Management of Decay Around the World and at Home
In research prioritizing sessions with apple and pear growers and packers in the Pacific Northwest (PNW), minimizing losses from decay remains one of the main concerns. The major rots that affect fruit grown in Washington occur in other fruit producing regions around the world. There has been a great deal of research on these rots both here and abroad. Management recommendations have been offered to aid growers, packers, and handlers in minimizing losses due to decay. Although it would be ingenuous to believe that we have all the answers to the problem, strategies have been put forth that have not been adopted by our industry, but have been instituted elsewhere. In this paper, I will explore some of the practices used in decay management and control by industries in a few of the growing regions of the world and compare them to practices used in the PNW. It is my belief that if other industries find it effective to use management practices that we are not, then we must ask ourselves, why?
Blue mold, gray mold, and Mucor rot are the major postharvest diseases of pome fruit that occur worldwide. All of these diseases are caused by fungi. Of these three diseases, blue mold is most common and may be caused by several species of Penicillium, especially P. expansum. Typical symptoms of blue mold are circular, tan colored lesions with sharp margins between the watery soft rot and healthy fruit flesh. Lesions often have a musty, earthy smell. When spores are present, they form a dense, powdery mass at the center of the lesion. These spore masses are bright green to blue colored and give the disease its name. The spores are easily wind disseminated and also frequently contaminate water systems in packing houses.
Gray mold is the second most prevalent decay and is caused by Botrytis cinerea. The disease causes a firm rot and usually has a white, cottony fungal mass growing on the surface (mycelium). Spores may develop on these mycelia and they take on a dusty, gray-brown appearance. Gray mold often forms "nests" as mycelia grow out from an initially infected fruit to surrounding fruit causing new infections. Black, scabby looking sclerotia are often seen on fruit, especially around the infection site. Spores are easily wind disseminated, but are not often recovered from water systems.
Mucor rot, caused primarily by Mucor piriformis, is less frequently encountered, but can be highly destructive. Lesions are watery with less distinct margins than those in fruit with blue mold. Fruit are relatively quickly decayed with the entire fruit often involved so that little is left of the fruit a few months after infection occurs. Similar to gray mold, mycelia are often present on the surface of diseased fruit, but hyphae are much coarser than those of B. cinerea. Spores are borne in tiny, dark gray balls at the end of vertical stipes. They are water and insect disseminated and can contaminate packing house water systems.
An integrated approach to disease management will have the greatest impact on reduction of losses. The tools and techniques that are used to develop plant disease control strategies fall into several broad categories. All available methods from each of these categories that meet the goals of a particular enterprise should be utilized when developing management strategies for disease control. In the case of postharvest diseases, eradication and protection are the primary means of control, but good management practices will also take advantage of whatever inherent disease resistance is in the fruit.
Categories of disease control are:
- Eradication. Eradication involves reducing, removing, eliminating, or destroying inoculum at the source. In postharvest systems this takes the form of sanitation, which should be the basis for any postharvest disease management strategy. This includes sanitation of field bins, packing house water systems, and packing and storage facilities.
- Protection. Another cornerstone of disease management involves the use of fungicides. These can be biological antagonists or synthetic chemicals. They should never be considered an alternative to good sanitation practices.
- Resistance. Growing disease resistant varieties of annual crops is the most common form of control utilizing resistance in agricultural systems. Some apple varieties have been released that are resistant to field diseases, especially apple scab. However, disease in plants, as in animals, is the exception rather than the rule and we can take advantage of some level of inherent resistance to postharvest disease. Good fertility and vigor management can help reduce decay losses. Studies by Dr. David Sugar showed decay was reduced in fruit with relatively high calcium to nitrogen ratios (Sugar 1994). Refrigeration also takes advantage of the relatively high level of resistance in unripe, pre-climacteric fruit as compared to ripe, senescing fruit. In a recent study, significantly more decay was found in Anjou fruit stored in bins at 34 ºF than at 31 ºF (Sanderson and Fuller, unpublished).
- Exclusion. This usually infers regulatory practices that prevent the introduction of a disease causing agent into an area where it is not currently present. Because the major decay causing fungi are present in all fruit growing regions, management strategies using exclusion are not an option.
- Avoidance. This involves timing practices such as planting or pruning during periods when pathogen inoculum is not present, weather conditions are unfavorable for disease development, or the host plant is not susceptible to infection. Avoidance also is not a useful option in postharvest disease management strategies.
- Therapy. Disease controls in this category are those that remove or destroy the pathogen after infection has occurred. This is not currently an option for postharvest diseases.
Given these broad categories of disease control, we can examine what measures are currently being employed by other fruit production industries and make comparisons to local practices.
Eradication (= Sanitation)
Bin sanitation. Decay fungi have been recovered from field bins in both the PNW and in Australia (Sanderson and Spotts 1995; Spotts et al. 1988), but the contribution of contaminated field bins to populations in drenches and flume water systems has not been established. To determine this relationship, we began a study to quantify the density of spores of species of Penicillium on field bins at harvest. On bins in some orchards, an average of as many as 8.3 x 107 spores of Penicillium spp. other than P. expansum and 3.3 x 106 spores of P. expansum were recovered per bin (Fig. 1). At these levels, if only 50% of the spores were washed from the bin in a drench, after 30 bins treated per 100 gal, there would be over 3000 spores/mL of Penicillium spp. other than P.
