Key Fruit Quality Aspects to Manage During Heat Waves
- Sunburn
- Internal heat damage
- Gel breakdown
- Over-ripeness
- Shrivel
Pre-Harvest
Growing Conditions and Cultivars
- Heat damage is most likely to occur on plums following heatwaves.
- Climate during the growing season affects when heat damage will occur, as plums acclimatize to warmer seasons.
- Heat waves likely to cause damage can be defined as:
- Temperatures of ~35°C for 3 consecutive days
- Temperatures of ~38°C for 2 consecutive days
- Temperatures of ~40°C for 1 day
- Orchards at higher risk:
- Plums grown on ridges and in sandy soil, especially 5–8 days before ripening.
- Orchards under drip irrigation with dry soil profiles prior to heatwaves.
- All plum cultivars may develop heat damage, but later cultivars (January–February) are more affected.
- More susceptible cultivars: Sapphire (early heatwaves), Fortune, and Laetitia.
Preventative Strategies for Heat-Related Disorders (Pre-Harvest)
- Timely action is critical: Many preventative measures must be taken before the heatwave occurs.
- Accurate weather prediction is essential throughout the growing season.
- As fruit matures, susceptibility increases. If heatwaves are expected, harvest as much fruit as possible that meets the maximum maturity standard before the heatwave.
- Irrigation: Irrigate to 100% of field water capacity at least 1 day (preferably 2–3 days) before the expected heatwave to buffer against increased evaporative demand.
- Pulsed irrigation: If possible, irrigate every hour for five minutes during the heatwave (especially 11:00–15:30) to reduce orchard temperatures and heat damage. If not possible (e.g., during drought), ensure soil is at field capacity before the heatwave.
- Harvesting during a heatwave: Restrict to the cooler part of the day (if maturity allows) and follow guidelines for field heat removal in the post-harvest section.
Note: Heat waves cannot be prevented, but their negative effect on plum quality can be reduced with correct precautionary measures.
Post-Harvest
Identifying and Managing Heat Damage
- External heat damage:
- Appears as discoloured areas on the fruit surface, typically from direct sunlight.
- These discolorations may sink into the flesh during storage.
- Remove affected fruit from the pack line; do not pack for marketing (e.g., Songold).
- Sunburn in African Delight:
- Manifests as yellow skin discolouration, which can turn brown during cold storage.
- Internal heat damage:
- Harder to detect, often causes greater losses.
- Occurs in the flesh (mesocarp), especially when ambient temperatures exceed thresholds above.
- Due to low vapour permeability, mature fruit cannot cool efficiently, and internal temperatures may exceed ambient by up to 15°C, effectively “cooking” the tissue.
- Fortune and Laetitia are particularly susceptible.
- Maturity and susceptibility:
- Fruit at optimum or post-optimum maturity are more susceptible than pre-optimum or green fruit.
- Fruit picked within 7 days after a heatwave often shows more symptoms than fruit picked later.
- Ripening after heat:
- Fruit picked after extreme heat ripens faster, increasing risk of gel breakdown, overripeness, and shrivel.
Management of Heat-Related Disorders (Post-Harvest)
Field Heat Removal and Packaging
- Delay before packing:
- For fruit packed on harvest day, a 4–6 hour delay between harvest and packing is recommended.
- Place bins in a draft in the shade to allow fruit to lose field heat and cool down.
- Cold store cooling:
- Place bins in a cold store at 12–15°C (above dew point) for fruit packed 24–36 hours after harvest.
- Maintain relative humidity above 90%.
- Avoid rapid chilling:
- Do not place fruit at -0.5°C for short periods (6–12 hours) before packing, as this increases moisture loss, softening, internal problems, and decay.
- Negative effects are less if the period at -0.5°C is extended to 48 hours or more.
- Field heat removal at -0.5°C is only recommended if fruit cannot be packed within 48 hours after harvest (note: may increase decay due to condensation).
- Preferred options:
- Pack fruit within 4–6 hours after harvest, or
- Remove field heat at 15°C and pack within 48 hours after harvest.
- Shrivel-prone cultivars:
- Use correct internal packaging to limit moisture loss (see Hortgro recommendations).
Forced-Air Cooling and Cold Storage
- Optimal cooling time:
- Target cold storage temperature of –0.5°C should be reached in 48 hours.
- Too rapid cooling increases internal disorders.
- Plums with suspected heat damage show lower internal damage when cooled in 48–72 hours, compared to 24 hours or less (Mariana Jooste & Arrie de Kock).
- Slower cooling does not reverse existing damage but helps control problems not yet visible at harvest.
- Inspection after cooling:
- Heat damage often manifests after cooling.
- Cut and examine fruit harvested after a heatwave, after cooling, to inspect internal quality before shipping.
- Where damage occurs:
- Usually in the mesocarp between the stone and the epidermis, but can also be on the shoulders or stem end.
- In African Delight, may appear as brown spots randomly in the mesocarp.
- Cut fruit through the equator, shoulders, and in quarters if pre-harvest temperatures were conducive to heat damage.
- Dual-temperature stored cultivars:
- Limit period at –0.5°C to a maximum of 10 days after harvest.
- Chilling injury (internal browning) may develop if this period is exceeded before warming to 7.5°C.
- Tolerance to –0.5°C before warming depends on fruit quality and cultivar.
- Cold storage duration:
- Because heat damage usually increases during cold storage, keep storage of plums exposed to extreme heat as short as possible.