Postharvest Integrated Pest Management
Today fruit growing and marketing are more complex. We have managed to separate fruit harvest from the fresh-fruit consumer by thousands of miles in distance and by as much as 12 months in time. Postharvest diseases and disorders have caused occasional major losses during storage and transit, but we control most problems by using chemicals that help to preserve quality and prevent decay. Now we are encountering consumer concerns about agrichemicals. This may engender tremendous changes in the fruit industry. This article focuses specifically on the apple industry, but many of the concepts are applicable to other fruit commodities.
The two biggest changes in the apple industry are efforts to minimize or eliminate traditional postharvest treatments and the shift in the Pacific Northwest away from Red Delicious to other new cultivars. Both changes will have an impact on postharvest disease management. New cultivars will undoubtedly bring with them new postharvest problems. The move away from traditional postharvest treatments will necessitate development of alternative control strategies for postharvest problems. This shift may be gradual or sudden depending on regulatory developments and activist pressure but is permanent and inevitable. As we eliminate traditional postharvest chemicals, the industry should recognize that there will probably be no more 'silver bullets' to solve postharvest problems as neatly as did the combination of benzimidazole fungicides and diphenylamine.
Biocontrols offer some promise as replacements for traditional agrichemicals, and I applaud the research efforts underway in the area of biocontrols. However, the prospects for commercial development of biocontrols are still uncertain. Compared to traditional fungicides, biocontrols are likely to be more sensitive to inoculum density and to variations in application method and storage conditions. Because each biocontrol is likely to have a narrow spectrum of usefulness as compared to the old broad-spectrum fungicides, the economics of developing, registering and marketing these niche products remains questionable. It is also unclear whether consumers will find biocontrols on their food any more acceptable than they have found traditional fungicides.
The good news is that I believe we can successfully store and market apples without using postharvest fungicides if we integrate postharvest disease management into all phases of production and marketing. Effective and consistent control of storage diseases requires careful attention to all of the following aspects of fruit production: 1) cultivar selection and orchard planning; 2) crop management in the field; 3) harvest management; 4) sanitation in the orchard, storage and packinghouse; 5) postharvest treatments; and 6) storage and marketing practices.
Cultivar selection and orchard planning are rarely considered part of postharvest disease management, but we all know there is a cultivar effect on postharvest problems. The Northwest apple industry is at a major turning point as large acreages are being replanted with new cultivars. As the industry invests in new cultivars, how many of the following points have been given appropriate consideration?
Susceptibility to Nutritional Disorders
Susceptibility to Moldy Core
Ability to Define Optimum Harvest Maturity
Crop management in the field is an essential part of producing healthy fruit with maximum natural resistance to postharvest diseases and disorders. Certainly, fruit with visible physiological problems will be prone to breakdown and decay during storage. More research is needed to determine how orchard nutrition and irrigation regimes affect fruit quality and susceptibility to decay. We already know that high calcium levels generally make fruit more resistant. Perhaps we can fine-tune other aspects of orchard management to improve storage quality of fruit.
In areas that receive summer rainfall, numerous fungi can infect fruit prior to harvest. Infected fruit that have not yet developed symptoms are said to have latent infections. Pinpoint scab, fly speck, bitter rot, black rot, brown rot and bull's-eye rot are all fungal diseases that can develop in storage if fruit have latent infections at harvest. Where these diseases occur, a combination of summer fungicide sprays and orchard sanitation measures (pruning out infected wood) is required. Postharvest fungicide treatments are usually not effective for controlling latent infections caused by these fungi. Harvest management involves harvesting fruit at appropriate maturity, minimizing bruising, and cooling fruit as rapidly as possible after harvest. Bruised fruit and fruit that are overly mature at harvest are more likely to develop physiological and decay problems during storage. Smooth orchards and driveways are essential for getting fruit from the orchard to storage with minimal bruising. Pickers and forklift operators must be repeatedly sensitized to the fact that fruit, even hard fruit, can be bruised and wounded during harvest and transportation.