Figure 1. Population density of species Penicillium recovered from field bins at harvest.
Figure 2. Population density of species Penicillium recovered from surfaces of field bins at harvest.
It is clear from the above discussion that bin sanitation should be a component in any decay management plan. In Australia, bin sanitation is an important eradication strategy. Most bins are sanitized before harvest either by pressure washing with hot water or a disinfectant, steam cleaning, or solarization. In addition, there is a relatively new system being developed that uses low pressure steam to pasteurize empty bins stacked in a cold store room. In the field, especially in pear orchards, bins are kept on trailers to prevent contamination by M. piriformis in orchard soil.
In the PNW, there is no systematic process for sanitizing field bins. Dr. Robert Spotts has researched a number of ways to sanitize wood surfaces and found that as little as 5 seconds exposure to steam was effective at killing spore of pathogenic fungi. Chlorine, sodium orthophenylphenate (SOPP), and quaternary ammonia compounds also were effective sanitizers.
Water system sanitation. It has been long recognized that spore of Penicillium spp. accumulate in water systems (Heald et al.
In Argentina, calcium hypochlorite at 100 ppm is recommended for dump tank sanitation. In Australia, much lower level of calcium hypochlorite are recommended (20 to 50 ppm), which our studies indicate is probably too low. Bromochlorodimethyl-hydantoin (BCDMH) also is registered for use at 5 to 10 ppm. BCDMH at 5 to 15 ppm is reputed to be comparable to 50 to 100 ppm of chlorine from hypochlorite for killing fungal spores. Chlorine dioxide derived from acidification of sodium chlorite is also used in Australia.
In the PNW, apple dump tank sanitation is based on use of sodium hypochlorite at 70 to 100 ppm free chlorine with a recommended pH of about 7.0. Chlorine dioxide is a strong oxidizer, but systems that were developed to inject gas into a side stream have not been sufficient to effectively sanitize dump tanks before serious off-gassing occurred. We observed better efficacy in systems that added sodium chlorite directly into acidified dump tanks, but this method has not been fully explored. Ozone also is a strong oxidizer, but is depleted very quickly on contact with organic matter. Because dump tanks are quickly loaded with organic matter, it is difficult to achieve the concentrations of ozone to effectively kill pathogen spores.
Although much is known about the use of chlorinated sanitizers, many packers in the PNW do not effectively use the available materials. For example, in surveys taken in 1996 and 1997, several packers were using sodium hypochlorite at levels of 20 ppm, which is ineffective for killing fungal spores. In laboratory tests, it took 5 minutes to kill spores of P. expansum at 100 ppm of free chlorine from sodium hypochlorite.
Cold store sanitation. Spores produced on decay lesions on fruit in storage can be blown around the rooms by the refrigeration fans and may be redistributed to new infection sites. These spores, in addition to those that persist on fruit surfaces and bins, also can contaminate water systems. Ozone generated with corona discharge systems is being used in more than 300 Kiwifruit and pome fruit storage rooms in Chile. Dr. David Sugar is conducting research in this area and reports decreased surface populations of fungi on fruit when used at low rates (0.7 ppm) for extended periods of time (D. Sugar, personal communication).
Current recommended practices in the PNW include the use of chlorine dioxide foam (3 to 5 ppm), which can be sprayed onto walls of cold store rooms. Use of fungicides, rather than biocides like chlorine, ozone, and other oxidizers or steam, is not recommended because of the high likelihood of developing strains of the decay fungi resistant to the fungicide.
Line/hard surface sanitation. Chlorine compounds, quaternary ammonia formulations, and steam are effective sanitizers for these surfaces. Ample unrecirculated rinses also help to flush away microbial contaminants. The maxim "Clean before you sanitize!" is very important. All sanitizers act on surfaces only. If colonized pieces of fruit or leaves are left behind, decay fungi may regrow from within and continue to contaminate the facility.
Protection = Fungicides
Fungicides for control of postharvest diseases may be applied preharvest, at harvest, or during packing. Ziram (zinc dimethyldithiocarbamate) is recommended as a preharvest spray and many years of data have shown some reduction in decay losses when a land application is made as close to harvest as possible. Currently, only two chemical fungicides, thiabendazole (TBZ) and captan (N-trichloromethylthio-4-cyclohexane-1,2-dicarboximide), and two biological fungicides, Aspire (Candida oleophila) and Biosave (Pseudomonas syringae), are registered for use in the PNW for control of diseases of pome fruit in postharvest applications. These fungicides show varying degrees of efficacy against the fungi causing blue and gray molds, but none are particularly effective against Mucor rot. Fungicide treatments made at harvest usually are applied as drenches. At packing, most fungicides are applied as line sprays.
