Published Ahead of Print 15 November 2013. 10.1128/AEM.02964-13.

2014, 80(3):849. DOI:Appl. Environ. Microbiol. Lisa A. Jones, Randy W. Worobo and Christine D. Smart WaterPathogens in Unfiltered Surface Irrigation UV Light Inactivation of Human and Plant

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UV Light Inactivation of Human and Plant Pathogens in UnfilteredSurface Irrigation Water

Lisa A. Jones,a Randy W. Worobo,b Christine D. Smarta

Department of Plant Pathology and Plant-Microbe Biology, Cornell University, Geneva, New York, USAa; Department of Food Science, Cornell University, Geneva, NewYork, USAb

Fruit and vegetable growers continually battle plant diseases and food safety concerns. Surface water is commonly used in theproduction of fruits and vegetables and can harbor both human- and plant-pathogenic microorganisms that can contaminatecrops when used for irrigation or other agricultural purposes. Treatment methods for surface water are currently limited, andthere is a need for suitable treatment options. A liquid-processing unit that uses UV light for the decontamination of turbidjuices was analyzed for its efficacy in the treatment of surface waters contaminated with bacterial or oomycete pathogens, i.e.,Escherichia coli, Salmonella enterica, Listeria monocytogenes, Clavibacter michiganensis subsp. michiganensis, Pseudomonassyringae pv. tomato, and Phytophthora capsici. Five-strain cocktails of each pathogen, containing approximately 108 or 109 CFU/liter for bacteria or 104 or 105 zoospores/liter for Ph. capsici, were inoculated into aliquots of two turbid surface water irrigationsources and processed with the UV unit. Pathogens were enumerated before and after treatment. In general, as the turbidity ofthe water source increased, the effectiveness of the UV treatment decreased, but in all cases, 99.9% or higher inactivation wasachieved. Log reductions ranged from 10.0 to 6.1 and from 5.0 to 4.2 for bacterial pathogens and Ph. capsici, respectively.

For decades, water has been a vector for human- and plant-pathogenic microorganisms (1, 2). Illnesses caused by water-

borne microorganisms can occur through the consumption offruits and vegetables that have come in contact with contaminatedwater (3, 4). Plant pathogens are also spread through water andcan lead to plant disease and yield losses. In the food productionenvironment, agricultural water is defined as any water that isused in the growing, harvesting or packing of produce where wateris likely to contact produce directly or to contact surfaces thatproduce are likely to come in contact with (5). Agricultural water,including that used for irrigation, pesticide or herbicide applica-tion, freeze protection, or produce washing, is under increasingscrutiny as a vehicle for food-borne human- and plant-disease-causing microorganisms. Irrigation water is one of the main ave-nues by which pathogenic microorganisms can reach produce,especially if the irrigation water is obtained from a surface waterreservoir. Irrigation water is typically taken from groundwater,surface water, or municipal sources. Water from groundwater andmunicipal sources is generally free of pathogenic microorganisms,but breaches can still occur. Surface water sources are consideredhigh risk for pathogen contamination because they are open tomany routes by which microorganisms causing plant disease orhuman food-borne illness can enter. Although surface water is ahigh-risk source, many growers continue to use surface water be-cause it remains the most feasible and economic choice.

Mitigation strategies may be necessary for growers to continueusing surface water for agricultural applications, particularly forleafy greens or produce that is intended to be consumed raw.Human-pathogenic bacteria can enter surface water sourcesthrough fecal material from wildlife and human activities or con-taminated runoff and debris. Three important bacterial speciesresponsible for many food-borne illnesses are pathogenic Esche-richia coli, Salmonella enterica, and Listeria monocytogenes, withillness occurring through consumption of contaminated freshfruits and vegetables. These pathogenic bacteria have been recov-ered from the environment, including irrigation sources, where

produce is grown (6, 7, 8, 9). Illnesses from these pathogens canlead to death and cause significant economic losses for growers ifthe bacteria are traced back to their farms.

