 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).
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.
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