Control of Major Postharvest Apple Diseases
A disease can be broadly defined as any departure from the normal condition of plants or plant parts which detracts from their appearance, interferes with their usefulness or reduces their value. Under this definition are included abnormalities caused by fungi (parasitic diseases), and functional disorders (physiological disorders).
In the orchard, diseases can be caused by fungi, bacteria, viruses or mycoplasmas. Postharvest parasitic diseases of apples in the Pacific Northwest are predominately caused by fungi. This article discusses strategies to control fungal diseases of apple that are of major concern to warehouse managers in the Northwest.
Disease prevention is essential to preserving fruit quality. Prevention begins in the orchard and includes using properly timed fungicide sprays, practicing orchard sanitation and harvesting at optimum maturity, using methods that minimize the bruising of fruit.
Common Diseases of Apples in Storage
Many postharvest diseases caused by fungi originate in the orchard. Therefore, some of the burden of control should fall to the orchardist. However, due to the volume of fruit being stored together from different orchards, the warehouse manager has to protect healthy fruit from disease. The following fungal diseases of apples have plagued warehouse managers. Blue Mold (Penicillium spp.), Gray Mold (Botrytis cinerea), Side Rot (Phialophora malorum), Bull's-eye Rot (Pezicula malicorticis) and Mucor Rot (Mucor piriformis). There have also been several incidences of diseases caused by Coprinus spp. For additional information, see Practices to Reduce Postharvest Pear Diseases.
Rapid Cooling and Storage
When it is received in the packinghouse, fruit should be placed into cold storage so that field heat can be removed as quickly as possible. Rapid removal of field heat has a great effect on both fruit quality and reduction in storage decay. Room loading and bin stacking procedures should be established to allow the rapid filling of rooms, and excellent air flow for cooling. Make sure that air flow through the bin runner space is not restricted by bin liners or by bins filled too full. Fruit respiration and fungal growth are both reduced at low temperatures.
Recent research with CA atmospheres indicates a reduction in the growth of certain fungi under low oxygen and high carbon dioxide regimes. Therefore, it becomes important to establish CA conditions as rapidly as possible in order to reduce postharvest decay and promote fruit quality. When it is received in the packinghouse, fruit should be placed into cold storage so that field heat can be removed as quickly as possible. Rapid removal of field heat has a great effect on both fruit quality and reduction in storage decay. Room loading and bin stacking procedures should be established to allow the rapid filling of rooms, and excellent air flow for cooling. Make sure that air flow through the bin runner space is not restricted by bin liners or by bins filled too full. Fruit respiration and fungal growth are both reduced at low temperatures.
Recent research with CA atmospheres indicates a reduction in the growth of certain fungi under low oxygen and high carbon dioxide regimes. Therefore, it becomes important to establish CA conditions as rapidly as possible in order to reduce postharvest decay and promote fruit quality.
Many warehouse managers now use drive-through drenchers to apply fungicide and scald chemicals to fruit before storage. This operation can be done rapidly and efficiently if several things are considered. Tanks containing the chemical solutions should be well agitated. See "Comparison of Benlate Drench Solutions and Apple Residues in Commercial Warehouses," Postharvest Pomology Newsletter 4(2):14-15. In this article Don George of the USDA-ARS points out that there is a large variation in the amount of Benlate actually applied to fruit. This can be caused by a lack of agitation in the tank or by using tanks which have square corners.
A second important aspect of fruit drenching is getting the solution where it is needed. It is critically important to make sure that the nozzles on the sides of the bins are spraying the chemicals through the forklift holes in the bins and not just against the sides of the bins.
A third point to remember is that the truck must remain stationary long enough so that every bin receives enough spray. Each bin must spend at least 30 to 60 seconds under the spray.
Fourth, make sure that the solutions are made up properly and that the spray remaining in the tank is discarded when dirty. Warehouses which use the filtering system on the drench tank have experienced less diphenylamine (DPA) burn because the solution is cleaner. It would be hazardous to extend the life of the solution beyond that prescribed on the label, especially if Mucor spores are present. It is possible to spread Mucor through the drencher, since none of the commonly used fungicides are very effective against this disease.
Disease prevention in the warehouse involves packinghouse sanitation to prevent the buildup of fungal spore populations. This includes daily removal of plant debris and damaged fruit, frequent monitoring of spores in the water in fruit handling equipment, and the thorough cleaning of equipment. Temperature and humidity play an important role in disease development. Fungi become more active at warmer temperatures. Finally, the proper use of fungicides and disinfectant chemicals can play an important role in disease control.
Sanitation is an extremely important factor in reducing postharvest diseases. Floors and walls should be cleaned regularly to reduce the number of spores. Equipment should be cleaned frequently. Chlorine, although tough on machinery, is an excellent killer of spores. Wash walls and floors with chlorine (1,000 ppm Cl2) regularly in areas that might become contaminated with fungal spores. Pay special attention to areas in which fruit juices might remain and dry. Chlorine has no residual activity, but is effective against a broad range of fungi. The use of Benlate or Captan (at label rates) will help reduce fungal growth. Clean storage rooms as soon as they are empty; a room with little air circulation and high humidity is a perfect breeding place for fungi.
Do not leave damaged fruit lying around a packinghouse. A single apple may have billions of fungal spores on its surface. A few badly infected apples rotting in a storage room can raise the spore level to 300,000 spores per cubic foot. Fruit which has dried and stuck to a bin can act as a source of inoculum. A few innovative packinghouse managers have instituted a regular program of washing bins after dumping.
