Carrot is an important crop in Israel grown over a total area of 2200 ha. Carrots are harvested and marketed fresh between November and June. During the remainder of the year the roots are marketed from cold storage. To extend shelf life and limit decay development, freshly harvested carrots go through a series of sanitation procedures. Initially, the carrots are washed with water jets to remove the bulk of dirt from the field. Next, they enter a polishing drum, which scrapes off remnants of dirt. This step is followed by 10 to 15 min of hydrocooling sanitation in chlorinated (200 ppm NaClO) and chilled (4°C) water in Jacuzzi baths. The sanitized and chilled carrots   are   then   transferred,  via  conveyor  belts, into troughs filled with the fungicide iprodione at rate of 0.1% (volume per volume, v/v) and are finally packaged in big bags of approximately 1,500 kg and placed in cold storage at 1°C until marketed.

In Israel, carrots may be stored for up to 6 months and are susceptible to decay incited by Sclerotinia sclerotiorum (Papoutsis and Edelenbos, 2021). Sclerotinia sclerotiorum (Lib.) de Bary is an ascomycete fungus, prevalent worldwide and known for its wide host range and aggressive nature (Purdy, 1979). In Israel the fungus is primarily active from fall to spring, where it affects various crops (Ben-Yephet et al., 1993), including carrot. The  fungus  is  commonly   carried   from   the   field  into storage on the carrots’ surface or in colonized crowns, where it develops over time into a watery soft rot, deteriorating the produce (Kora et al., 2003, 2005). Despite the vigorous sanitation practices, the fungus survives and decay develops, causing up to 7% loss annually.

Since the early 1990s and until recently, the fungicide iprodione was the sole fungicide in Israel used commercially to control postharvest decay of carrots incited by S. sclerotiorum. Iprodione is a Fungicide Resistance Action Committee (FRAC) code 2 group dicarboxamide, which affects signal transduction in fungi (FRAC code list® 2022; https://www.frac.info/). In Israel, the fungicide was approved for use to control white mold caused by S. sclerotiorum; gray mold caused by Botrytis cinerea; watery rot caused by Rhizopus stolonifer; early blight caused by Alternaria alternata, and other disease-causing pathogens of vegetable crops, horticultural crops, grapes, and citrus. Nonetheless, its use in Europe has been terminated and consequently, as of January 2021 the fungicide was phased-out in Israel.

The phase-out of iprodione placed the carrot industry with an immediate need for a replacement to sustain marketing. Therefore, it was suggested to examine the efficacy of the fungicide fludioxonil, which is used in Israel for the prevention of decay development in stored pomegranate (Porat et al., 2015). With this in mind, the present study tested the hypothesis that the fungicide fludioxonil can commercially replace the fungicide iprodione and limits decay incited by S. sclerotiorum in cold-stored carrots, as well as, extend storability and shelf life. The goal of the study was to provide evidence of efficacy for the purpose of approving the fungicide for commercial use by the Israeli Plant Protection and Inspection (PPIS) regulatory agency.

Statistical analysis

Statistical analyses were performed at α = 0.05 in JMP 16 (SAS Institute Inc., Cary, NC, USA). In the artificially inoculated trials, the area under the decay progress curve (AUDPC) values were average-rank transformed, as they arose from an ordinal scale, and were subjected to analysis of variance (ANOVA) utilizing the Kruskal-Wallis procedure. In the commercial trials, decay incidence values were root-squared and subjected to arcsin transformation followed by parametric ANOVA or two-tailed Student's t-test. ANOVA was followed by Fisher's protected least significant difference (LSD) test for treatment's mean separation. Values presented in the result section were back-transformed (Zar, 1999).

The goal of the present study was to provide evidence of fludioxonil's efficacy to control decay incited by S. sclerotiorum in stored carrots for the purpose of approving the fungicide for commercial use by the Israeli regulatory agency. Four replicated trials, two with artificial inoculations and two commercial, absent inoculations, were carried out. Artificially inoculated trial 1 extended for 58 days in cold storage. Of the three fludioxonil rates tested, only 0.1% (v/v) limited decay incited by S. sclerotiorum. Carrots treated with fludioxonil at 0.1% had 43 and 37% less (P<0.0001) decay than those exposed to chlorinated water or tiabendazole, but not less (P>0.05) than iprodione (Figure 1). Artificially inoculated trial 2 extended was for 70 days and presented a similar outcome (Figure 2). Carrots treated with fludioxonil at 0.1% (v/v) had 69 and 58% less (P<0.0001) decay than carrots treated with chlorinated water control or with the commercial reference tiabendazole, respectively. In both trials, thiabendazole proved ineffective in preventing decay caused the by S. sclerotiorum. The commercial trials were carried out in two different packing houses, managed by different commercial growers, utilizing carrots that were grown at two different locations - in the north or in the south of the country. The fungicide-treated carrots were stored for 187 and 167 days, which were 5.5 and 6 months, respectively. In both trials, carrots treated with fludioxonil at 0.1% (v/v) had 98 and 96% less (P<0.0001) decay than those exposed only to chlorinated water control (Figures 3 and 4). Furthermore, under the non-inoculated commercial conditions, fludioxonil was as effective as (P>0.05) iprodione (Figure 3).