The Food and Drug Administration (FDA) has recentlydrafted a set of food safety regulations as part of the Food SafetyModernization Act (FSMA), with a principal focus on fresh pro-duce due to its high risk for contamination. Produce consumptionaccounts for about half of the food-borne illness outbreaks eachyear, and many of these cases are due to the bacterial pathogens E.coli, S. enterica, and L. monocytogenes (10, 11). Much of the pro-duce focus of FSMA is on the prevention of contamination in thegrowing environment. These proposed regulations have broughtconsiderable attention to agricultural water and its role in thespread of disease-causing organisms. Testing of irrigation waterfor generic E. coli, an indicator of fecal contamination, will likelybe required for some growers, especially if the water is from asurface water source. If generic E. coli levels are above regulatorythresholds, the irrigation water would not be usable unless a mit-igation strategy is applied and further testing reveals generic E. colilevels to be below the threshold.

Much emphasis is being placed on human pathogen contami-nation of produce, but growers are also concerned about plant-pathogenic organisms, which can lead to large yield and economiclosses. A major mode of dispersal for many plant pathogens isthrough water which can become contaminated through infesteddebris, soil, or runoff. All major groups of plant pathogens, in-

Received 3 September 2013 Accepted 12 November 2013

Published ahead of print 15 November 2013

Address correspondence to Christine D. Smart, cds14@cornell.edu.

Supplemental material for this article may be found at http://dx.doi.org/10.1128/AEM.02964-13.

Copyright © 2014, American Society for Microbiology. All Rights Reserved.

doi:10.1128/AEM.02964-13

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cluding bacteria, viruses, fungi, nematodes, and oomycetes, havebeen found in irrigation water (2). Several plant-pathogenic iso-lates of the Gram-negative bacterium Pseudomonas syringae havebeen isolated from many different surface water sources (32, 33).In New York surface water, the pathogens that we have recoveredinclude the oomycete Phytophthora capsici and the Gram-positivebacterium Clavibacter michiganensis subsp. michiganensis (L. A.Jones and C. D. Smart, unpublished data).

Given that irrigation water is a major carrier for human andplant pathogens, a mitigation strategy that could deal with bothgroups would be of great benefit to any grower. There are cur-rently several methods available for the treatment of water, such aschlorine, ozone, UV, and filtration, but not all methods are suit-able for surface water sources due to its complexity and variability.Water quality parameters such as pH, turbidity, color, dissolvedsolids, and microbial load can adversely affect treatment efficaciesand can change seasonally or even hourly in surface water withweather events or human activities.

UV light has been used successfully for treating human bacte-rial and protist pathogens in drinking water (12). It has also beenused to successfully disinfect water contaminated with plant-pathogenic oomycetes and bacteria in nursery settings where re-cycling is a common method of water and nutrient conservation(13, 14). UV light treatment of drinking and nursery water can beeffective, since these waters are high quality, with low turbidity(�1.0 nephelometric turbidity unit [NTU]) and microbial loads.Previously, UV light was not considered suitable for the treatmentof surface water due to high turbidity levels (�1.0 NTU), whichcan block or absorb UV light, shielding pathogens from treat-ment. One UV treatment system, UV CiderSure (FPE, Inc., Roch-ester, NY), however, has been designed to overcome this problemand is capable of consistently achieving a minimum 5-log reduc-tion of E. coli O157:H7 in unfiltered apple cider. Apple cider is aliquid with a varying high content of solids and with high turbidityin the range of 1,000 to 2,400 NTU (15). The UV processing unit isdesigned to deliver the same UV dose to all pathogens by usingcomputational fluid dynamics and adjustable flow rates. In thisstudy, the UV processing unit designed to treat turbid liquids withhigh solid contents was evaluated for efficacy in decontaminatingsurface waters contaminated with bacterial and oomycete patho-gens, including E. coli O157:H7, S. enterica, L. monocytogenes, Pseu-domonas syringae pv. tomato, Clavibacter michiganensis subsp.michiganensis, and Phytophthora capsici.

MATERIALS AND METHODSWater sources. For this experiment, the UV inactivation of each pathogenwas tested in three water sources. Two of the water sources were fromactively used surface water irrigation sources, a creek (Tompkins Co., NY)and a pond (Ontario Co., NY). Phosphate-buffered saline (PBS) (137 mMNaCl, 2.7 mM KCl, 10 mM Na2HPO4, 2 mM KH2PO4) and reverse os-mosis (RO) water were used as low-turbidity water sources for all exper-iments with bacterial pathogens and Ph. capsici, respectively. Surfacewater was collected in the fall of 2012 and stored in 55-gallon drums(food-grade plastic). Surface water was stirred thoroughly, and then pH(HI 2211 pH/ORP meter; Hanna, Woonsocket, RI) and turbidity (2100Pportable turbidimeter; Hach, Loveland, CO) measurements were re-corded before use. All turbidity values were recorded in nephelometricturbidity units. RO water was produced as needed (Barnstead NanopureII; Thermo Fisher Scientific, Waltham, MA).

