Everything you need to know about growing and harvesting of the apple trees.
Some varieties are currently marketable only if they meet strict standards for red color intensity and coverage. There is a tendency for wholesalers to gradually increase color standards, thereby encouraging growers to select redder strains of previously acceptable bi-colored apples.
Red color is not an indicator of fruit maturity or quality, however. With few exceptions, the ground (background) color requirement for apples is light green, as yellowness is regarded as an indication of over mature or senescent fruit. Recently, consumers have preferred ‘Golden Delicious’ apples that have a white skin color, rather than green or yellow. Consumers demand fully green ‘Granny Smith’ apples without a red blush and 100% red color for ‘Red Delicious.’
A high quality apple in the marketplace is free from blemish, although there may be a greater tolerance for defects in certain markets such as organic outlets. Occurrences of physically induced damage such as bruising or stem-punctures and physiological and pathological disorders are not acceptable in any market.
The prevalence of these defects can be affected greatly by variety characteristics such as stem length, skin tenderness, softness of the fruit, and genetically based resistance to physiological and pathological disorders. The density of the flesh and the skin thickness can also contribute to resistance of fruit to bruising under normal handling conditions, and susceptibility to bruising can determine the commercial success of a variety.
A universal constituent of quality regardless of variety is firmness. Consumers demand apples that are crisp and crunchy. Other textural or flavor components are secondary. All apples are not required to have the same firmness values, and optimum values are dependent upon the characteristics of an individual variety. For example, a crisp ‘Granny Smith’ apple is often 80 to 98 N (18 to 22 lb-force) while a crisp ‘Golden Delicious’ is above 53 N (12 lb-force).
Sweetness and acidity vary by variety. For example, the acidity of ‘Granny Smith’ apples is high (0.8 to 1.2% malate) while that of ‘Red Delicious’ is low (0.2 to 0.4%). Similarly, sugar content of apples also varies by variety. ‘Fuji’ apples can have 20% or more SSC.
Ethylene Production and Sensitivity:
The apple is classified as a climacteric fruit, exhibiting increased respiration rates during maturation and ripening. This rise is associated with increases in internal concentrations of CO2 and ethylene, respiration and autocatalytic ethylene production. Endogenous ethylene production can vary greatly among varieties.
In general, early season varieties have high ethylene production rates and ripen quickly, while late season ones have low ethylene production rates and ripen slowly. The timing of the climacteric and ripening of apple fruit is advanced by exposure to ethylene.
Prevention or slowing of ethylene production, by affecting ethylene synthesis or perception, is a strategy for increasing fruit storability. This is achieved primarily by use of low storage temperatures and application of CA storage technologies. A new compound, 1-methylcyclopropane (1-MCP), is now available on a limited basis under the commercial name SmartFresh.TM1-MCP is structurally related to ethylene, has a non-toxic mode of action, and is applied at very low levels, with low measurable residues in food commodities.
Respiration Rates:
In general, early season varieties have high respiration rates, while late season ones have low respiration rates. The respiration rate of fruit is directly affected by temperature, and the respiratory climacteric is suppressed by storage temperatures below 10 °C (50 °F). The lowest temperatures for storage must be above freezing and those at which chilling injury will develop.
Temperature Summer apples Fall apples (mg CO2 kg-1 h-1)
(i) 0 °C 3to62to4
(ii) 5 °C 5 to 11 5 to 7
(iii) 10 °C 14 to 20 7 to 10
(iv) 15 °C 18 to 31 9 to 20
(v) 20 °C 20 to 41 15 to 25
To get mL kg-1 h-1 divide the mg kg-1 h-1 rate by 2.0 at 0 °C (32 °F), 1.9 at 10 °C (50 °F), and 1.8 at 20 °C (68 °F). To calculate heat production, multiply mg kg-1 h-1 by 220 to get BTU per ton per day or by 61 to get kcal per metric ton per day.
Maturity and Marketing Season:
In general, an apple fruit harvested less mature will have poor color and flavor and can be more susceptible to physiological disorders such as bitter pit and superficial scald. Fruit harvested over-mature tend to be softer, more easily damaged, may have water-core, and be more susceptible to diseases and physiological disorders such as senescent breakdown. The length of storage of apples generally can be increased by harvesting fruit before they are fully mature, but quality characteristics such as varietal flavor decrease as immaturity at harvest increases.
