INTRODUCTION

The New Zealand cherry industry is affected by post harvest fruit quality problems. These are mainly caused by improper post harvest handling and storage, which results in soft fruit, pitting, loss of skin brilliance, stem browning and shrivelling, and storage diseases. Fumigation with methyl bromide (MBr) exacerbates these problems. The following study investigated the use of a pre-harvest application of a plant growth regulator, gibberellic acid (GA), to improve post harvest and storage quality of Dawson, Stella, Bing and Rainier cherries. The effects of GA with fumigation and covering were also investigated for each cultivar.

In the past five years, poor fruit quality has been identified as a major problem with New Zealand (NZ) cherries arriving in Japan (NZ Trade Commission, 1986; 1988; NZ Trade Development Board, 1989; 1990). Poor quality is defined as soft fruit, diseased and decaying fruit, variability in colour and taste, low brix levels, small size, and poor storage. Many of the above factors contribute to short shelf life.

The long term viability of the NZ cherry industry depends on the above problems being addressed to improve the quality and shelf life of cherries. This will enable NZ to produce a top quality product which justifies the higher prices expected of the consumer.

Cherry soft fruit problems are attributed to cell wall characteristics which can be dependant on cultivar, nutrition, and water relations. Cell walls can be strengthened by the use of calcium chloride by pre and post harvest sprays, which increase fruit firmness (Lidster et al., 1978; 1979). Calcium sprays can extend storage and shelf life up to three weeks (Tukey, 1984). However, calcium sprays are not able to increase fruit size, and can contribute to chemical burn on leaves when applied as pre harvest sprays. The use of calcium as pre harvest sprays and post harvest dips is not adopted commercially in New Zealand. Fruit size and firmness aspects of cherry fruit quality can be improved by the use of gibberellic acid (GA_³). American and Israeli studies show that GA_³ increases fruit size and fruit firmness (Facteau, 1984; Sive and Resuiky, 1988; Kupferman, 1989). GA is now widely used in Washington State and Oregon, USA, and British Columbia, Canada.

In New Zealand, initial studies on GA were conducted by Facteau (1984) on cherry cultivars Merton Premier, Bing, and Dawson. The results showed that GA delayed harvest date but increased fruit firmness, weight and soluble solids. However, GA did not have any effect on fruit splitting. McLaren and King (1989, unpublished) obtained similar effects on firmness and maturity from studies conducted in Central Otago.

Export cherries to the Japanese market need to meet quarantine regulations primarily to prevent codling moth entering Japan. Methyl bromide (MBr) has been used by the industry to fumigate and disinfect the consignments of cherries, thus gaining entry into Japanese markets. However, the use of MBr contributes to soft fruit and poor storage and shelf life, thus inferior quality.

The use of GA by the American cherry industry to improve fruit quality has proved successful. There are, however, no known studies on the effect of MBr fumigation on GA_³ treated fruit and its storage and shelf life qualities.

Concerns have been expressed in the past five years by growers, exporters, and importers of reduced firmness and poor quality of MBr treated cherries. Therefore, the following study was conducted to investigate the effects of MBr fumigation on four sweet cherry cultivars grown under and outside artificial covers, and treated with GA.

RESULTS AND CONCLUSIONS

The application of GA at the beginning of stage three of fruit growth (approximately three weeks before harvest) significantly improved post harvest handling and storage quality of cherries. The response to GA was greatest in cultivar Bing, followed by cultivars Rainier, Stella and Dawson. A rate of 10 ppm GA was as effective as 20 ppm GA in improving cherry quality.

The results obtained from this research experiment relate to the specific climatic conditions and the age and type of cultivar. The conclusions are based on one years experimentation, but a minimum of two years experimentation is required for consistent results.

The main effect of GA was to increase fruit firmness, and this enabled the fruit to handle and store well.

GA reduced average skin colour, but this did not have a corresponding effect on °Brix. Therefore, GA treated fruit can be harvested at a grade lower than that specified in the colour grade standards. Rainier cherries did not develop the pink blush, but turned a medium straw colour.

