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WSU-TFREC/Postharvest Information Network/Filtration to Remove Spores of Penicillium Expansum from Water
in Pome Fruit Packinghouses

Filtration to Remove Spores of Penicillium Expansum from Water
in Pome Fruit Packinghouses


Blue mold caused by Penicillium expansum is a major decay of pome fruits. Spores of P. expansum are found in soil, on plant surfaces, and in air and are transferred to dump tank and flume water in packinghouses on contaminated wooden picking bins and fruit. Populations of P. expansum in water in pome fruit packinghouses vary considerably and are affected by the amount of fruit and bin contamination, the volume of fruit processed, and use of fungicides and disinfectants. In the Pacific Northwest, 44% of samples of dump tank water from apple and pear packinghouses contained between 10 and 100 spores of P. expansum per ml.1 In another study in Oregon and Washington, populations of P. expansum in the dump water varied throughout the packing season from 56 to 4,167 spores/ml.6 Previously, we found a direct relationship between concentrations of P. expansum conidia in water and decay of pear fruits.5

The two most common disinfectants used to reduce spore levels in dump tank and flume water are chlorine (sodium hypochlorite) and sodium o-phenylphenate (SOPP).1 Chlorine is inexpensive but corrodes equipment and will not kill spores lodged in injured tissues. SOPP occasionally injures fruit if not thoroughly rinsed. Other ways to reduce spore levels include ozone and chlorine dioxide, but these methods also have limitations and are not widely used.

Physically removing decay spores from water has received little attention. In Australia, a combination of sand, cellulose cartridge, and ceramic filters has been used successfully to remove fungal spores from irrigation water.2 In Canada, dump tank water in an apple packinghouse was passed through two Jacuzzi sand filters, and levels of P. expansum were reduced 59% to 88%.4 However, the filter system was considered too erratic to provide reliable water sanitation.

In the research reported below, a commercial cartridge filter system and two sand filters were evaluated to determine how efficiently they remove decay spores from drencher and flume water.


Two commercial sand filters (model Triton TR 140, Pac Fab, Inc., El Monte, CA) were evaluated in an apple packinghouse. Each filter contained 114 kg (251 pounds) of pea gravel and 227 kg (500 pounds) of #20 silica sand and had a filter area of 0.641 m2 (6.9 sq. ft.). Flume water was pumped through the filter at 530 L (140 gal.)/min. Water entering and exiting each filter was sampled weekly from 21 November, 1986, to 11 May, 1987. Each sample was dilution plated on acidified potato dextrose agar to determine the concentration of P. expansum spores in the water.

A commercial, multistage filtering system connected to a drive-through bin drencher at several apple packinghouses was evaluated. The drenching liquid was a water suspension of DPA at 2000 parts per million (ppm) and thiabendazole (Mertect 340-F) at 528 ppm. The first stage of filtration consisted of a 0.51 mm (0.02 inch) screen and liquid cyclone units (Maxicleaner unit, Rush Consultants, Wenatchee, WA) operated at 757 L (200 gal.)/min., continually recycling the liquid from tanks containing 1500 to 2000 gal. to remove leaves, stems, and other large debris. Periodically, after 2000 to 3000 bins had been processed, the liquid in the drencher holding tank was emptied into another storage tank and the suspension pumped through a final set of filters (Zero discharge unit, Rush Consultants) at 37.9 L (10 gal.)/min. These filters consisted of four elements in step-down series: 100 and 45 µm nylon mesh filter bags, a pair of 5 µm polypropylene cartridge filters, and a terminal pair of either a 1.0 µm polypropylene or 0.45 µm glass fiber cartridge filters. Samples were taken before and after filtration on five occasions where 1.0 gm terminal filters were in use and on three occasions with 0.45 µm filters. Samples were dilution plated, incubated, and colonies of P. expansum enumerated as described above.


The average number of P. expansum spores per ml in water before and after filtration through sand filter unit #1 was 547 and 725, respectively, and the difference was statistically significant (P=0.05). For sand filter unit #2, 458 spores/ml were detected in water before and 441 spores/ml after filtration; the difference was not significant (P=0.05).