Fruit harvested early in the harvest window for long-term storage (6 to 12 mo) tend to have less flavor than those allowed to ripen further on the tree but are acceptable if minimal flavor requirements are met along with good texture. The challenge is to decide appropriate harvest periods for each marketing period, which can range from immediate on-farm sales to marketing across the globe after 11 mo of storage.
A wide range of indices has been tested over many years as possible indicators of harvest maturity. Several common methods have been outlined by Lau (1985). Ethylene, measured commonly as internal ethylene concentration, and the starch index, in which the degree of starch hydrolysis is estimated, have become the most widely used maturity indices. Since a rise in ethylene production is associated with initiation of ripening, it has been suggested that ethylene production or internal ethylene concentration (IEC) should be a major determinant of harvest decisions.
However, relationships between ethylene production and optimum harvest dates can be poor, and the timing, or presence, of increased ethylene production is a function of cultivar, and within a cultivar is greatly affected by factors such as growing region, orchard within a region, cultivar strain, growing season conditions, and nutrition. Therefore, ethylene production may not be relevant for determining harvest of some cultivars.
For some bi-colored apples, background color is considered an important harvest index. In some cases, state regulations have been established to set minimal harvest maturities, e.g., in California the starch index for ‘Granny Smith.’ While ethylene production is regarded as the only physiological indicator of apple fruit maturity, fruit of some varieties and growing regions are often harvested well before autocatalytic ethylene production occurs, and therefore this measurement is commercially irrelevant.
However, in some regions ethylene is used to determine when fruit are too mature to be candidates for long-term storage. It is generally recognized that no single maturity index is appropriate across all varieties; growers have learned to rely on a combination of indices.
Other indices such as flesh firmness and soluble sugar content are quality indicators rather than maturity indicators, as they are influenced greatly by orchard factors, e.g., fruit exposure to light. During the pre-harvest season, firmness falls and sugar content continues to increase. However, these quality indicators provide information that can be important to fruit performance in storage. Both indices are increasingly used in the marketplace as quality criteria by wholesalers, especially in Europe.
The three major apple-growing regions in the U.S., Washington, New York and Michigan, operate apple maturity programs in conjunction with their land grant universities. Currently in Washington State individual packinghouses conduct their own maturity programs in line with their marketing strategies. In each region, a wide range of maturity and quality indices are collected, and the optimum harvest period (harvest window) is established for each variety.
The strength of these programs lies not in reliance on absolute maturity indices, but on discussion with industry personnel on changes in maturity and quality occurring over the harvest period. In this way, full participation of extension personnel, growers and storage operators can ensure that fruit of appropriate storage potential are directed towards short, medium or long-term storage.
U.S. grades are U.S. Extra Fancy, U.S. Fancy, and U.S. No. 1, based primarily on color requirements, but also on freedom from decay, disorders and blemishes, as well as firmness of fruit. These federal guidelines have been adopted by many states, but states may have additional grading and branding laws.
Information pertaining to any state can be obtained from the local state department of agriculture. Washington State packers follow the grade standards of either U.S. Federal regulations or special Washington State Grade Standards promulgated by the Washington State Department of Agriculture (WSDA) in conjunction with USDA.
Sizing is usually carried out by weight or fruit diameter but is independent of grade. Requirements for fruit size vary greatly by market, but in general larger sizes bring greater returns. Most fruit are packed into bushel cartons, usually 40 lb (18.2 kg), depending on variety, and sold by count (fruit per carton). Apples are most often packed on 4 to 5 soft fiberboard trays made from recycled newspaper.
In some cases, the tray may be made of soft polystyrene. Cartons are often unvented. However, unvented cartons on pallet stacks will cool slowly, detrimentally affecting product longevity. Venting to improve cooling rates of fruit is becoming more common.
A two-layer carton that is wider, known as the 60 x 40 pack, is becoming more common in the U.S. and for export to Europe. It has the advantage of minimizing fruit handling as the cartons are placed directly onto display racks at retail.
