Practices to Reduce Postharvest Pear Diseases
The fruit grower plays a critical role in determining the quality of fruit delivered to the consumer. This is true even in the area of diseases that show up in the packinghouse. Growers must begin control procedures in the orchard for fruit diseases which appear long after harvest. Preventing wounds, which are the sites for disease infection, is a critical responsibility of the grower.
Postharvest diseases cost everyone money. Disease reduction in the orchard is less costly than cullage after storage. Cullage means slow movement of fruit in the packinghouse, an expensive job of repacking, or even rejection of lots in the marketplace. We will review preharvest factors affecting postharvest decays of pears and discuss postharvest control within the storage and packinghouse.
The major postharvest diseases of pears are caused by fungi. Especially important in the Pacific Northwest are the diseases Gray Mold (Botrytis cinerea), Blue Mold (Penicillium expansum), Coprinus rot (Coprinus spp.), Mucor rot (Mucor piriformis), side rot (Phialophora malorum), and bull's-eye rot (Pezicula malicorticis). In most cases orchard sanitation and sprays will significantly reduce the amount of diseased fruit in the warehouse.
Gray mold (Botrytis cinerea) is a common decay of Anjou pears. This fungus enters through punctures and wounds. Minimize injury to fruit to reduce the amount of decay from this fungus. However, Botrytis also enters through the stem ends of Anjou pears, since the tissue at the tip of the stem remains alive even after the fruit has been picked. Researchers at the Mid-Columbia Research Station hoped that Botrytis infection could be reduced by drying stem tissue. Pears were held up to 2 weeks at room temperature or 4 months in cold storage. Unfortunately, the stems did not heal. Stem ends apparently remain a site for infection even long into the storage period. Botrytis spores on the stem end can grow down the stem and into the fruit flesh, causing decay and eventually Botrytis nest-rot.
The source of Botrytis spores is in the orchard. Fungus grows and sporulates abundantly on dead and dying plant material found in orchard cover crops, especially during cool, moist weather. Botrytis spores are formed in clusters and can become airborne. Millions of very small spores can form in a short time. In addition to causing stem end decay and the infection arising in wounds, Botrytis rot has the ability to move from fruit to fruit during the storage season. It can spread over time from infected fruit to surrounding healthy fruit and form a cluster or nest of decay. Hence, this disease has been called nest-rot.
Blue Mold caused by Penicillium expansum is a common and destructive rot found on fruits in storage and at the market. Blue Mold spores, like Gray Mold, can be airborne in tremendous numbers. See also the article by Dr. Peter Sanderson in this issue of the Journal.
Stem and neck rot develops from stem infections in fleshy stemmed varieties such as Anjou and Comice. Losses from this disease have increased since use of polyethylene box liners has extended the storage season for pears. The amount of decay that develops on a single fruit depends upon the length of the storage period. It may involve only the stem, the stem and a small area at its base, or the entire upper half of the fruit. High humidity within the polyethylene box liner favors the development of the white to bluish-green fungal mass of spores on the surface of infected tissue.
Pinhole rot occurs mainly on Winter Nelis, a pear variety with large, prominent lenticels. It first appears as numerous minute spots of decay scattered over the surface of the fruit; infection apparently occurs at the lenticel. As the disease progresses, the spots increase in size and finally coalesce, and the entire fruit decomposes.
Blue Mold is generally considered a wound parasite, but it can penetrate through lenticels, particularly those near bruises. Late in the storage season when fruit has become weakened by ripening and aging, most varieties are susceptible to lenticel infection by Blue Mold. This type of infection may result when rotted pears are handled carelessly during repacking. Environmental conditions such as moisture, ventilation and temperature directly influence the development of decay. The atmospheric moisture necessary to prevent pears from shriveling is sufficient for Blue Mold development. Lack of ventilation due to tight packing and lack of air space in storage increases the moisture around the fruit and slows the rate of cooling, making conditions favorable for fungus development.
Fungus diseases develop more rapidly at temperatures higher than the usual storage temperature for pears. Pears that are delayed going into storage, cooled slowly in storage, stored till late in the season, or held at warm temperatures after removal from storage are particularly subject to infection. Disease is not necessarily prevented or arrested even at 30°-32°F. Rotten spots continue to enlarge, and even new infections can be initiated at these temperatures. Decay proceeds slowly in the early part of the storage season when fruit is firm and somewhat resistant, but during long periods of storage it can cause serious losses.
