A report on effective disinfestation of persimmon pests while reducing chilling injury
The New Zealand Persimmon Industry faces several challenges in exporting persimmons to countries such as Japan. One of the major obstacles is the need to disinfest crops of all pests of quarantine importance. Methyl bromide fumigation is currently employed, but can substantially reduce fruit quality and profitability of the persimmon export crop. In addition, persimmons develop chilling injury when exposed to temperatures below 12oC. The incidence of chilling injury is currently reduced by storing persimmons in a low oxygen atmosphere using modified atmosphere packaging (polybags). However, using polybags is expensive and time consuming and the need for complete reliability in establishing low oxygen atmosphere makes this an involved process.
HortResearch has made a major commitment to find alternatives to methyl bromide disinfestation by developing non-chemical disinfestation treatments for persimmons, which will not reduce fruit quality. Research has already been undertaken using controlled and modified atmospheres and more recently entomologists and physiologists have examined the effect of heat. Heat, in the form of hot water or hot air, is currently used worldwide for quarantine disinfestation of a variety of crops, including mangoes and papayas, and is proposed for a number of other horticultural crops. One limiting factor in the use of heat is the thermotolerance of the crop, which is often less than that of associated insects.
Fig 1: Longtailed mealy bug and lightbrown apple moth larva on persimmon
Fruit physiologists at HortResearch have found persimmon fruit, unlike other fruit crops, to be relatively heat tolerant using both hot water and hot air. In one experiment Fuyu persimmons were treated in hot water at temperatures between 47 to 54oC and stored at 0oC for 6.5 weeks in air (no polybag) to simulate sea freight. Fruit were then left for 5 days at 20oC to simulate consumer handling of the fruit, before being assessed for fruit quality. The results showed that heat treatments for 2 hours at 47oC reduced the incidence of chilling. In comparison, non-treated fruit stored at 0oC in air were severely damaged by chilling injury. While treatments at 47oC show promise in reducing chilling injury, the temperature during treatment needs to be accurately controlled since exposure to higher temperatures may cause browning of the skin, and in extreme cases browning of the fruit flesh.
Similar results in reducing chilling injury were obtained when persimmons were treated with hot air at temperatures between 45oC and 49oC, and then stored and assessed in the same manner as the hot water treated fruit. Exposure of fruit to 47oC for 2.5 to 5 hours reduced chilling injury and loss of juiciness while increasing fruit firmness. However, low levels of external browning were observed.
Fig 2: Immersion of treatment containers in hot water bath.
Based on these promising results, HortResearch entomologists have tested the thermotolerance of insect pests of quarantine concern associated with persimmons. Insect thermotolerance is tested by treating insects on fruit with hot water or hot air for a range of times and calculating an estimate of the time when 99% of the insects have died. We have tested the response to heat treatments of lightbrown apple moth, which has been shown to be the most thermotolerant lepidopteran leafroller present in New Zealand, and longtailed mealy bug. Insects on persimmons were dipped in hot water, or treated in the HortResearch High Air Flow Controlled Atmosphere and Temperature fruit treatment facility (HAFCAT).
Tests of these pests in hot water showed that the longtailed mealy bug is the more thermotolerant pest. A treatment time of 74 minutes at 44oC was needed for 99% mortality, and this decreased to 15 minutes at 54oC. However, at 47oC, which is the temperature most beneficial to fruit, both quarantine pests were killed with 34 minutes immersion. This time is well within the limits of the fruit quality.
Results from experiments in the HAFCAT indicated that longtailed mealy bug was again more difficult to kill than lightbrown apple moth. It took about 12 hours treatment at 44oC to obtain 99% mortality, but this could be reduced to 4 hours at 50oC. At 47oC, the insect tolerance was estimated at 4.5 hours which is within the 5 hours best tolerated by the fruit. Experiments are currently being undertaken to reduce this time even further by combining hot-air treatment with coolstorage, which is likely to result in significantly higher insect mortality.
Heat treatments are an environmentally friendly disinfestation treatment, and can kill leafroller and mealy bug insects, as well as reducing persimmon chilling injury. The use of such treatments would remove the need for methyl bromide fumigation on arrival in markets, and the use of polybags for fruit exported via sea freight. Consequently heat shows great potential for increasing both market access and profitability for New Zealand growers.
This research was funded by the Foundation for Research, Science and Technology, with a contribution from the New Zealand Persimmon Industry Council. For further information please contact: Peter Dentener, Mt Albert Research Centre, HortResearch, Private Bag 92 169, Auckland. Tel: (09) 815 4216 Fax (09) 815 4239.