Most apples are sold loose, although fruit are increasingly available in polyethylene bags of 3, 5, or 10 lb (1.4, 2.3, or 4.5 kg). These bags were originally used for marketing smaller fruit, but are now used for all qualities and sizes. Bags are most often sold in warehouse-type retail stores.
Consumer packages in which 2 to 6 apples, or a combination of fruits, are shrink-wrapped are becoming more popular in some retail outlets. Shrink-wrapped packages reduce the time consumers spend in the produce section, and also reduce loss caused by consumer sorting and handling of individual fruit.
The rate of cooling of apple fruit affects retention of quality, but its importance varies according to variety, harvest maturity, nutritional status of the fruit and storage history. It is very important to rapidly cool apple varieties that mature in the early part of the harvest season (summer varieties) since they will soften more rapidly than those that mature in the later part of the harvest season. Within a variety, apples tend to soften more rapidly at later stages of maturity than earlier stages. Effects of slow cooling are magnified as storage length increases.
Therefore, inadequate investment of resources at harvest to ensure rapid fruit cooling may not be apparent until late in the storage period when fruit may not meet minimum firmness standards for marketing. For example, a 1-day delay at 21 °C (69.8 °C) before cooling results in a 7 to 10 day loss of storage-life for ‘McIntosh.’ The effects of delays before cooling of fruit, irrespective of timing of CA conditions.
Apple fruit can be cooled by room cooling, forced-air cooling, or hydro- cooling. Forced-air cooling and hydro-cooling systems can be used to rapidly reduce fruit temperatures, but they are not widely used for apples in the U.S. Room-cooling, in which normal air flow within the storage room cools the fruit, is the predominant method in most regions. However, air flows around rather than through bins of fruit, and therefore this method is slow and inefficient.
Rapid cooling is often difficult to accomplish when rooms are filled rapidly and refrigeration capacity was not designed for a large fruit load. This problem can be overcome in two ways. First, fruit can be separated and loaded into a number of rooms for pre-cooling before being moved into long-term storage. A second option is to load only the quantity that can be handled by the existing refrigeration system.
When refrigeration capacity is a limiting factor, no more than two stacks of bins should be placed across the width of the storage room each day, and that should be reduced to one stack if the air temperature in the room is not down to 0 °C (32 °F) by the next morning. Faster cooling will be obtained if bins are placed in the downstream discharge of the evaporator with pallet runners oriented in the same direction as the air flow.
Additional bins of fruit should be stacked, no more than two high, in unfilled refrigeration rooms to cool overnight before loading into the CA room the next morning. These stacks should be placed randomly throughout the unfilled room to maximize air exchange with the fruit. Capacity to cool fruit is dependent on refrigeration capacity and room design. A qualified refrigeration engineer should assist in the development of a cooling program.
Maximizing quality maintenance of fruit requires attention not only to temperature immediately after harvest, and during storage, but also during packing, transport, and retail display. This combination of events is sometimes described as the “cold chain,” highlighting the importance of maintaining the links from harvest to consumer.
Excellent discussions of cooling can be found in Postharvest Technology of Horticultural Crops, Commercial Cooling of Fruits, Vegetables and Flowers, and Bartsch and Blanpied (1990).
Optimum Storage Conditions:
Apple producers have learned that apple fruit respond dramatically to both temperature and atmosphere modification. Rapid temperature reduction and the exacting maintenance of low temperatures close to the chilling point of the variety can provide good to medium quality product following 3 to 6 mo of storage and in some cases longer. However, modern commercial warehouses couple temperature management with CA for long-term storage of apples.
Regular Air Storage:
The recommended conditions for commercial storage of apples are -1 °C to 4 °C (30.2 to 39.2 °F) and 90 to 95% RH, depending upon variety. The acceptable duration of air storage has become shorter over the last several years as quality standards in the market have increased. Also, short-term CA storage is becoming more common as the period available for sale of air-stored fruit has decreased.