Rapid cooling after harvest is essential, both for maintaining apple firmness and quality and also because cold temperatures slow development of decays. Some fungi, such as those causing brown rot, black rot, and bitter rot, cannot grow or grow only very slowly at cold storage temperatures. If these fungi are found causing storage decays, most of the decay development probably occurred between harvest and the time the fruit were cooled to less than 40°F. The one exception to the need for rapid cooling would be cases where pears are heat-treated immediately after harvest to eliminate certain fungal pathogens.
Keep fruit and bins from contacting soil. Many decay organisms are soil inhabitants picked up on the bottom of bins when bins are set directly on the soil. Mucor is an especially troublesome soil contaminant because it is not controlled by postharvest chemical treatments. Keeping bins on sod is one means to avoid soil contact, but the sod barrier is often ineffective when bins are pushed or when the forklift picks up soil with the bin. A better solution might be a stiff, reusable slip-sheet under bins in the field to keep them totally out of contact with the soil.
If postharvest drenches or water flumes are used, keep the solutions as clean as possible. Concentrate on keeping soil out of drench solutions. In drive-through drenching systems, trucks or trailers may need to be pre-rinsed with a high-pressure washer to remove mud or dust before the load is taken through the drencher. Drench solutions should be changed or filtered when they become dirty. When changing solutions, all soil residues must be removed from the tank. Drencher tanks where fungicides are applied should have rounded corners because mud tends to entrap fungicide and settle into the corners of square tanks. If we could eliminate all soil contact and all water handling and drenching prior to CA storage, I suspect we would eliminate the majority of our postharvest problems in apples.
Disinfect badly contaminated bins before they are re-used. It probably is not economical to disinfect all apple bins between seasons. However, bins that contained badly decayed fruit (such as cull bins) and bins with visible fungal growth on the wood should be disinfected before they are stored for the summer. Penicillium will survive hot, dry weather in bin wood. Inoculum recycled into the storage on contaminated bins has already been exposed to postharvest chemicals and therefore is more likely to be resistant to benzimidazole/DPA treatment. Cleaning up this source of inoculum may help to reduce the proportion of benzimidazole-resistant isolates in storage.
Decayed culls and leaf debris should be removed from packing sheds daily. A single decayed apple can produce millions of Penicillium or Botrytis spores that will be blown about the storage with only the slightest movement of air. Most decays will not sporulate very much in CA storage, but spores develop very quickly if fruit are left at warm temperatures. Removing culls daily gets them out of the storage before they produce a full complement of spores.
Keep packinghouses clean. I see no reason why a packinghouse should not be kept just as clean as a kitchen. Clutter collects dust and dust collects spores that are redistributed to fruit. Packinghouse operators should remember they are in the food-handling business.
Remove or eliminate drops in the orchard. Although many decay fungi survive in soil, there is increasing evidence that these fungi cannot grow in soil unless they have a nutrient source. Wind-fall fruit appear to be the major nutrient source for at least several of the decay fungi. Removing wind-falls removes this nutrient source and should help to keep inoculum levels low in the orchard. Unfortunately, we really know very little about the ecology of postharvest decay fungi within orchards and the impact of orchard populations on incidence of postharvest decays.
Postharvest chemical treatments are still commonly used on apples to control storage scald, blue mold and gray mold. The future for postharvest chemical treatments may be limited, but I will discuss the use of benzimidazole fungicides and diphenylamine (DPA) because these compounds are still being used and are useful on apples. Few, if any, new fungicides are expected to receive registrations for postharvest use on pome fruits during the next five years. Thus, we are unlikely to find a fungicide replacement for the benzimidazoles within the foreseeable future.
Blue mold and gray mold, caused by Penicillium and Botrytis, respectively, are the most common and most important decay problems on apples. At least a dozen species of Penicillium can cause blue mold, but Penicillium expansum is by far the most important. Both Penicillium and Botrytis invade fruit primarily through wounds. Botrytis can also move from infected fruit into adjacent fruit within a bin and cause the condition known as "nesting." In modern cold storages, nesting is not common unless fruit are bruised, wounded, overly mature, or injured by freezing.
Storage scald, a physiological disorder, is usually controlled by applying an anti-oxidant postharvest treatment of either diphenylamine (DPA) or ethoxyquin just prior to placing fruit into CA storage. The drenching solution picks up spores of Penicillium and Botrytis from bins, leaves, soil and apple fruit and disperses this inoculum to wounds in the fruit. Therefore, when drenches are used for scald control, fungicides are also needed to control blue mold and gray mold. If scald inhibitors are not used and fruit are properly harvested and handled, apples can probably be stored without using postharvest fungicides.
