Biological Control of Postharvest Fruit Pathogens in Papaya

First of all, since the area of biological control is so vast, I will be restricting my talk to the area of biocontrol of postharvest fruit pathogens. Fungicides are a primary means of controlling postharvest diseases. However, as a result of public concern about the presence of synthetic chemicals in our food supply and environment, several fungicides have been banned by the U. S. Environmental Protection Agency, or have been voluntarily withdrawn from the market for postharvest use (Wisniewski and Wilson 1992). The papaya industry has also experienced the temporary loss of registration of the use of Dithane fungicide . We now face an urgent need to develop new and effective methods of controlling postharvest diseases, not only for papaya, but for other commodities as well. Sanitation and exclusion can help reduce inoculum level of pathogens; the use of non­ . selective chemicals (sodium carbonate, sodium bicarbonate, active chlorine, and sorbic acid) , and heat treatments can lower the disease pressure on harvested commodity. Minimizing injury to the commodity during harvesting and postharvest handling, and maintaining the commodity at storage conditions that optimize host resistance, will also aid in suppressing disease development after harvest (Wisniewski and Wilson 1992). And , recently, attention has been focused on biological control of postharvest diseases as an alternative to the use of fungicides. What is biological control? Biological control of plant disease is defined as " the decrease of inoculum or the disease-producing activity of a pathogen accomplished through one or more organisms, including the host plant, but excluding man." (Kenneth F. Baker 1987) The area of biological control of postharvest diseases has been revolutionized by Pusey and Wilson (1984), and Wilson and Pusey's studies .(1985) on the biological agent, Bacillus subtilis, a bacterium which was applied directly to peaches after harvest to control brown rot , Monilinia fructicola . Since then, there have been numerous reports of other microorganisms that control postharvest diseases of . various commodities (Table 1). Commodities that have been reported to use biocontrol agents include: apple, apricot, citrus, cherry, grape, nectarine, peach, pear, pineapple, plum, and strawberry. The microorganisms used include bacteria, yeasts, and fungi. Some of the organisms will be elaborated on later. What are some of the characteristics of an "ideal" postharvest biocontrol agent? " The ideal postharvest biocontrol agent is (1) genetically stable, (2) effective at low concen­ trations (3) not fastidious in its nutritional requirements (not be too "restrictive," or re­ quiring of "exotic" ingredients), (4) amenable to production on inexpensive growth medium with a long shelf life, (5) easy to dispense (6) able to survive adverse environmental conditions (that is, compatible to commercial handling and storage practices, including low-temperature and con­ trolled-atmosphere storage), (7) effective against a wide range of pathogens on a variety of commodi­ ties (to make it "cost effective" and increase its market value) (8) safe to human health,and (9) nonpathogenic to the host (Wisniewski and Wilson 1992). . How does . the biocontrol agent work? What are possible modes of action? Except for the production of antibiotic zones by the biocontrol agent in petri dishes when challenged with the pathogen, the mode of action of many of the biocontrol agents is poorly understood. When antibiotic production is not a factor, the mode of action probably involves a complex syndrome of characters, including nutri­ ent competition, site exclusion, attachment of the antagonist (biocontrol agent) to the pathogen, induced resistance in the host , and direct para-: sitism of the pathogen (Wisniewski and Wilson 1992). Biological control of postharvest diseases of fruits and suggested modes of action are detailed in Table 2. Under antibiotic production, except for the fungus Trichoderma sp., all of the antagonists are bacteria. Under nutrient competition and or induced resistance, Pseudomonas syringae which controls blue mold of apple, · and Enterobacter cloacae which controls rhizopus rot of peach, are bacteria. Acremonium breve is a fungus, and Pichia