Quantification of the effect of near-harvest temperatures on pitburn and heat damage in cold stored plums and prediction method development

Pitburn, a dark discolouration of the inner flesh around the stone and internal browning (IB) are both symptoms of exposure to heat wave conditions prior to harvest. The aim of this research is to ascertain the impact of near harvest temperatures on internal disorders in cold stored plums and test the Laetitia internal heat damage prediction method. With global warming becoming prominent, knowledge on management of pitburn and heat damage in plums will be critical to be more economically viable in future.

Temperature loggers were installed to measure air and fruit pulp temperatures prior to anticipated heat waves for a period ranging from 5 days to four weeks prior to harvest. Experimental site selection was based on the likelihood of heat waves combined with availability of susceptible cultivars. In 2022, four orchards each of Fortune and Laetitia were studied, six orchards of Fortune and six orchards of Laetitia were studied in the 2023 season, while five orchards of each cultivar were included in the 2024 study. Fruit pulp temperatures (halfway into the mesocarp) were measured on the east and west side of trees in each orchard in direct sun positions, with ambient air temperature and relative humidity also recorded. At harvest, the Laetitia internal heat damage prediction model (6h forced air cooling [FAC], 3d at -0.5 ^°C, 2d at 20 °C) was applied using a 60-fruit sample.

Fruit was sampled between optimum and advanced maturity (still in optimum range), and from the east and west side tree canopies for nine of the ten orchards in 2024. In one orchard – a north and south sampling strategy was required. Fruit samples were evaluated for quality at harvest, after six weeks cold storage according to sea export regimes and after subsequent shelf-life.

In 2024, as in the previous season, the fruit pulp temperatures were between 5 ° to 10 °C higher than air temperatures at the hottest time of day. In most orchards over the three years of the study, the pulp temperatures of fruit on the west side were higher or similar to the east side. It was evident that Fortune plums are susceptible to IB after cold storage and that this may be due to the timing of heat waves just before harvest. It is postulated that a short time between heat wave and harvest leads to inadequate preconditioning of the fruit to the heat and concomitant stress. Over the three seasons of research, the internal heat damage prediction model did not prove accurate enough for commercial certainty.

Generally, relative humidity varied between approximately 30% during the heat of the day to 98% at the coldest night temperature and was similar to the last season. In both ‘Fortune’ and ‘Laetitia’. orchards, shade nets reduced direct sunlight and external heat damage to fruit, but might lead to slightly higher temperatures in the orchard canopy.

The data confirmed that it is difficult to predict internal heat damage in Fortune plums. However, for Laetitia any temperatures above 30 °C within 2 days of harvest can lead to internal heat damage expressed as IB. Temperatures above 35 °C in the period 10 days before harvest have a strong possibility to cause internal heat damage. Laetitia plums are very susceptible to internal heat damage and therefore, it is critical to apply step-down cooling when necessary.

Overall the research to date showed that the Laetitia internal heat damage prediction model works as an indicator of possible damage, but is not accurate enough to make commercial decisions. It appears that internal heat damage is not solely associated with the high temperatures of heat waves, but also on the physiological stage of the fruit on the tree at the time of the heat event.