Coprinus RotAnother fungal disease is Coprinus rot, which is often mistaken for bull's-eye rot. Coprinus rot has appeared in both Hood River and in Wenatchee. This low temperature organism (mushroom fungus) will nest and spread like Gray Mold. Spores come from a mushroom in the orchard and appear to infect fruit during the last month before harvest. One major difference between Coprinus rot and bull's-eye rot is the presence, in cold storage, of a cobweb-like, white fungal growth on the fruit surface in Coprinus rot.
Mucor RotMucor is a soil-borne fungus that grows well even during the winter. It is found in varying amounts from orchard to orchard and varies in quantity depending upon the time of year. For example, immediately after harvest the spore count in orchard soil increases. The Mucor fungus is found in debris and litter on the soil surface and most occurs in the top 2 inches of soil. Some orchards have high levels of Mucor which is related to high soil moisture and an abundance of fruit on the ground. When the bottoms of bins are in contact with contaminated soil, a large number of Mucor spores can be brought into the packinghouse in and on the bins.
Pears which had fallen on the ground were examined for evidence of fungal spores. During harvest most of the fruit on the ground had begun to rot with Gray Mold. One month after harvest most of the fruit was being decayed by Mucor. One method of reducing the number of spores on the orchard floor would be to pick up any of the early maturing fruits (Bartletts) lying on the ground. These fruits provide nutrients for the buildup of high levels of fungal spores, which may contaminate and infect later harvested Anjou or Bosc pears. Rodents such as mice and squirrels, as well as insects and rain, are factors in spreading decay organisms throughout the orchard.
Mucor spores are not easily airborne. This is in direct contrast to Botrytis and Penicillium spores. To reduce the amount of spores going into the packinghouse, growers can put a layer of gravel or wood chips on the soil surface to insulate the bottom of the bins in the loading area. Thoroughly rinsing the bottom of the bins with water to remove contaminated soil before the bins go to the packinghouse also would reduce the number of spores.
Side RotSide rot has been a problem in the Medford, Oregon, pear-growing district for the past several years. Though the primary causal fungus, Phialophora, has been found on decaying pears in Washington, it is not currently an economic problem there or in the Hood River district. Side rot has been found on Anjou and Comice pears, but the most serious losses have occurred on Bosc. It is a problem of long-term storage; infections become visible in late December or January, and incidence of decay increases as the storage season continues.
Research at the Southern Oregon Experiment Station has shown that side rot lesions can be caused by two fungi, Phialophora malorum and Cladosporium herbarum. Typically dark brown, dime-size decay lesions separate cleanly from adjacent healthy flesh. The color and texture of the decayed tissue vary with the amount of drying due to skin breakage. Both of these fungi are relatively slow-growing, weak pathogens which apparently must wait for fruit to weaken through age before infecting. Cladosporium is sensitive to thiabendazole (TBZ), while Phialophora is not. Most side rot in fruit treated postharvest with thiabendazole is caused by Phialophora.
Pathologists at the Mid-Columbia Research station studied changes in susceptibility of fruit to decay throughout the growing season. In summary, fruit becomes most susceptible to the fungal decay organisms during the last month before harvest. However, infection by the bull's-eye rot organism can occur any time from petal fall to harvest.
Pears were wounded and inoculated throughout the growing season with the different decay-causing fungi. Fruit was harvested and placed into storage for 7 to 8 months. The decay was generally less than 10% on fruit sprayed with fungal spores a month or more before harvest (Table 1). Fruit treated with decay organisms during the last few weeks before harvest was seriously decayed during storage. Consequently, growers should time chemical control programs to cover fruit at least 2 to 3 weeks before harvest, as it loses its resistance to decay.
Harvest maturity is critical. Studies on Bosc pears have shown dramatically that more decay occurs on later picked fruit. By delaying harvest 2 weeks after commercial harvest, scientists found a significant rise in the amount of infection in nonwounded fruit that was sprayed with the fungal suspensions.