Temperatures for air-stored fruit are affected by sensitivity of the variety to low temperature disorders. While lower temperatures usually result in firmer and greener fruit, some varieties such as ‘McIntosh’ can develop core browning, soft scald and internal browning when held at temperatures below 3 °C (37.4 °F).
However, these disorders typically develop only in fruit kept for more than several months, so risks of low-temperature injury are low for fruit kept in short-term storage (2 to 3 mo). An additional factor to consider in selecting storage temperatures is the impact of temperature on RH requirements. It is easier to maintain RH > 90% at 1 °C (33.8 °F) than 0 °C (32 °F). Final decisions should be based on experience with a variety and advice of extension personnel.
Most apple varieties are not sensitive to chilling temperatures and should be stored as close to 0 °C (32 °F) as possible. However, varieties that are susceptible to low temperature disorders should be stored at 2 to 3 °C (35.6 to 337.4 °F). Temperatures also should be increased for fruit stored in low O2 CA, since lower temperatures increase risk of low O2 injury.
Temperature in storage rooms should be monitored throughout the storage period using thermocouples throughout the room. It is dangerous to rely on a single thermometer at the door, as temperatures within stacks and throughout the room may be lower or higher than indicated by such readings. Faster fruit ripening and greater refrigeration usage result when fruit temperatures are too high.
Excessive temperatures after packing due to lack of cooling or developing during transport to market can negatively impact quality at the consumer level. Fruit temperatures can increase during packing; failure to remove heat may result in subsequent loss of firmness during transport.
Controlled Atmosphere (CA) Considerations:
Apples are the predominant horticultural commodity stored under CA conditions, but the gas composition and storage temperature conditions are specific to variety, growing region, and sophistication of the equipment available for monitoring and controlling the atmospheres. Interactions occur between O2, CO2 and temperature.
For example, low storage temperatures increase fruit susceptibility to low O2 injury. Also when very low O2 levels are utilized, levels of CO2 should be reduced to prevent CO2 damage.
The wide range of recommended atmospheres has been reported by Kupferman (1997) and reflects the above factors as well as strategies employed by different industries. Information about CA recommendations for varieties in any growing region should be obtained from local extension personnel.
Until the mid-1970’s, 8 to 10 days were often required to load a CA room and a further 15 to 20 days were needed for fruit respiration to lower O2 to 2.5 to 3%. Fruit quality resulting from these conditions gradually became unacceptable in the marketplace. Rapid CA is now standard practice in many apple industries.
Nitrogen flushing equipment enables O2 in CA rooms to be reduced to less than 5% within a day or two of harvest, although 4 to 7 days from the harvest of the first fruit and placing into the storage room with CA conditions is considered “rapid CA.”
For certain varieties, fruit core temperatures must be reduced to predetermined thresholds before application of CA. Even when rooms are filled over extended periods, O2 concentrations are usually lowered by flushing with N2, and it is becoming more common to use N2 flushing for re-sealing rooms that are opened briefly to remove some of the fruit required for marketing. Nitrogen used for flushing is either purchased in tanks or generated on site.
An RH of 90 to 95% is recommended for apples to prevent shrivel. The major causes of dehydration are small coil surface areas and/or frequent defrosting. When CA rooms are designed, the refrigeration engineer should demand the largest coil size feasible for the room.
Operators have been reducing the number of defrost cycles to an absolute minimum to optimize RH in the room. Some operators reduce the O2 to the minimum safe level and then raise the temperature to 1 to 2 °C (33.8 to 35.6 °F) to minimize the need to defrost.
Some storage rooms are outfitted with high pressure water vapor systems that add moisture to the room and are suited for operation at around 0 °C (32 °F). The air distribution system should be designed to prevent condensation of water droplets on fruit to prevent decay. The use of plastic rather than wooden bins, or poly tubes (bin liners) inside wooden bins, has also helped minimize shrivel of ‘Golden Delicious.’
Once fruit have been cooled and CA conditions established, CA storage regimes fall into one of three categories, depending on level of equipment and technology involved.
Standard CA involves conservative atmosphere conditions used with minimum risk of gas-related injuries. Control of these atmospheres may be manual by daily reading and adjustment, or via computer controlled equipment. The margin of safety is large enough so that fluctuations in gas concentrations in manually adjusted storages should not cause fruit injury.