Human and plant pathogen strains. Six pathogen species, i.e., E. coli,S. enterica, L. monocytogenes, Ps. syringae, C. michiganensis, and Ph. capsici,

were used in this study. Five strains of each pathogen were used to preparea five-strain cocktail for UV inactivation experiments (Table 1). The E.coli, S. enterica, and L. monocytogenes strains are clinical or food isolatesobtained from M. Wiedmann’s food safety laboratory at Cornell Univer-sity. After storage at �80°C, human bacterial pathogens were passed oncethrough tryptic soy broth (TSB) (Hardy Diagnostics, Santa Maria, CA).All Ps. syringae, C. michiganensis, and Ph. capsici strains were obtainedfrom the Smart lab and were isolated from field samples collected in NewYork. All Ps. syringae and C. michiganensis strains were isolated from to-mato. Ph. capsici strains were isolated from pepper (strain 0664-1), pump-kin (strains 0759-8 and MMZ-4A), zucchini (strain 0752-15), and butter-nut squash (strain 06180-4).

Media and culture conditions. E. coli, S. enterica, and L. monocyto-genes were maintained on tryptic soy agar (TSA) (Hardy Diagnostics,Santa Maria, CA), while Ps. syringae, C. michiganensis, and Ph. capsici weremaintained on King’s B (KB) agar (16), D2ANX agar (17), and PARP agar(18), respectively. For preparation of the five-strain cocktails, E. coli and S.enterica were grown for 18 h at 37°C; L. monocytogenes was grown for 24 hat 37°C in TSB, and Ps. syringae was grown at 28°C for 18 h in KB broth.Clavibacter michiganensis was grown for 48 h at 28°C in Luria-Bertani(LB) broth (19). All bacteria were incubated on a rotary platform shaker at250 rpm. Phytophthora capsici was cultured on 15% V8 agar for 7 days atroom temperature (25 to 28°C) under continuous fluorescent light forsufficient sporangium production. Zoospore suspensions were producedaccording to the protocol used by Dunn et al. (20).

UV radiation and inactivation. UV radiation (254 nm) was deliveredto water samples using a thin film CiderSure unit (UV CiderSure 3500;FPE, Inc., Rochester, NY) for all experiments in this study. Water is ini-tially drawn through 1-in.-diameter tubing and then dispersed through athin, 5-mm-thick cylinder surrounding 8 central UV lamps. The maxi-mum penetration depth of UV irradiation is set at the thickness of thetreatment cylinder at 5 mm. The UV irradiance penetrating through thewater sample is measured by sensors in the treatment cylinder, oppositethe side of the UV lamps. The unit contains two UVX-25 sensors (UVP,Inc., Upland, CA) that measure UV irradiance penetrating the liquid sam-ple every 50 milliseconds. The flow rate automatically adjusts according toa preprogrammed algorithm based on information gathered from thesensors to overcome differences in water quality parameters (i.e., solidcontents, turbidity, and color) of water being processed and delivers thesame UV dose of 14.2 mJ/cm2 to all samples (21). The efficacy of UVinactivation for each pathogen species was tested separately in three watersources. A five-strain cocktail of a pathogen species was added to 1-literwater samples. Multiple-strain cocktails are commonly used in humanand plant pathogen studies for more comprehensive assessments becauseit is common for more than one strain to be present in a food-borne illnessor plant disease outbreak (22, 23). Bacterial cocktails were produced byadding 10 ml of stationary-phase culture of each strain together and thor-

TABLE 1 Pathogens used for UV inactivation studies

Species or subspecies Strains or serovars

Human pathogensEscherichia coli O157:H7 933, 2722, ATCC 43895, ATCC

35150, ATCC 4389Salmonella enterica subsp. enterica Hartford, Montevideo, Rubislaw,

Gaminara, CubanListeria monocytogenes 2812, 2289, L99, 104025, F2586-VI

Plant pathogensPseudomonas syringae pv. tomato 09150, 09110, 09084, 0761, 0578Clavibacter michiganensis subsp.