Table 1. Decay in attached Anjou pear fruits inoculated monthly during the growing season in 1980 and 1981.
|Week of inoculation before harvest||Percent decay* caused by|
|Botrytis cinerea||Mucor piriformis||Penicillum expansum||Pezicula malicorticus|
|* Decay is the total from evaluations conducted monthly during the growing season, during storage, and after a 1-week ripening period. Each value represents the mean of 50 fruits. Researchers made 2 needle punctures per fruit through drops of inoculum. Fruit was inoculated 2 days before harvest.|
Three factors are of primary importance in designing a fungicide spray program. These factors include 1) when spores of a particular disease organism are present in the greatest quantity, 2) when fruit is most susceptible to infection and decay, and 3) when environmental conditions most favor infection.
Certain postharvest rots occur when infected flower parts are trapped in the calyx end of the fruit soon after bloom, i.e., calyx-end infections by Botrytis. A spray of Ziram, Manzate-200, or Dithane M-45 within 10 days of petal fall helps reduce infection. Growers in areas with bull's-eye rot may need a second fungicidal spray if it rains in August. Preharvest sprays of Ziram also help reduce side rot incidence.
Practices to Reduce Decay
During the winter months, prune trees to eliminate low hanging branches which might set fruit in contact with cover crops or lie on the ground. These fruit can easily come in contact with soil-borne spores and become infected as a result of the high humidity in the microclimate of the cover crop.
During the summer months, it is important to keep weeds and grass under control. Spores can be released from the cover crop, which also provides high humidity for germination. In particular, Gray Mold and other Botrytis species grow well on weakened or dead plant material in the orchard. Periods of rainy weather or excessive irrigation promote the growth and sporulation of these fungi, which account for a general increase in the incidence of Gray Mold in wet years. Conversely, too little water may promote dusty conditions, which result in spread of the soil-borne spores of Mucor, Penicillium, and Botrytis.
At harvest, growers can do a number of things to reduce postharvest decay. Injury to fruit during harvest and packing is probably the most critical factor leading to postharvest decay. Harvesting fruit at the proper maturity is also extremely important. Pears harvested on the immature side will abrade easily on the packingline. Overmature fruit or fruit harvested late in the maturity range has reduced storage life and is more susceptible to postharvest diseases. Fruit is most susceptible to diseases as it approaches maturity.
Proper handling becomes critical in preventing decay and bruising. Pickers should not pick up "grounders" (fallen fruit), since that fruit is likely to be infected as well as ripening prematurely. Volatiles produced by these fruit stimulate the ripening of adjacent fruit and reduce storage life.
Avoid harvesting wet fruit, as it likely will have spores adhering to the surface which may germinate and infect. Allow fruit to dry before harvesting.
Most postharvest rot organisms are soil inhabitants and can be picked up on the skids or sides of bins. Mow the cover crop or use sawdust or wood chips under bins rather than allowing them to touch the soil. Do not skid bins on the orchard floor, load the bins roughly, or allow drivers to speed through the orchard. Urge pickers to handle fruit delicately to prevent bruising. Finally, immediately take picked fruit to the packinghouse where it can be cooled rapidly.
Control in the Packinghouse
Control of postharvest diseases in the packinghouse is based on spore load reduction through sanitation and killing spores with fungicides. Minimize damage to fruit by thorough padding of surfaces and overall maintenance of equipment.
Controlling Spore Load in the Dump Tank
Pear packinghouses use either chlorine or SOPP (sodium ortho-phenylphenate) in the dump tank and flumes. Chlorine 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 penetrated. Chlorine, however, lacks the ability to provide long-term coverage of fruit in storage or on its way to market and cannot penetrate wounds well. Postharvest Pomology Newsletter, Volume 2, Number 4, contains an extensive discussion on using chlorine in the packinghouse.
The concentration of spores in a dump tank can be critical in terms of control of fungal diseases in the packinghouse (Table 2). Several organizations are available to monitor the number of spores in a dump tank. If monitoring is used, 100 spores/ml should not provide a problem in a packinghouse; however, spore levels over 300/ml should be avoided.
Table 2. The effect of dirt on the ability of chlorine to kill fungal spores.
|50 ppm (dirty water)||75|
|50 ppm (tap water)||0|
|* Combination of Mucor, Botrytis and Penicillium spores.|
The pH of a solution in which chlorine is used will influence the amount of killing that chlorine provides. Flotation salts dramatically raise the pH to the alkaline area in most cases. Operators are warned not to acidify or reduce the pH of chlorine when used with sodium silicate, since the flotation salt solution will form a gel and solidify. Disposal of 3,000 gallons of "Jell-O" can be a problem.