Low O2 CA storage requires that fruit be kept at O2 < 2%, but above the concentration at which fermentation will occur. Non-descriptive terms such as ‘ultralow’ are sometimes used but should be avoided in favor of describing specific O2 percentages. The safe O2 concentration varies by variety and region.
‘Delicious’ apples from British Columbia, Canada, for example, can be stored safely at 0.7% allowing control of superficial scald without use of diphenylamine (DPA). Fruit of the same cultivar from other growing regions may show injury when stored at these low O2 levels.
Strains within a variety can also vary in sensitivity. An extreme case is the Marshall strain of ‘McIntosh’ where O2 < 4 to 4.5% are not safe whereas 2 to 3% O2 is acceptable for other ‘McIntosh’ strains. In general, it is necessary to increase storage temperature when low O2 CA storage is used. A number of guidelines have been developed for safe operation of long-term CA storage.
Using these techniques, it has also been possible to minimize risk of low O2 injury in the northeastern U.S.:
1. Apply low O2 CA storage only to apples harvested early in their harvest window. Over-mature fruit can be damaged by low O2 storage.
2. Avoid apples from orchard blocks that average fewer than five seeds per apple. Low seed count can be a problem with some varieties.
3. Reduce core temperatures to 1 to 2 °C within 2 days after harvest for ‘McIntosh’ and ‘Empire.’ (In Washington, cooling can take longer as it is done under CA, with the exceptions of ‘Fuji’ and ‘Braeburn’).
4. Decrease O2 < 5% within 7 days after harvest, except for ‘Fuji’ and ‘Braeburn.’
5. Raise storage temperatures from 0 to 2 °C can reduce risk.
6. Automatic gas analysis/control equipment to eliminate O2 fluctuations that may lead to low O2 injury.
7. Avoid use of postharvest drench with DPA where possible; it’s application has been associated with low O2 injury, e.g., for varieties with low scald risk.
Low Ethylene CA Storage:
Apple fruit are climacteric, with autocatalytic ethylene production often beginning close to harvest. However, rates of ethylene production can vary greatly among apple varieties. An important physiological effect of CA storage is inhibition of either ethylene production or its action due to lowered O2 or increased CO2.
Low ethylene CA storage has been evaluated as a method for reducing superficial scald, as a safe substitute for low O2 CA storage, and for retarding flesh softening and other forms of senescence. Low ethylene CA storage (< 1 ppm or 1 μL L-1) was used successfully in New York for storage of the naturally low ethylene-producing ‘Empire’ apple, but it has been replaced by low O2 storage.
In general, low ethylene CA storage has not proven successful for maintenance of fruit quality if levels of ethylene gas within the fruit cannot be controlled, and generally the return on investment in this technology has been poor.
Other methods used in conjunction with CA storage to maintain quality of apple fruit include treatments using short-term stress levels of low O2 or high CO2. In varieties including ‘Granny Smith,’ ‘Delicious,’ and ‘Law Rome,’ O2 of 0.25 to 0.5% for up to 2 weeks has resulted in control of superficial scald.
High CO2 (15 to 20%) treatments before application of CA storage were used for maintenance of firmness of ‘Golden Delicious’ apples in northwestern North America, but generally are no longer recommended due to fruit damage.
Under commercial conditions, fruit from CA rooms should be sampled at monthly intervals to detect development of any storage problems and therefore reduce the chances of major fruit losses. Sampling should be carried out by placing representative samples of fruit near a sampling port in the door of the CA room. Samples should be kept in mesh bags rather than plastic bags to prevent false positive readings for scald.
The selection of CA atmospheres and temperature must take into consideration the variety and in some cases, as mentioned above, the strain of a particular variety, in addition to where it was grown. Experience in Washington has shown that varieties can be divided into two types – those tolerant of high CO2 and those that are not.
‘Gala’ and ‘Golden Delicious’ are CO2-tolerant varieties which also benefit from rapid reduction of atmosphere. In Washington, fruit with moderate pulp temperatures can be placed into a low O2 environment without danger of CO2 damage.
