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 are both symptoms of exposure to heat wave conditions prior to harvest. The aim of this research is to ascertain 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. 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 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 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 canopies of trees 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.

As in previous seasons, the fruit pulp temperatures reached between 5 ° to 10 °C higher than air temperatures at the hottest time of day. In most orchards over the three years of study, the pulp temperatures of fruit on the west side fruit were higher or similar to the east side. It was reconfirmed that the Fortune plums were more susceptible to IB after cold storage and this may have been due to the timing of the heat waves just before harvest. It is postulated that the short time between heat wave and harvest results in inadequate preconditioning of the fruit to the heat and concomitant stress. In this third season of research, the internal heat damage prediction model worked well, but still wasn’t a definite answer, but more an early indication. The 2023 seasons research indicated that this prediction model didn’t work well on Fortune plums. Generally, relative humidity varied between approximately 30% during the heat of the day to 98% at the coldest night temperature and was similar to last season. Shade net orchards reduced direct sunlight external heat damage to fruit as seen for both the Fortune and Laetitia orchards, but might lead to slightly higher temperatures in the orchard canopy.

Data needs further analysis to gain a better understanding of the impact of near harvest temperatures on plum fruit quality. Solutions to mitigate fruit damage and reduce the risk due to heat damage from heat waves need to be formulated and tested.

Overall the research to date showed that the Laetitia internal heat damage prediction model works well as an indicator of possible damage, and can be used as a tool. 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.