The level of spores of P. expansum in commercial drencher water before and after filtration through the four-element cartridge filter unit was 1153 and 1123 spores/ml, respectively, when the terminal filter was 1.0 µm. This represented a removal of 3% and was not significant (P=0.05). However, 92% of spores were removed from drencher solutions filtered through the four-element units when the terminal filter was 0.45 µm (617 vs. 50 spores/ml); removal was significant (P=0.03).


Sand filters were relatively ineffective for direct reduction of conidia of P. expansum from flume water. In the packinghouse, the one sand filter did not reduce the spore load significantly, and the other filter appeared to add spores to the system. Although sand filters are backwashed regularly, they still may build up organic matter such as pieces of fruit, leaves, and stems which may serve as a food base for growth and sporulation of fungi inside the filter unit. In addition, build-up of organic matter may cause nonuniform water flow or "channeling" through the filter, which results in decreased efficiency. Sholberg4 reported that a sand filter system in an apple packinghouse did not always work efficiently and was not able to lower the propagule concentrations of P. expansum enough to remove the risk of infection. However, less chlorine was required in filtered water than in nonfiltered water to maintain the desired concentration. Filtering also prolonged the use of dump tank water and lengthened packing runs before tank cleaning was necessary.

The cartridge filter system has potential for effective removal of decay spores and thus increased decay control. However, effectiveness is closely related to pore size of the terminal filter. The unit, when used with a commercial drencher and the 1.0 gm terminal filter, removed only 3% of P. expansum conidia. The 1.0 µm filter is nominally rated, and pore size varies considerably.

The cartridge filter system with the 0.45 µm terminal filter effectively removed spores in both laboratory and commercial tests, reducing the spore level by 92% to 99%. However, if this system is to become more useful for spore removal and decay control in packinghouse drenchers, dump tanks, and flumes, the capacity must be increased well beyond the current 10 gal./min. rate. In addition, terminal filters of 1.0 µm remove about 20% and 0.45 µm filters an even greater but unknown amount of thiabendazole, the most common fungicide used in drencher water for decay control, and fungicide concentration must be monitored and adjusted continually.

As with the sand filters, the cartridge filter units effectively increase the time that drencher suspensions can be used. Because drencher water may contain DPA, calcium chloride, and thiabendazole, reducing the amount of waste water generated (and which must be disposed of) is a positive benefit of filtration. During harvest, bottoms of bins often are covered with soil and debris.3 Filter life, and possibly filtration rate, may be increased by improved bin sanitation.

We thank the Winter Pear Control Committee for partial funding of this research. Use of trade names in this article does not imply endorsement of the products named or criticism of similar products not mentioned by Oregon State University.

Literature Cited

  1. Bertrand, P., and Saulie-Carter, J. 1979. Postharvest decay control of apples and pears after immersion dumping. Ore. State Univ. Agric. Exp. Sta. Spec. Rep. 545. 9 pp.

  2. Darling, D. D. 1977. A mobile filtration plant to eliminate fungal spores in irrigation water for plant propagation. Aust. For. Res. 7:273-274.

  3. Michailides, T. J., and Spotts, R. A. 1986. Factors affecting dispersal of Mucor piriformis in pear orchards and into the packinghouse. Plant Dis. 70:1060-1063.

  4. Sholberg, P. L., and Owen, G. R. 1990. Populations of propagules of Penicillium spp. during immersion dumping of apples. Can. Plant Dis. Surv. 70:11-14.

  5. Spotts, R. A. 1986. Relationships between inoculum concentrations of three decay fungi and pear fruit decay. Plant Dis. 70:386-389.

  6. Spotts, R. A., and Cervantes, L. A. 1986. Populations, pathogenicity, and benomyl resistance of Botrytis spp., Penicillium spp., and Mucor piriformis in packinghouses. Plant Dis. 70:106-108.

Robert A. Spotts and L. A. Cervantes

Mid-Columbia Agricultural Research and Extension Center, Oregon State University, Hood River

Tree Fruit Postharvest Journal 4(1):16-18
June 1993

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