michiganensis11015, 10-4, 0767, 09085, 0690

Phytophthora capsici 0664-1, 06180-4, 0759-8, 0752-15,MMZ-4A

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oughly mixing. One milliliter of the cocktail was added to 1 liter of waterto give approximately 108 to 109 CFU/liter, and 10 ml was added to 1 literof water to give approximately 109 to 1010 CFU/liter. Zoospores of Ph.capsici were enumerated using a hemacytometer (Hausser Scientific, Hor-sham, PA). Equal numbers of zoospores from each strain were combinedto produce a five-strain cocktail for inoculation of 1-liter water samples at104 and 105 zoospores/liter. The higher inoculum levels were included toensure the assessment of the upper inactivation rates. Three 1-liter sam-ples were prepared from individual cocktail preparations for each patho-gen concentration and water source pairing. Subsamples (1 ml) weretaken immediately from inoculated 1-liter water samples for enumerationbefore UV inactivation. Samples were gently mixed prior to subsampling,UV inactivation, and filtering. The 1-liter water samples were processedimmediately through the UV irradiation unit, and treated samples werecollected in sterile 1-liter bottles and filtered for enumeration. Noninocu-lated samples from each water source were also processed for each patho-gen. All UV inactivation experiments were repeated once, for a total of six1-liter samples for each pathogen concentration and water source pairing.

Enumeration. Pathogen populations in both inoculated and nonin-oculated samples were enumerated before and after UV inactivation onselective or semiselective media. Before UV exposure, bacteria (CFU/liter)were enumerated by serial dilution plating, and Ph. capsici zoospores(zoospores/liter) were enumerated with a hemacytometer. After UV inac-tivation, 1-liter samples were filtered using sterile magnetic filter funnels(Pall, Port Washington, NY). For bacterial pathogens, 47-mm, 0.45-�m-pore-size filters (Thermo Fisher Scientific, Waltham, MA) were used. ForPh. capsici, 47-mm, 5.0-�m-pore-size filters (EMD Millipore, Billerica,MA) were used and placed onto the corresponding semiselective mediumfor the enumeration of the pathogen of interest. One or more filters wereused per 1-liter sample to prevent clogging. Enumeration of E. coli wasperformed on Violet Red bile agar (VRBA) (Hardy Diagnostics, SantaMaria, CA); samples were incubated at 37°C for 18 h. Salmonella entericaCFU/liter were enumerated on bismuth sulfite agar (Criterion, Santa Ma-ria, CA); samples were incubated at 37°C for 18 h. Listeria monocytogenesCFU/liter were enumerated on Oxford medium base amended with mod-ified Oxford antimicrobial supplement (Difco, Franklin Lakes, NJ); sam-ples were incubated at 37°C for 24 h. Pseudomonas syringae CFU/liter wereenumerated on Pseudomonas agar base containing a C-F-C supplement(Oxoid Limited, Hampshire, United Kingdom). All Ps. syringae was trans-ferred to KB agar for confirmation of CFU/liter (production of fluores-cein). Clavibacter michiganensis CFU/liter were enumerated on D2ANXwith incubation at 28°C for 48 h. Phytophthora capsici zoospores wereenumerated on PARP and were incubated at room temperature (25 to28°C) for 5 days; mycelial growth was transferred to 15% V8 agar andincubated for 28°C for up to 7 days for morphological conformation ofPh. capsici.

Calculations and statistics. The percent inactivation [(N0 � N)/N0]was calculated for each pathogen concentration by water source pairing.Pathogen counts were also converted into logarithmic units, and log re-duction was calculated as log(N/N0), where N corresponds to the after-treatment count and N0 to the initial count. Data were analyzed byanalysis of variance using R statistical software (24). Tukey’s honestlysignificant difference (HSD) was used to determine significant differences(� � 0.05) between log reduction means or percent inactivation means ofall pathogen concentration and water source pairings.

RESULTSUV inactivation of pathogens by water source. The efficacy ofUV inactivation of each of the six pathogen species was analyzed inthe three water sources, which varied in pH and turbidity. The pHand turbidity of the water sources were monitored throughout theUV inactivation experiments, and the average and range of bothparameters are presented for each water source in Table 2. The ROwater had an average neutral pH (6.96), while the PBS, creek, andpond water exhibited alkaline pH levels (8.01, 8.32, and 8.21, re-

spectively). The pH did not vary more than 0.19 pH units in anywater source. The turbidity of the RO water and PBS were consis-tent at 0.1 NTU. The turbidity in the creek water was higher andmore variable and ranged from 3.0 to 4.4 NTU. Turbidity was byfar the highest and most variable in the pond water, which rangedfrom 15.8 to 22.7 NTU.