GA did not have any influence on fruit weight, as observed by few other researchers. It is possible that the full sink potential from increased cell size was not realised.

GA increased fruit splitting in Bing and Rainier cherries. Dawson and Stella cherries are usually more resistant to splitting and were not affected by GA. The economic analysis of losses from splitting caused by GA shows that the marketable yield can be reduced in Bing cherries by 11.9% (10 ppm GA) and 18.9% (20 ppm GA) compared with not applying any GA. However, this loss has to be traded off against the quality improvement and possible weight gains from late harvests of GA treated fruit.

GA enhanced overall fruit quality of all the cultivars except Dawson. The incidence of pitting was reduced with the use of GA. Skin brilliance was enhanced with the use of GA, but GA did not have any effect on improving stem colour, except in cultivar Rainier. Stem integrity was not affected by GA. Skin brilliance was generally enhanced by GA, and this is an indicator of freshness. It is important to minimise storage diseases, otherwise importers and retailers lose confidence in the product and also sustain economic losses. GA helped to reduce the incidence of disease only in Rainier cherries.

Fumigation was conducted with a high rate of MBr and at a high temperature. Fumigation reduced fruit firmness before and after storage, and deteriorated the storage qualities of cherries of all four cultivars. Post storage evaluations showed that fumigated fruit was of unpresentable and unmarketable quality. Although significant GA fumigation interactions were not common, it is concluded that GA treated fruits were relatively better than non-GA treated fruits when fumigated. Improvements to fruit quality from GA treatments may result with lower rates of MBr and temperature than is currently used.

Covering generally reduced fruit firmness and had no distinct advantage in improving fruit quality of GA treated and fumigated fruit. However, covering did help to reduce splitting in most cultivars.

GA has definite beneficial effects on cherries, by improving firmness and storage quality of the fruits. These studies show that there is no distinct advantage in applying GA at 20 ppm. The industry may wish to adopt a 10 or 20 ppm rate of GA application. If a 20 ppm rate is adopted, then the cost of the extra 10 ppm should be determined. Research indicates GA application at the beginning of Stage Three (approximately 21 days before harvest) reduces flowering in both spur and terminal wood. This may help to reduce crop load in the subsequent year and possibly result in an increase in fruit size and weight. The long term effects of GA on cropping need to be monitored. Most cherry trees in Marlborough have been treated with a growth retarding chemical (Cultar). As GA induces vegetative growth, its effect on Cultar treated trees needs to be monitored.

RECOMMENDATIONS

The following recommendations are made based on the results of a single field experiment:

1.  The use of GA on cherries should be adopted by the industry.

2.  GA should be applied three weeks prior to harvest
    (beginning of stage three of fruit growth) immediately after pit hardening.

3. GA should be applied at a rate of either 10 or 20 ppm.
    The 10 ppm rate was as effective as the 20 ppm rate.

4. Consumer perception of maturity should be changed to account for delayed skin
    colour development.

5. A cost benefit analysis should be conducted to ascertain the use of GA and the resulting
    crop loss through fruit splitting.

6. Fumigation with MBr should be avoided as much as possible.
    Where fumigation is mandatory, steps should be taken to reduce the rate of MBr
    and the fumigation temperature at least to the standards adopted by the
    USA cherry industry (48 gm-³ at 6 ºC).

7. Because of problems associated with wet fruit (from rainfall and hydrocooling) an
    investigation into the use of alternate methods to hydrocooling for rapidly
    removing field heat should be made. A viable alternative is the use of forced air cooling.

Balasubramaniam, R. and R. Agnew. 1990. The effect of Giberellic Acid on post-harvest handling and storage quality of methyl bromide fumigated and artificially covered cherries of cultivars Dawson, Stella, Bing and Rainier. Report for the Marlborough Fruit Producers Limited. MAF Technology, Marlborough Research Centre.