POTENTIAL USE OF CHLORINE DIOXIDE TO PREVENT THE SPREAD OF FUSARIUM BASAL ROT DURING THE HOT WATER TREATMENT OF DAFFODIL BULBS

Historically, formaldehyde has been used extensively by bulb growers to prevent the spread of basal rot (Fusarium oxysporum f. sp. narcissi) during the hot water treatment (HWT) of daffodil bulbs. A number of restrictions have been placed on the use of formaldehyde in the United States which have limited the number of growers that are able to use formaldehyde. During the past five years, a research program to identify potential formaldehyde replacements has been proceeding at WSU-Puyallup. One potential is chlorine dioxide, which has been used increasingly as a replacement for chlorine in the fruit and vegetable industries. Research has shown that the activity of chlorine dioxide in the presence of high levels of organic matter, such as that found in dump tanks, is not diminished as rapidly as chlorine. In addition, research has shown that chlorine dioxide has a higher biocidal activity on a ppm basis against several microorganisms in comparison with sodium hypochlorite, iodine, quaternary ammonium compounds, glutaraldehyde, and phenol. Results from our HWT basal rot trials indicate that chlorine dioxide at 5 to 10 ppm effectively controls Fusarium inoculum levels during HWT of daffodil bulbs and thus protects bulbs from the spread of basal rot. There was also no indication of any adverse effects on the growth of bulbs following treatment. INTRODUCTION Ornamental bulbs represent an important high value minor crop in western Washington. Over 90% of the field grown daffodils, tulips, and bulbous iris produced in the United States are grown on approximately 809 ha in western Washington (Gould, 1990; Krause,1991; De Hertogh and Le Nard, 1993). One of the most important disease management problems currently facing the bulb industry in the United States is the loss of formaldehyde. Hot water formaldehyde treatments (HWFT) have been used extensively by bulb growers since the late 1930's to control a number of important insect pests, nematodes and fungi associated with harvested bulbs (Boerema, et al., 1989; Byther and Chastagner 1993; Chastagner and Byther, 1985; Courtney ,1963; Dickens ,1979; Gould and Byther, 1979; Hague, 1972; Price and Briggs, 1977; Tompsett, 1975). Although the addition of formaldehyde gives some slight improvement in nematode kill, its primary use is to prevent the spread of basal rot in the dip tank (Byther and Chastagner, 1993; Chastagner and Byther ,1985; Dickens ,1979; Gould and Miller, 1971; Gould and Byther, 1979; Moore, 1980; Price and Briggs, 1977; Winfield, 1970). Basal rot is caused by the fungus Fusarium oxysporum f. sp. narcissii and occurs wherever daffodils are grown (Byther and Chastagner, 1993; Boerema and Hamers, 1989; Chastagner and Byther, 1985; Dickens, 1979). HWFT have been an important component of growers basal rot disease management programs. Diseased stocks are generally treated for two to three years until the level of disease is reduced. Without formaldehyde, the temperature of the water is not high enough to prevent the spread of basal rot inoculum from diseased to healthy bulbs Proc. 8th Int. Symp. on Flowerbulbs Eds. G. Littlejohn et al. Acta Hort. 570, ISHS 2002 268 during hot water treatment (HWT). Agricultural uses of formaldehyde have been severely restricted in the U.S. by the Environmental Protection Agency. Currently, only a limited number of growers have third-party registrations which allow them to continue to use formaldehyde during the hot water treatment (HWT) of bulbs. The restrictions and potential loss of formaldehyde will have its greatest impact on daffodil growers because of the need to use HWT for the control of stem and bulb nematode, Narcissus bulb fly, various bulb mites, over summering inoculum of various fungal pathogens, and basal rot (Byther and Chastagner, 1993; Courtney, 1963; Gould and Byther, 1979; Hague ,1972; Tompsett et al. ,1977). The inability to control the spread of basal rot during HWT has the potential to result in the total loss of basal rot-susceptible but economically important cultivars such as ‘Golden Harvest’ and ‘Dutch Master’. The Pesticide Management Division of the Washington State Department of Agriculture estimated that without a suitable replacement, the loss of formaldehyde would result in a $16 million loss to bulb growers in Washington. Although growers can currently use nematicides to control stem and bulb nematodes, they have very few alternatives to formaldehyde to prevent the spread of Fusarium inoculum during HWT of bulbs. Linfield (1997) and Hanks (1992) have screened the activity of a number of general biocides and fungicides for their effectiveness in killing Fusarium chlamydospores in vitro. Research at WSU-Puyallup has identified several formulations of chlorothalonil, benomyl, and thiobendizole that are effective in controlling the spread of basal rot during HWT of daffodils (Chastagner, unpublished). Even with these alternatives, growers are still faced with the problem of maintaining minimal concentrations of these materials in dip tanks and disposal of the tank effluent after treatment. Ideally it would be desirable to have a system that automatically monitors and maintains the concentration of material in the dip tank and be able to neutralize it prior to its disposal. One potential is to use chlorine dioxide. Chlorine dioxide is being used increasingly as a replacement for chlorine in the fruit and vegetable industries and a general water disinfectant in many industrial applications. This general biocide appears to have the potential to serve as a replacement for formaldehyde during the HWT of daffodil bulbs (Kaczur and Cawlfield, 1993; Roberts ,1994; Roberts and Reymond, 1994; Spotts and Peters, 1980). The activity of chlorine dioxide in the presence of high levels of organic matter, such as that found in dump tanks, is not diminished as rapidly as chlorine (Benarde et al., 1965). In addition, chlorine dioxide has a higher biocidal activity on a ppm basis than sodium hypochlorite, iodine, quaternary ammonium compounds, glutaraldehyde, and phenol (Tanner, 1989). Working with several postharvest pathogens of tree fruits, Roberts and Reymond (1994) showed that spores of Cryptosporiopsis perennans were killed when exposed to 1 ppm of chlorine dioxide for 30 seconds. Spores of Mucor piriformis were killed after a 4 minute exposure at 1 ppm or 30 second exposure at 3 ppm and spores of Penicillium expansum are killed after a 2 minute exposure at 3 ppm or a 30 second exposure at 5 ppm. Spores of Botrytis cinerea were killed after a 2 minute exposure at 5 ppm. Roberts and Reymond (1994) concluded that a concentration of about 3 to 5 ppm chlorine dioxide in the dump tank water should provide an effective control of spores and thus reduce the potential for various postharvest diseases. CH2O International (Olympia, WA) is currently marketing automated systems to generate chlorine dioxide for a variety of agricultural uses. Using sensors and a computer controlled mixing system, their Fresh-Pak chlorine dioxide system is able to generate and maintain specific concentrations of chlorine dioxide for use in dipping tanks and wash lines. This system produces chlorine dioxide by mixing sodium chlorite with hydrochloric acid. During the past three years a series of test have been conducted to evaluate the effectiveness of chlorine dioxide in preventing the spread of Fusarium inoculum during the HWT of daffodil bulbs.