Articles
DISINFESTATION OF NEW ZEALAND FLOWER THRIPS FROM STONEFRUIT USING HOT WATER
Non-chemical postharvest treatments are being investigated as an alternative to preharvest insecticides.
Hot water treatments have shown promise in small-scale trials, with all stages of NZFT being controlled by water dips at 50°C for 2 min.
This treatment has not caused damage to apricots in the past, even when fruit quality has been affected by rain close to harvest.
In January 1996, a purpose built, commercial-scale hot water bath and drier were installed in line with the fruit grader in the packing shed at Clyde Research Centre.
Lines of export apricot (cvs. ‘Sundrop’ and ‘Valleygold’), peach (cv. ‘Improved Flavourcrest’) and nectarine (cv. ‘Fantasia’) were treated at 48° or 50°C for 2 or 3 min, after they had been removed from coolstorage.
Each hot water treatment included a sample that was dried before packing (fan drier at 60°C for 1 min), and one which was not (fruit were packed wet). Hot water treatments at 50°C were extended on peach and nectarine to 5 and 7 min respectively.
Trays of 25 fruit were packed from each of 4 size grades, coolstored at 0°C for 3 weeks, then held in the laboratory at 20°C for a further 6 days.
All stored samples were assessed for postharvest rots; and 10 fruit per tray were assessed for quality attributes.
To determine the effect on NZFT, samples of 500 peaches or nectarines, which had been naturally infested with NZFT in the field, were treated for 2 and 3 min at 48 and 50°C and the effects on the mortality of all instars were assessed.
No internal damage was detected in fruit heat-treated within the first 2–3 min; but blemishes on the skin, which had been acquired during picking, turned brown on some apricots during the 50°C for 3 min treatment and were evident when the fruit was being packed.
After storage, brown pock marks appeared on the skin in the same treatments.
All the cultivars tolerated 48°C for 3 min and 50°C for 2 min.
However, apricots (in a rain-affected season) were damaged by 50°C for 3 min and were more sensitive to hot water treatment than peaches (5 min) and nectarines (7 min). Drying the fruit after treatment at 50°C for 2 min increased the damage to the skin of ‘Valleygold’ apricot, but reduced the incidence of brown rot (Monilinia fructicola) in both the apricot cultivars and the nectarine.
Drying reduced all postharvest rots in ‘Valleygold’ apricot.
Postharvest rots, including brown rot, were reduced by the combination of hot water treatment (50°C for 2 min) and drying in all cultivars except ‘Sundrop’. The incidence of postharvest rots was lowest after 2 min at 50°C, and increased as heat treatment was increased to 7 min.
The internal temperature of fruit after 2 min in water at 50°C increased by approximately 9°C in apricots (80–100 g) and by 6°C in the larger peaches and nectarines (130 g).
The treatments, 48°C for 3 min and 50°C for 2 min gave similar results against all instars of NZFT. Fruit treated for 2 min in water at 50°C, lost 97% of their adults into the water, while 99.80% of adults, 100% of larvae and 99.65% of eggs were killed.
It is concluded that hot water treatments at 50°C for 2 min or 48°C for 3 min show promise for the disinfestation of NZFT on export stonefruit.
The 1400 litre bath treated 600 kg fruit/hour.
This can be scaled up to meet requirements for current packing shed systems.
The drier has the potential to reduce postharvest rots on fruit treated with hot water, but may require modification for use on apricots, especially in seasons when the skin has been affected by heavy rain close to harvest.