Tests have been run on the fungicidal effects of various flotation salts. Most of the flotation salts have no fungicidal properties, that is, they do not kill fungus spores. However, sodium ligninsulfonate prevents germination of fungal spores when used alone. When it was combined with SOPP in the laboratory tests, no decay spores germinated. Ligninsulfonate has a number of problems which must be considered. First, SOPP measurements are difficult due to the color of the solution. Second, fruit must be thoroughly rinsed following treatment to avoid injury. Operators should be aware that ligninsulfonate and chlorine are not compatible and should not be mixed.
Heat Treatment in the Dump Tank
Heat treatment of pear dump tanks is another method of reducing spore load. Over the past several years we have been experimenting with heat sterilization of the dump tank for those using SOPP in the system. It appears that 130°F for 20 to 25 minutes kills spores in the dump tank.
The procedure is to lay a Styrofoam cover over the tank at night after all the fruit is out and to turn on the boilers to raise the temperature. In commercial trials it took about 4 hours to bring the tank up to the 130°F level. The boilers were then turned off and the Styrofoam was removed. By morning the water was back to 70°F, so the fruit could be run without injury. Water loss due to heating was about 10% and SOPP loss about 25%.
We have done these trials with both silicate and ligninsulfonate solutions. The tank containing ligninsulfonate was reheated once weekly for 3 to 4 weeks, during which period of time it was not dumped and spore counts remained low.
Thus, this method reduces the number of times during the season that tanks must be emptied. However, organic matter and other debris eventually accumulate in the tanks and the tanks require cleaning.
Heating also will sterilize infected fruit at the bottom of the tank. The calculated cost to clean, empty and refill the tank was about $800, while the cost to heat sterilize the tank was about $200. Good ventilation of the packinghouse during heating is important.
Spores of decay pathogens that survive dump tank SOPP or chlorine treatments or which contaminate the fruit after it leaves the dump tank may be prevented from infecting by application of a fungicide line spray. Commonly, the benzimidazole fungicide thiabendazole (TBZ) is applied but, unfortunately, not all postharvest pathogens are controlled by this fungicide. The following table lists common pear decay pathogens according to their sensitivity to benomyl.
Table 3. Sensitivity of common pear decay pathogens to thiabendazole.
|Penicillium (Blue Mold)||Mucor|
|Botrytis (Gray Mold)||Phialophora (side rot)|
|Pezicula (bull's-eye rot)||Alternaria|
Tank mixtures of TBZ + Captan can improve control to a small extent but do not significantly expand the range of protection. However, the use of Captan after harvest on pears sprayed with oil during the summer or wrapped in oil paper can develop blotchy discoloration on the skin.
In recent years concern has been raised about the development of resistance to benomyl in decay pathogens. Resistant strains have been found in all major pear-growing districts. A close examination of this potential problem has been made in the Hood River district, where records show the incidence of resistant strains has been stable for the past several years.
Postharvest FungicidesFungicides such as thiabendazole (TBZ) and Captan are of tremendous importance in decay control. The fungicides are often applied in a line spray, after fruit crosses the sorting tables, and often in combination with wax. Frequent use of benomyl or thiabendazole in the orchard has resulted in a buildup of resistant strains of Botrytis and Penicillium in many parts of the world; thus, orchard use of these fungicides should be avoided. Limiting benzimidazole fungicides to packinghouse applications is critical to preserve its effectiveness. Sanitation to prevent increase and spread of resistant strains is also important. In addition, new fungicides are being evaluated for their potential in controlling postharvest pathogens.
Low Oxygen StudiesIn a cooperative test with Dr. Paul Chen, 1% oxygen was tested against air storage for its effectiveness in controlling stem end decay. Two years of research have shown that there is a significant reduction in stem end decay in pears stored in 1% oxygen. The reason for this appears to be that a 1% oxygen atmosphere keeps the stems in good condition and slows the progress of the fungus down the stem.
Dr. Eugene Kupferman(1), Robert Spotts(2); and David Sugar(3)
(1)WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
(2)OSU-Mid-Columbia Experiment Station
(3)OSU-Southern Oregon Experiment Station
Tree Fruit Postharvest Journal 6(2):18-23