Dump Tank Sanitation
On a hydro presize line, monitor the number of spores per unit of water and use appropriate fungicide treatments to prevent the inoculation of healthy fruit as they go over the line.
A recent study by Dr. Bob Spotts (OSU, Hood River) attempted to define the number of spores in a dump tank which could cause infection of pears. As the relationship between the concentration of spores and the amount of decay is a complex one, he was not able to establish a simple threshold spore concentration for industry use. He was able to draw some helpful conclusions. Managers should reduce spore concentrations to the lowest possible level in dump tanks and flumes, because any viable fungal spores can lead to decay when wounds are present. Previous studies by Spotts and others have shown wide ranges in the number of spores found in packinghouse dump tanks. He suggests the use of chlorine or sodium orthophenylphenate (SOPP). Spotts found that a spray rinse of the fruit was helpful in reducing the spore concentration on the fruit.
Spotts reported that both chlorine and SOPP appear to reduce the level of Penicillium spores in the dump tank. Chlorine reduced the germination of Mucor piriformis and Penicillium spores. SOPP, however, was ineffective in reducing the population of Mucor piriformis spores. Some spores of Penicillium and Botrytis are resistant to benomyl in the Mid-Columbia region. Resistance to benomyl has not increased over the last 5 years, principally because benomyl has not been used in apple or pear orchards in the Hood River area.
Acidified Dump Tank Water
A number of packinghouse managers have asked which fungicide to use in dump tanks in which the pH has been lowered to remove calcium deposits from the fruit. The calcium comes from overhead irrigation water in the orchard. To improve the appearance of the fruit, an acid is added to the tank. Using chlorine in a dump tank where the water is out of the neutral range (6.5 to 7.5 pH) is not recommended. Water which is acid (lower pH) or basic (higher pH) will drive the chlorine off into the air. The packinghouse will have a strong odor of chlorine and will be an unpleasant place to work. The use of SOPP is not recommended either, since fruit damage may occur if the pH of an SOPP solution is not basic. Benlate might be used, but I know of no studies which prove its effectiveness and lack of fruit damage at low pH levels. Probably the best strategy would be to use acid only in the worst cases of calcium buildup and to change the water frequently.
Some warehouse managers have heated the dump tank water to improve the quality of the wax application. Using SOPP under those conditions may be hazardous. Lower the rate of application to prevent fruit injury.
Commonly Used Fungicides
Few fungicides are available for postharvest application to apples in Washington State. Restrictions placed by other countries further limit the choices. The newly revised Export Manual, prepared by the Northwest Horticultural Council, contains excellent information on current regulations. The manual was written for northwest fruit packers. Ask your importer for current regulations as well. Here are a few notes about the limitations and use of the commonly available fungicides.
Chlorine. Chlorine has no residual activity but does reduce spore germination. It should only be used in solutions where the pH (acidity/alkalinity) is around neutral (pH 6.5 to 7.5). Acidification of sodium silicate solutions used for flotation of pears results in gel formation. At lower pH levels, chlorine is unstable and will be driven into the atmosphere as a gas. At higher pH levels, chlorine is less active against fungal spores. Check the pH when checking the chlorine level. Meters or test strips are available and easy to use. Chlorine can be tied up in dirt and organic matter and become ineffective. Be sure to keep checking the level of available chlorine when the tank gets dirty. Chlorine chemistry is fairly difficult. A handy review is available in Using Chlorine in the Packinghouse. Spotts has found that the addition of a surfactant can be highly useful. Chlorine is very effective against spores of Botrytis, and Penicillium and somewhat effective against Mucor. It kills spores rapidly and is inexpensive. It has a tendency to corrode equipment.
This class of fungicides includes TopsinM, MertectR and BenlateR (benomyl). These are the most powerful fungicides. The decision not to use them in the orchard has greatly extended their life in the packinghouse. They are often used as prestorage drenches, sometimes with Captan. Combining fungicides of different classes delays the time when the benzimidazoles will lose their effectiveness. The benzimidazoles have residual activity, so they provide protection while the fruit is in storage. They are highly effective against Penicillium and Botrytis. They are not effective against Alternaria (Moldy Core), Pezicula (Side Rot), Rhizopus or Mucor.
This fungicide is widely used by homeowners and commercial agriculture alike. It has good residual activity, but its activity is reduced at pH levels away from neutral (pH 7). The use of Captan is not recommended with DPA or in cases where wet fruit will be placed immediately into CA in which a Molecular Sieve will also be used. It is lethal to Alternaria and Rhizopus, but less effective against Penicillium. Its use on fruit bound for certain export markets is limited.
SOPP (sodium orthophenylphenate)
SOPP is sold as Stop-Mold "F" or Steri-Seal "D." It is highly effective against Penicillium and Botrytis, and is stable when in contact with organic matter. It is noncorrosive to machinery, but can injure fruit that stays in contact with the solution. Rinse fruit immediately after treatment with SOPP. It can irritate skin. There have been reports of SOPP causing fruit injury when the dump tank water is heated, but it may be possible to avoid injury by reducing the concentration of SOPP. Avoid using SOPP in the flumes, since the fruit remains in the flumes long enough to be injured.
Dr. Eugene Kupferman, Postharvest Specialist
Parts of this article are adapted from earlier papers by Dr. Charles F. Pierson, USDA (retired) and Dr. Eugene M. Kupferman
WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, Vol. 4, No. 3