Rapid CA is valuable because it helps retain fruit firmness and acidity better than slowly established CA on these varieties. Washington grown ‘Gala’ and ‘Golden Delicious’ can be stored as low as 1.0% O2 with CO2 levels up to 2.5% at 1 °C (33.8 °F). If the temperature is lowered below this point, O2 is raised. Regular storage is 0 °C (32 °F).
‘Fuji,’ ‘Braeburn’ and ‘Granny Smith’ are varieties in the CO2-intolerant category. Their cells are densely packed and air exchange within the fruit is therefore reduced. In Washington, these apples must have the flesh temperature close to the storage temperature before the O2 is reduced. These varieties have a tendency to develop internal browning, a CO2 damage symptom that is associated with a natural predisposition of the variety (and pre-harvest factors as well as storage regime).
CO2 should remain well below the O2 level at all times, and temperatures should be slightly elevated. For example, fruit stored at 1.5% O2 are stored with CO2 below 0.5% at 1 °C (33.8 °F) if fruit are appropriately mature at harvest. It is not advisable to store waxed fruit in boxes with poly-liners in CA, as this can hinder air exchange within fruit.
‘Red Delicious’ is somewhat CO2-tolerant and is also tolerant of rapid CA. However, producers have not seen the dramatic positive effects of rapid CA on ‘Delicious’ that have been noted on ‘Golden Delicious’ or ‘Gala.’ ‘Delicious’ fruit soften more rapidly in a bin than on the tree, so CA should not be delayed after harvest. Typical regimes for CA of non-water-cored ‘Delicious’ are 1.5% O2 and up to 2.0% CO2 at 0 to 1 °C (32 to 33.8 °F).
State regulations on CA storage cover both the safe operation and use of the legal definition of “Controlled atmosphere” for stored apples. Regulations include the rate of establishment of CA conditions, the maximum level of O2 permitted and the length of time fruit are in CA. Since regulations vary from state to state, operators need to contact their state Department of Agriculture.
Many precautions must be taken to assure the safe operation of CA storage rooms. Operators must be aware of the risks of working with O2 levels below those needed for survival. Death can be almost instantaneous. Additional precautions must be taken when working with CA generators to avoid implosion or explosion hazards. In short, workers must be thoroughly trained. Contact your local extension office for information about safety in CA.
Softening, yellowing, respiration, loss of titratable acidity, and sometimes a reduction in SSC, as well as development of several physiological disorders, are delayed or inhibited by 1-MCP application. Responses of fruit to 1-MCP may be affected by cultivar and fruit maturity. Volatile production by apples also is inhibited by 1-MCP, being consistent with the view that volatile production is regulated by ethylene. Consumer studies on acceptability of 1-MCP treated fruit will be required to ensure flavor is not unacceptably compromised.
Retail Outlet Display Considerations:
Keeping apple fruit cold reduces metabolic rates and maintains fruit quality, but the trend in most new retail outlets is not to have refrigeration in display tables. Moreover, typical displays in U.S. supermarkets employ stacking of loose fruit with increased risk of bruising. There is some movement towards use of display cartons such as the 60 x 40 box, which reduces the need to handle fruit and ensures more rapid turnover of product.
The main postharvest diseases of apples that develop in storage are blue mold caused by Penicillium species and gray mold caused by Botrytis cinerea. Blue mold is the most common and destructive of all the rots.
Most blue mold decays are caused by P. expansum, but P. solitum, P. commune, and P. crustosum are also common postharvest pathogens of apples. Penicillium species enter fruit primarily through cuts, stem punctures, and bruises. However, some cultivars of apples can also be invaded via the stem during long-term CA storag.
Numerous other pathogens can also appear in stored apples. Some postharvest pathogens infect fruit in the field but remain latent or quiescent until after apples are harvested and placed into storage. These include the Colletotrichum species that cause bitter rot, Botryosphaeria species that cause black rot and white rot, and Pezicula malicortis, the cause of bull’s eye rot. Postharvest decays initiated in the field must be controlled using fungicides or other disease management strategies during the growing season.
