Postharvest Strategies that Reduce Risk of Pome Fruit Decay
Factors that Influence Decay
Postharvest losses in pome fruit vary each year according to weather, fruit condition, and length of storage. The most obvious weather factor that leads to increased decay of fruit in storage is rain at harvest. This almost always results in increased Bull's-eye rot. Pinpoint scab may be a problem in years with wet weather in August. In 1997, Botrytis cinerea infection was especially frequent due to a wet spring. Hail and wind damage are weather related problems that often reduce fruit to the status of culls. Condition at harvest has a tremendous effect on how long fruit can be stored without decay. Mineral content of the fruit as determined by maturity at harvest, and nutritional status have much to do with how well fruit will store before they are infected by fungi. This is reflected in the fact that decay increases with the length of time fruit are stored. Losses are greater in processed fruit because growing, harvesting and storage practices are handled differently than for fresh-market fruit. For example, they may be harvested when over mature or slightly damaged and are often cooled more slowly and kept for longer periods in non-controlled atmosphere storage. Therefore, it is clear that decay is a complicated problem and requires a thorough understanding of how and when fruit are attacked by fungi in order to develop control strategies.
Preharvest Control Strategies
Strategies for decay prevention that could affect a wide range of fungal pathogens must be considered before fertilizers are applied. For example, incorrect nutrition, as provided by high nitrogen, makes fruit more susceptible to decay. On the other hand, trees sprayed with calcium chloride during the growing season retards decay in stored fruit, although it probably does not prevent infection. A trial at Summerland, British Columbia in which Braeburn and Fuji apples were sprayed six times with calcium chloride (5.0 g/L) showed that calcium reduced the size of blue mold rot lesions. In a study on blossom infection of Jonagold, Fuji, Braeburn, Gala and Spartan apples it was found that floral parts consisting of stamens and pistils were infected at bloom by Botrytis cinerea. Bloom sprays with fungicides that prevent infection by B. cinerea may also help in decay control. Fungicide sprays applied one to two weeks before harvest reduce decay. Several studies have been done on the effectiveness of preharvest Ziram and show an average reduction in decay of about 25 to 50% with a single application1.
Postharvest Control Strategies
TBZ and Biocontrols
Postharvest fungicides and biological controls are most useful for preventing infection of wounds. Fungicides such as thiabendazole (TBZ) and Captan and biological controls such as Aspire (Candida oleophila isolate 1-182) and Biosave II (Pseudomonas syringae strain ESC-11) protect wounds from infection. It is becoming apparent that these biological controls are more effective when integrated with thiabendazole (TBZ) fungicide treatements1.
An important strategy to reduce contamination of fruit by fungal spores is sanitation. This involves storing fruit in clean bins, taking steps to keep the packing house free of airborne fungal spores, and maintaining flume water and dump tank free of fungal pathogens. Chlorine (100 ppm) can do a very effective job of killing spores in a dump tank if the concentration of chlorine is correct, the amount of dirt in the water is minimized, and all areas of the fruit are penetrated2. Several organizations are available to monitor the number of spores in a dump tank. Spore levels of 300/mL should be avoided.
Research at the Pacific Agri-Food Research Centre, Summerland, British Columbia starting in the early 1990's showed that acetic acid vapor was very effective in killing fungal spores of postharvest pathogens. These results indicated that acetic acid vapor could be a possible alternative, or could be used in conjunction with chlorine for disinfesting fruit.
Fumigation of fruit to control decay is not an entirely new concept. Sulfur dioxide is used to prevent postharvest decay in table grapes. The tolerance of grapes to sulfur dioxide is unique among fresh fruits and vegetables. Many other compounds have been tested as fumigants for decay control but none have gained widespread use. Acetic acid, present at a concentration of 4% or greater in vinegar has long been known for its preservative and flavoring properties. As a natural compound found throughout the biosphere, it poses little or no residual hazard and is a generally-regarded-as-safe (GRAS) compound. Important chemical and hazard characteristics of acetic acid are given in Table 1. Acetic acid is a monofunctional organic acid not as strong as most mineral acids but is still sufficiently corrosive to warrant significant safety precautions.