The fungicide iprodione was used in Israel for nearly 30 years for the prevention of decay development in stored carrots, caused by S. sclerotiorum. As a result of its elimination in Europe, it had been phased-out in Israel as of January 2021, leaving the carrot industry without an effective replacement, prohibiting long term storage of carrots for both the local market and export. The present study succeeded in identifying the fungicide fludioxonil at a rate of 0.1% (v/v) as an effective replacement. As a result of the study presented here, fludioxonil was regulatorily approved by the Plant Protection and Inspection Services (PPIS) in Israel, and entered commercial use.

The fungicide fludioxonil  is  a  synthetic  analog  of  the bacterial antifungal compound pyrrolnitrin, which is produced by bacterial species from the genus Pseudomonas (Arima et al., 1964; Gordee and Matthews, 1969). Fludioxonil is characterized to FRAC code 12 (Fungicide Resistance Action Committee; https://www.frac.info), disrupting osmotic signal transduction via improper activation of the Hog1-type mitogen-activated MAP/Histidin Kinase (Yoshimi et al., 2005) and association of the Ssos4 gene (Li et al., 2021). Hence, causing osmotic stress in fungi and interfering with mycelial growth. Duan et al. (2014) demonstrated that in exposure to fludioxonil, S. sclerotiorum had an increase in mycelial membrane permeability, increased activity of the stress related enzymes phenylalanine ammonia lyase and peroxidase, and accumulation of glycerol. As well, a decrease in oxalate, which is associated with pathogenicity and in extracellular polymeric substances levels associated with the decomposition of organic matter (Op De Beeck et al., 2021).

Currently, fludioxonil is used in Israel exclusively and extensively for minimizing decay in stored carrots. This overwhelming application of fludioxonil poses significant management risks for insensitivity development and resistance build-up in the  population  of  S.  sclerotiorum.

To date, the information in the literature does not indicate resistance to fludioxonil in wild-type populations of S. sclerotiorum. Nonetheless, in vitro tests with S. sclerotiorum mutants suggested a moderate risk for insensitivity development (Kuang et al., 2011). Low sensitivity to fludioxonil has been reported in Penicillium digitatum and P. italicum with maximum inhibitory concentration (MIC) of ≥100 µg/ml and EC₅₀ values of 0.1 to 1 and 1 to >100 µg/ml, respectively (Schirra et al., 2005). Resistance to fludioxonil also was reported in Botrytis cinerea from a strawberry field in Virginia, USA (Fernández-Ortuño et al., 2013). The resistant isolate grew in vitro on a discriminatory rate of 0.1 µg/ml and had an EC₅₀ value of 0.26 µg/ml. On the contrary, the sensitive isolates were inhibited by the discriminatory dose with an EC₅₀ of 0.01 µg/ml. Furthermore, the resistant isolate cause disease when strawberry fruits were artificially inoculated and treated with the recommended field application rate of 2.5 ml/liter fludioxonil. Keeping in mind that B. cinerea and S. sclerotiorun are closely related genetically (Amselem et al., 2011) and characterized to the fungal family Sclerotiniaceae, it is not unlikely for development of fludioxonil resistance genotypes in the latter. This information stresses the need to avoid the exclusive use of fludioxonil in Israel for preventing decay in cold-stored carrots and identify additional means for decay management. The focus may be additional chemical fungicides that could be used in alternation with fludioxonil or in mixtures. But, primarily bio-fungicides and other eco-friendly GRAS materials, such as plant extracts, essential oils, biostimulants and effective food additives (Elsherbiny and Taher, 2018; Papoutsis and Edelenbos, 2021) that will provide a sustainable decay control.

The present study reported the successful identification of the fungicide fludioxonil at a rate of 0.1% v/v as a replacement of iprodione for decay management in stored carrots in Israel. As a result of this research, fludioxonil was regulatorily approved and incorporated into commercial use. The reports of the potential for resistance development to fludioxonil in phytopathogenic fungi is alarming, requiring to continue exploring new materials to providing the carrot industry with additional means for decay control.

The authors have not declared any conflict of interests.

The authors express their gratitude to Dr. Eli Shelavin, for assisting with the development of the inoculation procedure. The authors recognize Mr. Matan Gur Arie for his technical assistance.