A disorder known as 'Braeburn browning disorder' (BBD), a CO₂-related disorder, can develop in the flesh of Braeburn apples during storage (Elgar et al., 1998). BBD has characteristics that are typical of other CO₂ injuries such as brownheart and associated cavity formation in the flesh. Disorder symptoms can be induced by elevated CO₂ and can be further aggravated by depressed O₂ partial pressures in the storage atmosphere, and by methyl bromide fumigation. However, BBD can occur both in fruit prior to harvest and also during air storage (unpublished industry observations).

Susceptibility of Braeburn apples to BBD is potentially damaging for commercial confidence in the cultivar. Disorder occurrence can be erratic as a result of unknown preharvest factors which influence susceptibility, and identification of these factors will lead to greater confidence in the cultivar by marketers. However, relatively little information is available in the literature about preharvest effects on either external or internal CO₂ injuries in apple fruit: most occurrences of these injuries are associated with incorrect storage atmospheres, sometimes because of poor ventilation during air storage, but usually during CA storage. Research therefore has focused on identification of safe atmospheres and defining postharvest strategies such as keeping CO₂ concentrations low during the early stages of storage when the risk of injury is highest, treatment with diphenylamine, or delaying the period after harvest before the fruit are exposed to CA storage (Elgar et al., 1998).

Preliminary studies have indicated that in New Zealand, Braeburn fruit grown in colder or more southern regions, or in colder or higher altitude districts within a region, have a higher incidence and severity of the disorder (unpublished observations). Experiments also have indicated that large variations in BBD incidence can occur between orchards within a region, and between trees on a single orchard. Several possible contributing factors to greater fruit susceptibility to disorder development were identified; these included later harvest of fruit, light crop load, and in addition, low blush fruit often appeared to show disorder development more readily than highly blushed fruit. The objective of this study, therefore, was to assess the importance of these factors on BBD incidence. We have examined effects of harvest date, crop load and blush level on susceptibility of fruit from four orchards in each of two districts to BBD development. We have confirmed that the incidence and severity of BBD is highly variable between fruit from different districts and orchard blocks. This variability has been studied by investigating effects of harvest date, blush and crop load on fruit maturity, minerals, skin permeance and BBD incidence.

BBD incidence was higher in late-harvested fruit than early-harvested fruit, in fruit grown on trees carrying a low crop load than on trees carrying a high crop load, but blush intensity had no influence on susceptibility to the disorder. Fruit maturity factors were affected by region, harvest date, blush type and crop load, but no consistent relationship between these factors and BBD occurrence was found. Ca, Mg and/or K concentrations were influenced by harvest date, blush type and crop load. Skin permeance to gas exchange was affected by growing region and blush type, but not harvest date or crop load. Overall, no physiological or mineral factor measured in this study related strongly to susceptibility of fruit to BBD. Fruit from orchards that have a history of developing BBD, or from trees that have adverse harvest date, regional, crop load, microclimate and seasonal influences should be segregated at harvest, and excluded from any CA storage regime. Alternatively, crop load needs to be managed during fruit growth to reduce the occurrence of biennial bearing.

Reference

Prepared for HortNET - June 1998