A quantitative detection method for Penicillium digitatum and P. italicum in citrus packinghouses.

Resistance of Penicillium digitatum Sacc. and P. italicum Wehmer to the benzimidazole fungicides, thiaben dazole (TBZ) and benomyl has led to economic losses in the California and Florida fresh citrus industry. The standard air sampling technique of exposing benzimidazole-amended and nonamended potato dextrose agar petri plates for between 1 to 3 minutes is useful in determining spore presence and the level of resistance in lemon packinghouses. Because orange and grapefruit packinghouses generally have fewer Penicil lium spores present, the standard assay method which is qualitative may not accurately indicate a potential decay problem. A quantitative air sampling method was de veloped that has a high collection efficiency and high sampl ing volume. Sporeload can now be recorded as number of blue-greens per standard cubic foot of air. In 1980, the contributing editor of Florida Grower & Rancher stated, "getting produce to the customer is as im portant as producing it, since that's the operation that brings returns to the grower" (3). Decayed citrus fruit re sults in market losses and many of these decays occur after harvest. Prevention and/or the reduction of postharvest Proc. Fla. State Hort. Soc. 98: 1985. 211 decay and subsequent market losses are what growers, packers and shippers concern themselves with on a daily basis. Providing a postharvest environment that maintains the product in good condition is a major component in a pest management system. The most prevalent post harvest decay organisms for citrus are the fungi Pencillium digitatum (green mold) and P. italicum (blue mold) (10). These molds exist in the air and are transported by air currents to healthy fruit (6,2). It is estimated that one spore infecting a wound-bearing fruit can produce as many as 100 million spores in 10 days under optimum environmental conditions (2). Hall & Bice (6) reported decay in a packinghouse is proportional to the concentration of spores in the air and on equipment sur faces. To further compound the problem, these Penicillium species develop fungicide resistance (5,8,9,11) and these, too, proliferate in a packinghouse environment. Murdock (12) determined that these airborne spores could be assayed using microbiological methods in a lemon packinghouse operation. Trie technique was to simply ex pose petri dishes filled with potato dextrose agar for a given exposure time and count the spores that had "settled out" on the petri dish. This technique was refined by others (4,1,2) to determine not only spore load but resis tant spore population. The agar was refined to enhance the growth of blue/green molds and the media was amended with the appropriate fungicides to determine re sistance (7). The technique has become an important tool to the packinghouse manager in his decay control strategy. He uses the data to assist in evaluating packinghouse sanitation programs and to evaluate appropriate fungicide treatment based on the resistivity assay. The settling plate technique, however, is a qualitative indicator of airborne contamination load. Many factors af fect its performance, exposure time; air currents; place ments of petri dishes; nearby packinghouse personnel; and media depth. A need was therefore established to deter mine quantitatively airborne contamination and thereby establish a uniform assessment of the packinghouse envi