Blue mold and gray mold have been controlled since the early 1970’s using postharvest applications of benzimidazole fungicides. Thiabendazole (TBZ), benomyl, and thiophanate-methyl were all registered for postharvest use on apples until the postharvest labels for benomyl and thiophanate-methyl were withdrawn in the early 1990’s.
TBZ is usually applied immediately after harvest in combination with the antioxidant DPA. TBZ may be applied a second time as a line spray or in wax as apples are packed.
Benzimidazole-resistant strains of P. expansum and B. cinerea were discovered in apple storages during the mid- to late-1970’s. However, postharvest application of a benzimidazole plus DPA continued to control blue mold and gray mold because most benzimidazole-resistant strains of the pathogen were highly sensitive to DPA.
During the mid-1990s, the incidence of blue mold began to increase in some apple packinghouses where the predominant strains of P. expansum had developed resistance to benzimidazole-DPA combination. Gray mold is still controlled by the benzimidazole-DPA combination, presumably because it does not recycle on field bins as readily as does P. expansum, and it therefore has been subjected to less selection pressure for fungicide resistance.
Captan has a postharvest registration but has proven only moderately effective for controlling P. expansum and B. cinerea. Captan residues are not acceptable in some export markets. The new fungicide fludioxonil is very effective for controlling P. expansum on apples. Fludioxonil is not yet approved for use on apples in the U.S., but it may receive an EPA registration in the near future.
Much effort has been devoted to development of bio-controls for postharvest diseases of apples. The controlled postharvest environment theoretically should allow selection of bio-control agents particularly suited to those environments. Many of the bio-control agents selected and developed to date have proven very effective in controlled tests, but commercialization of bio-controls has been slow.
Product developers view the market potential for postharvest treatments as relatively limited because the products are applied in a closed environment rather than being sprayed over thousands of acres as are conventional fungicides used to protect crops in the field.
Furthermore, liabilities involved in postharvest treatment of apples are considerable both because of the value of the stored crop and because apples as a commodity have previously been spotlighted in debates relating to food safety issues.
In addition, devising shelf- stable formulations of bio-control agents has been difficult. Some currently registered products must be kept frozen until use. Activity of bio-control organisms may be compromised by excessively high pathogen inoculum or by presence of other compounds in postharvest treatment mixtures.
Bio-controls generally cannot provide eradicant activity against established infections. Thus, infections by P. expansum that occur during harvest can be controlled with TBZ fungicide that is applied several hours later whereas most bio-control agents are ineffective if the pathogen is already established in the infection court when the bio-control is applied. Using combinations of bio-controls and reduced rates of TBZ may be more effective than using either product alone.
When such combinations are used, the chemical fungicide may provide eradicant and short-term protectant activity necessary to prevent decays until the bio-control agents become established in wounds or other infection courts Bio-controls provide some protection against TBZ-resistant strains of the pathogens for which no other controls are currently available.
Regardless of the postharvest fungicide or bio-control strategies that may evolve from current research, good sanitation will remain essential for reducing disease incidence. Inoculum for blue mold recycles from year to year on contaminated field bins and in packinghouses and cold storage rooms.
Recycling of P. expansum on field bins results in repeated exposure of the same pathogen strains to postharvest fungicide treatments, thereby contributing to more rapid selection of fungicide-resistant strains.
Badly contaminated bins should be cleaned and disinfested (steam-cleaned) before they are re-used for a new crop. Plastic bins may carry less inoculum than wooden bins. Plastic bins also have the advantage of reducing bruising and abrasion where apples contact the sides of bins. Careful fruit handling, rapid cooling after harvest and storage at recommended temperatures also help limit postharvest decays.
These vary widely according to marketplace and country and guidance should be sought from the local Department of Agriculture.
Suitability as Fresh-Cut Product:
As for other fruit, methods to reduce senescence and browning processes in cut apples are being investigated and commercial products are available. However, potential for improvement is large.
Additional Information and Websites:
Many states have workshops and materials available. Washington, Michigan and New York offer regular CA workshops and proceedings are available from Michigan State University and Cornell University, respectively. Contact your local extension office or horticulture society to obtain information for your state. In addition, newsletters are available.