The average percent inactivation and log reduction values forall pathogens by water source can be found in Tables S1 to S6 inthe supplemental material. Percent inactivation for all pathogensby water source was 99.9% or greater. The log reductions rangedfrom 10.0 to 6.1 and from 5.0 to 4.2 for bacterial pathogens andPh. capsici, respectively (Tables S1 to S6). In all water sources, C.michiganensis consistently had larger total numbers of CFU/literafter UV treatment than the other bacterial pathogens (Tables S1,S3, and S5). Escherichia coli and S. enterica had similar numbers ofCFU/liter after UV treatment in all water sources and were con-sistently lowest of the bacterial pathogens (Tables S1, S3, and S5).Pseudomonas syringae and L. monocytogenes had intermediatenumbers of CFU/liter after UV treatment among the bacterialpathogens; they performed similarly to each other in PBS (TableS1), but in the creek and pond water, L. monocytogenes had moresurviving CFU/liter than Ps. syringae (Tables S3 and S5).

UV inactivation of pathogens by species. (i) E. coli O157:H7.One hundred percent inactivation was achieved for the lower con-centrations and 99.9% inactivation was achieved for higher con-centrations in each water source (see Tables S1, S3, and S5 in thesupplemental material). No significant difference in log reductionwas found among water sources at the lower concentrations (Fig.1). For the higher concentrations, the log reduction was not dif-ferent between PBS (10.0) and creek water (9.5); however, the logreduction was significantly less (7.3) in pond water (Fig. 2). NoCFU were recovered in uninoculated controls before or after UVinactivation.

(ii) S. enterica. At the lower concentrations in PBS and creekwater, 100% inactivation was achieved, while 99.9% inactivationwas achieved for the lower concentration in pond water and forthe higher concentrations in each water source (see Tables S1, S3,and S5 in the supplemental material). At the lower concentrations,there were no log reduction differences between PBS (8.8) andcreek (8.6) water or between creek and pond (8.3) water. A signif-icant difference was found between PBS and pond water (Fig. 1).For the higher concentrations, there were significant differences inlog reduction between each pair of water sources (Fig. 2). No S.enterica CFU were recovered in uninoculated controls before orafter UV inactivation.

(iii) L. monocytogenes. For all concentrations and watersource pairings, 99.9% inactivation was achieved (see Tables S1,S3, and S5 in the supplemental material). Significant differences inlog reduction were observed among all water sources at both the

TABLE 2 pH and turbidity values for water sources used in UVinactivation experiments

Water source

pH Turbidity (NTU)

Avg Range Avg Range

PBS 8.01 7.88–8.10 0.1 0.1RO 6.96 6.90–7.00 0.1 0.1Creek 8.32 8.26–8.37 3.9 3.0–4.4Pond 8.21 8.17–8.36 19.6 15.8–22.7

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lower and higher pathogen concentrations (Fig. 1 and 2). No L.monocytogenes CFU were recovered in uninoculated controls be-fore or after UV inactivation.

(iv) Ps. syringae. For all concentrations and water source pair-ings, 99.9% inactivation was achieved (see Tables S1, S3, and S5 inthe supplemental material). Log reductions at the lower concen-trations were different in pond water (6.9) than in PBS (8.1) orcreek water (7.7), but not between PBS and creek water (Fig. 1).Log reductions at higher concentrations were different for eachwater source (Fig. 2). Background Pseudomonas organisms werepresent in uninoculated pond water (55 CFU/liter before UV in-activation and 0 CFU/liter after UV inactivation). For inoculatedsamples, background Pseudomonas organisms were included inthe values for before and after UV inactivation.

(v) C. michiganensis. For all concentrations and water sourcepairings, 99.9% inactivation was achieved (see Tables S1, S3, andS5 in the supplemental material). Differences in log reductions atthe lower concentrations were found between PBS (7.5) and creek(6.9) or pond (6.8) water but not between creek and pond water(Fig. 1). Differences in log reductions were found between eachwater source at the higher concentrations (Fig. 2). No C. michi-ganensis CFU were recovered in uninoculated controls before orafter UV inactivation.

