerbated the problem of superficial mould symptoms occurring.