How to prevent diseases of fruit in storage

Postharvest disease control is a challenge in Washington State, as fruits often have a far longer postharvest life than they had on the tree.

By Dr. Eugene Kupferman

The Washington State fruit industry is oriented to the production of apples, pears, and cherries. The major apple varieties are Red Delicious, Golden Delicious, Fuji, and Granny Smith. The major pear varieties are d'Anjou, Bartlett, and Bosc. The cherry varieties are principally Bing and Rainier.

Apples and pears are shipped from the time of harvest to the end of the longest possible storage period. Thus, Red Delicious are stored in controlled atmosphere (CA) and shipped over 12 months, and Golden Delicious, Fuji, and Granny Smith are shipped for 10 to 11 months following harvest. Even Gala are stored in CA and shipped over a six- to seven-month period. The industry attempts to supply apples and pears year round, principally to large supermarkets around the world. This is unlike the fruit industries of New Zealand, Chile, or South Africa.

From the perspective of postharvest disease control, this presents severe challenges, for the fruit have a far longer postharvest life than they had on the tree. It is a well-known fact that resistance to diseases decrease as the fruit ages.

An additional challenge in control of decay is that Washington-grown fruit is shipped all over the world. Long journeys are common, and conditions are not always optimal to conserve quality and minimize decay. More than 35% of Washington State's apples are exported, with the largest share going to Asia and Latin America.

Is postharvest disease control a packer problem or a grower problem?

Washington growers have realized that the economic control of postharvest disease is a partnership between grower and packer. Neither can do the whole job. Postharvest disease is a misnomer, as it should be termed "postharvest-expressed" diseases, as the disease-causing organisms originate in the orchard.

Major postharvest diseases

The major diseases of apples and pears in Washington are fungal in origin. The climate in the fruit-growing district is very dry, so bacteria are not normally a problem. The fungi include:

Blue mold (Penicillium spp. especially P. expansum) is one of the most common problems. Fortunately, it is not a nesting fungus and therefore will not move from an infected fruit to an adjacent sound fruit unless the fruit is bruised or punctured in some way. In pears, blue mold moves into the fruit through the stem, which remains juicy and succulent. Stems in apples are normally drier. Blue mold easily moves from fruit to fruit in water systems.

Gray mold (Botrytis cinerea) is a very common decay-causing fungus. It too enters through wounds and can move in water systems. It also often enters pears through the stem. Unfortunately, gray mold can nest, and whole bins or boxes of fruit can become infected. This fungus can grow at low temperatures. Spores of Botrytis get in and on the fruit in the orchard. Thus, orchard control and fruit quality are especially important.

Mucor rot (Mucor piriformis) is a soil-borne fungus that grows well even in the winter. It is found in varying amounts from orchard to orchard. Orchards with a high amount of soil moisture and decaying fruit will often have mucor. When bins are in contact with soil, it is possible to bring huge amounts of spores into the packing house.

Bull's-eye rot (Pezicula malicorticis) usually appears in storage, but inoculum comes from cankers on the tree. It is a slow-growing fungus characterized by concentric rings. Control should begin in the orchard with eradication of cankers found in the bark of the tree.

Moldy core (Alternaria alternata) is not normally a problem in the dry growing region of Washington but can be found when rainy weather occurs during the petal-fall period.

Thus, there is a relatively short list of postharvest diseases affecting apples and pears in Washington and Oregon. It should be easy to control since there are few decay-causing organisms. However, as the list shows, each pathogen requires different control measures.

Postharvest decay is an economic problem for the fruit industry for several reasons:

There are very few effective fungicide controls.

The public would prefer not to have fruit treated with fungicides after harvest as it likely that there is some residue on the fruit at time of consumption.

There is increasing resistance or a building up of nonsusceptible spores, of the common fungi to the remaining fungicides.

Growers currently follow a multiphase approach to minimizing decay organisms. This starts with an understanding of the biology of the enemy, and the work begins in the orchard.

Orchard factors affecting decay

Pathologists have determined that, although fungal spores may be present in the orchard environment during the growing season, the fruit becomes susceptible only during the last few weeks prior to harvest.

However, with some organisms, infection may occur any time from petal fall to harvest. Washington growers watch the weather at petal fall to decide whether to apply a fungicide to those blocks that have a history of bull's-eye rot, as the fungus may infect at that time and lay quiescent until the fruit matures.

Students of pathology realize that three things must be present in order to have infection:

the pathogen;

the host in a susceptible state; and

the correct weather to promote infection.

When all three of these things are present, the risk of infection is greatest.

There are some cultural practices that growers can employ to minimize decay. These include:

Prune trees to eliminate low hanging branches, which might set the fruit in contact with grass.

Keep weeds and grass mowed, as spores can be released from the cover crop, especially in conditions of high humidity.

Keep dust down in the orchard to prevent the spread of soil-borne spores.

Keep nutrients in balance with the fruit. High nitrogen and low calcium promote decay and poor fruit quality.

Make sure the fruit has high calcium levels to help the fruit resist fungal invasion.

Time harvest so fruit is of optimum maturity at harvest. Fruit of advanced maturity is more susceptible to decay.

Handle fruit properly at harvest, as fungi often enter through wounds.

Never include fruit for storage that has fallen on the ground or has been in contact with grass or soil.

Avoid harvesting when the fruit is wet.