The benzimidazole fungicides (Benlate, Topsin M and Mertect) were extremely effective for controlling blue mold and gray mold when they were first introduced. However, isolates of Penicillium and Botrytis with resistance to the benzimidazoles are now common in most apple storages in the Northeast. (No one has surveyed the situation in Washington State.) In the early 1980s we discovered that most isolates of Penicillium expansum that are resistant to benzimidazoles show increased sensitivity to DPA. Other researchers have shown that the same is true for Botrytis. Thus, the combination of a benzimidazole fungicide and DPA has generally controlled P. expansum even after resistance to the benzimidazoles has developed because DPA kills most of the resistant isolates.
Some of the other Penicillium species which can cause blue mold are less sensitive to the benzimidazole/DPA combination. Of 345 Penicillium isolates collected from water dumps in four packinghouses in the Hudson Valley, we found 81% were P. aurantiogriseum and only 19% were P. expansum. Ninety-five percent of the 277 P. aurantiogriseum isolates were resistant to both benzimidazoles and DPA, whereas only 11% of the P. expansum isolates had this dual resistance. Penicillium viridicatum is frequently found in Washington, but the sensitivity of P. viridicatum to the benzimidazole/DPA combination is unknown. Fortunately, P. viridicatum and P. aurantiogriseum are much less pathogenic in apples than is P. expansum. Although they can cause small decays around wounds, they are unlikely to cause the large soft decays typical of P. expansum.
Where postharvest treatments are deemed necessary to control storage scald, I believe the benzimidazole/DPA combination is likely to continue providing good protection against both sensitive and resistant strains of P. expansum and Botrytis. A small percentage of P. expansum isolates may be resistant to the benzimidazole/DPA combination, but these isolates and the other Penicillium species can probably be kept in check by using good sanitation measures. Loss of registration for DPA, however, would probably make the benzimidazole fungicides much less effective because isolates of Botrytis and P. expansum with high resistance to the benzimidazoles would no longer be controlled. (Ethoxyquin has no effect on benzimidazole-resistant Penicillium.) Intelligent storage, packing and marketing practices are required to avoid damaging fruit during storage and to assure that fruit with poor keeping quality is marketed first. One common-sense aspect of intelligent storage is careful monitoring of storage temperatures and atmospheres to assure that fruit are not damaged by freezing, by salt from brine solutions, or by high carbon dioxide or low oxygen levels in CA storage. The need for attention to these details would seem self-evident, but problems with temperature and atmosphere regulation still result in occasional major losses.
Any time fruit are handled in water, they can be inoculated with decay-causing fungi if fungal spores are allowed to collect in the flume water. The spore load in water flumes should be kept low either by changing water regularly or by treating water to kill or remove the spores. Chlorination has been the traditional means of disinfecting flume water, but filtering, heat treatment or ozonation may prove more acceptable in the future. Because water is such an effective accumulator and carrier of fungal spores, a water-free handling system would have real advantages if such a system could be designed to handle fruit without causing bruising.
Packingline equipment must be properly designed and adjusted to minimize bruising. Development of the small portable impact monitor at Michigan State University may provide the data to stimulate better design of packing equipment.
In summary, I think we must actively pursue development of fruit production, storage and packing operations that allow us to market fruit with no chemical residues. Managing postharvest diseases and disorders without agrichemicals will be more difficult, but it should be feasible if we integrate postharvest considerations into all aspects of fruit production and handling. Our two biggest challenges are 1) to improve sanitation during harvest and packing, and 2) to develop bulk handling methods that minimize bruising, wounding and exposure to fungal spores. With apples, I think we may be able to eliminate postharvest fungicides as soon as we can eliminate scald-inhibitor drenches. Phasing out postharvest treatments for pears and stone fruits will be more difficult, but it may be feasible for arid production areas in California and the Pacific Northwest.
David A. Rosenberger
Plant Pathologist, Cornell University Hudson Valley Lab
Tree Fruit Postharvest Journal 1(1):8-12