The activity of acetic acid towards microorganisms is related to pH, carbon chain length, and the inherent susceptibility of the organism3. The undissociated form of the acid, as when it is a vapor, is primarily responsible for its anti-microbial activity. It is thought that acetic acid disrupts the cell membrane interfering with transport of metabolites and maintenance of membrane potential.
Preliminary trials on fruit contaminated with fungal spores were conducted in chambers constructed from 12.7 L cans4. After the fruit (4 apples or 4 pears) were sealed in the chambers, glacial acetic acid (99.9% acetic acid) was evaporated, and the contents were left for 1 hour when the chamber was opened. The fruit from the chamber was handled under sterile conditions, injured with a sterile glass rod (approximately 1/16-inch diameter) and incubated at 20 °C (68 °F) until sufficient rot developed in the control fruit. As little as 2.0 mg/L (740 ppm) acetic acid was required to disinfest the fruit contaminated with Botrytis cinerea (Table 2). Penicillium expansum spores were killed by double this concentration of acetic acid vapor.
Acetic acid fumigation trials have also been conducted with larger quantities of fruit. Table grapes were fumigated with acetic acid and stored for 6 weeks at 5 °C (41 °F)5 in 1994 and 1995. Decay by Penicillium expansum and Botrytis cinerea was effectively controlled by both acetic acid and sulfur dioxide fumigation (Table 3). This indicated that acetic acid vapor could be used as an alternative to sulfur dioxide in grape fumigation.
Fumigation of apples in small wooden bins naturally contaminated with fungal spores after storage at 1 °C for 8 months, showed that acetic acid vapor would sterilize the apple surface without causing phytotoxicity. For example in an experiment conducted in May, 1996 wounding 90 unfumigated Red Delicious apples led to 72% decay compared to only 7% in the fumigated fruit.
Recently, research has been initiated on fumigation of pears contaminated with spores of decay fungi. The pears (33 lb/bin) are placed in small wooden bins modelled after standard bins, and three bins are fumigated in a 1 m3 chamber with three similar bins of fruit not fumigated acting as control fruit. Results indicate that fumigations conducted at 0 °C (32 °F) with pears contaminated with high numbers of Penicillium expansum spores can be disinfested with acetic acid vapor. If levels of acetic acid vapor are not kept below 1500 ppm/hr the fruit surface can be burnt by the acid vapor. Generally phytotoxicity is not a problem in pears unless the fruit is heavily contaminated with P. expansum spores and a higher dosage of acetic acid vapor must be used, or the fruit is over mature and very sensitive to burning.
Research is presently focused on determining the optimum concentration of acetic acid vapor needed to destroy common fungal pathogens on four pear cultivars. The effect of single and repeated fumigations on decay control in processing fruit, fumigating bin lots of fruit, and methods to accurately measure concentration and prevent corrosion of cooling equipment.
Several strategies are necessary to minimize decay in stored apples and pears. An integrated approach is essential. Sanitation is an important component of this integrated approach. The use of chlorine in combination with the sanitation of bins and the packing house environment are important. Acetic acid fumigation is a new technology that could possibly aid effectiveness of chlorine or be used instead of chlorine where sanitation is necessary but chlorine cannot be used.
1Sugar, D., and Spotts, R. 1995. Preharvest strategies to reduce postharvest decay. Proceedings Washington Tree Fruit Postharvest Conference: 31.
2Kupferman, E., Spotts, R., and Sugar, D. 1995. Practices to reduce postharvest pear diseases. Washington State University Tree Fruit Postharvest Journal Vol. 6, No. 2, 18-23.
3Davidson, P. M., and Juneja, V.K. 1990. Antimicrobial Agents. pp. 83-137 In Food Additives. eds. A.L. Branen, P.M. Davidson, and S. Salminen. Marcel Dekker, Inc. New York.
4Sholberg, P.L., and Gaunce, A.P. 1995. Fumigation of fruit with acetic acid to prevent postharvest decay. HortScience 30: 1271-1275.
5Sholberg, P.L., Reynolds, A.G., and Gaunce, A.P. 1996. Fumigation of table grapes with acetic acid to prevent postharvest decay. Plant Dis. 80:1425-1428
Dr. Peter L. Sholberg, Plant Pathologist
Agriculture and Agri-Food Canada
Pacific Agri-Food Research Centre, Summerland, British Columbia
14th Annual Postharvest Conference, Yakima, Washington
March 10-11, 1998