Mow the staging areas or use wood chips where bins are held to minimize contact with soil. Do not skid bins along the ground.

Postharvest factors affecting decay

When fruit have been harvested, take the bins to the packing house to be cooled rapidly, as cold temperature is one of the best methods of reducing the rate of growth of fungi. Rapid cooling is essential for quality maintenance as well as decay control.

Once the fruit has arrived at the packing house, the management has certain responsibilities. Temperature and storage conditions are critical; so is sanitation.

In Washington, bins are made of either plywood or plastic. In the debate about whether it is better to have wooden bins or plastic bins, there are supporters on both sides. Wooden bins are less expensive; they do not slide off trucks, forklifts, or hillsides. Plastic bins are easier to clean, the fruit cool more quickly due to greater vent space, they stack more precisely, and they may not harbor as many spores.

Regardless of the type of bin, it must be cleaned prior to being sent to the orchard. A bin with a piece of rotten fruit can provide billions of spores to a water system or the air in the packing house. Most Washington packers clean the bins only when the fruit is floated at the time of packing. Research has shown that steam cleaning can reduce spore numbers significantly. Cleaning bins with steam has proven very effective in reducing viable spores in research trials. It would be a mistake to use a fungicide on bins, as it will only help to reduce the effective life of the fungicide, because this will speed the selection of nonsusceptible spores.

Sanitation

There is a difference between "cleaning" and "sanitation." Cleaning is the removal of particulate matter, while sanitation is disinfestation of a surface. Sanitation without first cleaning is not very effective and often a waste of effort. There are several methods of cleaning. The easiest is often hot water delivered through a pressure hose. This should be done daily.

Once the surface has been cleaned by physical means, it should be sanitized. Washington packers spray quaternary ammonia compounds (QUATS), chlorine, or chlorine dioxide foam to sanitize surfaces. There are other proprietary compounds, most of which are based on these active ingredients. Packing lines are cleaned and sanitized daily in most sheds. Rooms are cleaned when empty of fruit, at least annually.

In Canada, fumigation of apples in storage with low levels of acetic acid has been tested for disease suppression. Although the results look promising, acetic acid vapor can injure fruit or workers if appropriate safety measures are not taken.

Reduction of spore count in water systems

Water systems can carry tremendous volumes of spores, and water is the most effective method of spreading decay. Therefore, it is critical to reduce spores within a water system.

The water system that the fruit contacts first after harvest is in the drencher. Research has shown that the spore count climbs rapidly as more bins of fruit are treated. It is important that the drench tank solution not be extensively recycled or extended. It is important to include a fungicide in the water system with DPA (diphenylamine) or calcium to prevent decay organisms from moving from infected fruit to sound fruit.

One packer is heating the drencher to 70°C using a type of pasteurization system with good results on decay control. This procedure has not appeared to reduce the viability of the fungicide or DPA.

Other packers have used a double drenching system in which the fruit are first exposed to a nonrecirculating chlorine rinse, which is then allowed to drip-dry prior to drenching.

The dump tank in the packing line is the next potential source of inoculation. This is the dirtiest water in the packing house, and packers usually use 100 parts per million of chlorine in this water to reduce spore load. Some Washington packers have tested the use of ozone-creating devices to reduce dump tank spore load, but these have not proven to be effective.

Alternatives such as heating the water at night can reduce spore load. Heating of water containing chlorine will drive off the chlorine into the atmosphere and possibly create a worker hazard. It is possible for the first bin of fruit processed in the morning to infest the water with viable spores. Thus, this system has its limitations.

A study in Australia of pasteurization at 60 to 80°C reduced spore viability to acceptable levels in a lemon packing line. I do not know if this is being done commercially. A commercially available constant flow bypass heater has thus far not proven effective in Washington. Heating dump tanks at night is being done commercially in the Hood River district of Oregon for pears.

There is some indication that acidifying the water (pH 3.0) in the dump tank may help reduce the number of viable fungal spores. Washington packers have achieved suppression of decay organisms with certain compounds containing phosphoric acid when used in dump tanks. This is under study. Note that acid cannot be combined with chlorine.

Postharvest fungicides

Washington apple packers rely upon the fungicide thiabendazole (TBZ) for control of the common postharvest fungal diseases. This chemical is a close relative of Benlate (benomyl), and there is cross-resistance among the benzimidazoles. We have observed that approximately 30% of Penicillium spores in apple orchards are not susceptible to control by TBZ.

Biological fungicides, those evolving from natural fauna in the orchard biosphere, are currently being tested commercially. There are several promising yeasts or fungi that exhibit positive traits for control of pathogenic fungi.

Conclusion

Sanitation is the cornerstone of any effective postharvest decay reduction program. Sanitation must be a partnership between grower and packer, and it must begin in the orchard. Once fruit are infected, there is no effective way to eradicate an infection within a fruit. Once sporulation has occurred, it is difficult to control the spread of spores and increased infection.

Grower and packer must be vigilant to minimize the number of spores that enter the postharvest system. It is only by working together that decay can be minimized.

Dr. Eugene Kupferman is a
postharvest extension specialist at Washington State University's Tree Fruit Research and Extension Center in Wenatchee.

Copyright 1999, Good Fruit Grower,105 South 18th Street, Suite 217, Yakima, Washington 98901
Voice (509) 575-2315, (800) 487-9946, Fax (509) 453-4880

How to prevent diseases of fruit in storage

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