Drenches. There are two reasons that most packers drench fruit at harvest. The first is to apply an antioxidant, either diphenylamine (DPA) or ethoxyquin, to prevent superficial scald and the second is to apply a fungicide. Early applications of fungicides are important for minimizing decay losses. In drenching experiments with Anjou pears, fruit were treated with TBZ either at harvest or at weekly intervals up to 5 weeks after harvest. Primary gray mold infections were reduced by 70% to 90% when fruit was treated within 2 weeks of harvest.
Although there are overwhelming reasons to drench fruit in order to apply postharvest fungicides and antioxidants as quickly as possible and with minimal handling of fruit, drenching with recirculating systems is inherently risky. Spores of pathogenic fungi accumulate in drench mixtures and are dispersed to infection sites on fruit (Sanderson 1999). Resistance or insensitivity to fungicides by strains of pathogenic fungi can result in greater decay losses in drenched fruit than in undrenched fruit. For example, in the PNW we sometimes trade gray mold, which is effectively controlled by TBZ, for blue mold and Mucor rot, which are not as effectively controlled by TBZ. In recent work, we found between 30% to 50% of isolates of P. expansum recovered from drenches were resistant to TBZ. Furthermore, M. piriformis, which also collects in drench mixtures is not sensitive to this class of fungicides.
In Australia, about one-third of the apple crop is prewashed in either a fresh water or chlorinated water. When fresh water is used, it is not recirculated; chlorinated water is. Fruit is then drenched with DPA and a fungicide. A few packing houses in the PNW also have adopted this practice and our work has shown that it can be effective at helping to reduce decay. When using chlorinated prewashes, great care must be taken to adequately flush off the chlorine in the subsequent drench. In our tests, large quantities of Anjou pear fruit were burned when treated with a predrench of chlorinated water (150 to 200 ppm chlorine from sodium hypochlorite) followed by a fresh water rinse. Little or no burn was encountered when that treatment was followed by a TBZ drench.
Many other fruit producing countries can use fungicides that are not registered in the United States. In Australia, the benzimidazole fungicides, Carbendazim and TBZ, and iprodione (Rovral) are registered. In Argentina, Benomyl, TBZ, Topsin M, Carbendazim, as well as Rovral and Imazalil are registered. Their strategy is to use fungicides with mixed modes of action when possible. The Chileans use the same strategy. Importantly, in both Argentina and Chile fruit are segregated and treated for the market to which the fruit is bound.
As stated above, of the conventional fungicides, both captan and TBZ are registered in the PNW. Our trials demonstrated greatly increased disease control efficacy when those two fungicides were combined, over that of TBZ alone (Sanderson 1999). However, few packers are willing to use captan because of import restrictions, especially by Taiwan, and they are unwilling to segregate fruit for different markets.
New fungicides. Fortunately, some new fungicides are being evaluated and look promising for postharvest use. EXC 9001, a new biological fungicide (Crytococcus infermo-miniatus, discovered by Dr. Tara Chand-Goyal on pear fruit from Yakima, Washington) will hopefully be registered within a year. Initial tests with EXC 9001 show evidence of good efficacy against a broad spectrum of postharvest pathogens. Fludioxonil (Scholar) and fenhexamide (Elevate) are being tested in the IR-4 program this year. Scholar has shown broad spectrum efficacy against postharvest pathogens, whereas Elevate is very effective against B. cinerea, but not against Penicillium spp. or M. piriformis. A new compound, TM 41501, also has shown a broad spectrum of efficacy against postharvest pathogens.
Although these materials look exciting and have show potential, it must be kept in mind that an integrated approach to disease management will be most effective. It is imperative that packers use the existing fungicides to optimize their efficacy and minimize problems from resistant strains of pathogenic fungi.
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Heald, F.D., Neller, J.R., Overley, F.L., Ruehle, G.D., and Luce, W.A. 1928. Arsenical Spray Residue and its Removal From Apples and Pears. Wash. Agric. Exp. Stn. Bull. 226. 100 pp.
Sanderson, P.G. 1999. Fungicidal drenches for control of postharvest decay. Good Fruit Grower 50:53-56.
Sanderson, P.G. and Spotts, R.A. 1995. Postharvest decay of winter pear and apple fruit caused by species of Penicillium. Phytopathology 85:103-110.
Spotts, R.A. 1986. Relationships between inoculum concentrations of three decay fungi and pear fruit decay. Plant Dis. 70:386-389.
Spotts, R.A., Holmes, R.J., and Washington, W.S. 1988. Sources of spores and inoculum concentrations related to postharvest decay of apple and pear. Aust. Plant Pathol. 17:48-52.
Sugar, D. 1994. Reducing postharvest decay in pears through integrated management. Proc. Wash. Tree Fruit Postharvest Conf. 10:60.
Peter G. Sanderson, Ph.D.
Washington Tree Fruit Research Commission
1719 Springwater St., Wenatchee, WA 98801
16th Annual Postharvest Conference, Yakima, WA
March 14-15, 2000