(vi) Ph. capsici. One hundred percent inactivation wasachieved at the level of 104 zoospores/liter in RO and creek water,and 99.9% inactivation was achieved in pond water (see Tables S2,S4, and S6 in the supplemental material). At 105 zoospores/liter,99.9% inactivation was achieved in the three water sources (Tables

FIG 1 Average log reductions for the lower inoculum levels of each pathogen in each water source. (A) E. coli O157:H7; (B) S. enterica; (C) L. monocytogenes; (D)Ps. syringae pv. tomato; (E) C. michiganensis subsp. michiganensis; (F) Ph. capsici (no error bars are shown because all samples resulted in 100% inactivation).Different letters indicate significantly different groups (� � 0.05).

FIG 2 Average log reductions for the higher inoculum levels of each pathogen in each water source. (A) E. coli O157:H7; (B) S. enterica; (C) L. monocytogenes;(D) Ps. syringae pv. tomato; (E) C. michiganensis subsp. michiganensis; (F) Ph. capsici. Different letters indicate significantly different groups (� � 0.05).

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S2, S4, and S6). No significant difference in log reduction wasfound between water sources at 104 zoospores/liter (Fig. 1). At 105

zoospores/liter differences in log reductions were found for eachwater source (Fig. 2). No Ph. capsici isolates were recovered inuninoculated controls before or after UV inactivation.

DISCUSSION

The surface water sources for this study were chosen because theywere actively used irrigation sources that are representative of thepH and turbidity of surface waters in New York State as deter-mined by an irrigation water survey conducted by Jones andSmart (unpublished data). For UV inactivation experiments, wa-ter sources were monitored for pH and turbidity to ensure unifor-mity of water sources during experimentation. Additionally, thevalues of these two water quality parameters are important whenconsidering a water treatment option. Chlorination is one of themost effective and economical water disinfection methods but isnot recommended for water with a pH above 7.5 or water withhigh levels of particulates, due to low levels of hypochlorous acidformation and binding to organic matter, respectively. The pHvalues of the surface waters in this study were on average 8.32 and8.21 for the creek and the pond, respectively, making these sourcespoor choices for chlorine disinfection. Studies have found that pHis not a significant factor in UV treatment efficacy (25, 26).

Turbidity, on the other hand, has been found to adversely af-fect UV treatment, but the relationship between turbidity leveland UV efficacy is not consistent. Water components that influ-ence turbidity have variable UV-blocking and -absorbing quali-ties, and therefore turbidity can be used only as a general guidelineto determine UV transmittance. In general, as turbidity increases,UV transmittance and bactericidal efficacy decrease (27, 28). Theaverage log reduction results from this study support this trend, astests conducted with more turbid water were generally less effec-tive at inactivating pertinent challenge pathogens than those witha less turbid source. In some cases, for example, with E. coli and Ph.capsici, there was no significant log reduction difference betweenthe 0.1-NTU (PBS or RO) water source and the 3.9-NTU (creek)source. This may be due to the fact that complete or almost com-plete inactivation occurred in the less turbid water, making the logreduction equal to the initial pathogen concentrations and notwholly representative of the full measure of UV efficacy. In NewYork State, the turbidity of surface water used for irrigation iscommonly between 1 NTU and 20 NTU, but it can vary consid-erably throughout the year (Jones and Smart, unpublished data).The creek water had an average turbidity of 3.9 NTU and the pondof 19.6 NTU; these values are representative of the range of tur-bidities of the majority of surface water sources in New York.

The percent inactivation for all pathogens and water sourcepairings was found to be 99.9% or greater. These data show thatUV light as a mitigation strategy can be effective against a broadspectrum of pathogens in complex surface water sources. Whenanalyzing percent inactivation data, differences in UV efficacyamong water sources are not apparent, but with examination ofthe average log reduction data, we can begin to discern how UVefficacy is affected by the different water sources. For the bacterialpathogens, log reductions ranged from 10.0 (E. coli in PBS) to 6.1(C. michiganensis in pond water). No validation standards havebeen developed for the treatment of bacterial or oomycete patho-gens in surface water that is intended for irrigation. To be recog-nized as a valid method for treatment of juices by the FDA, the

treatment must obtain a 5-log reduction of a pertinent pathogenin juice (29). The log reduction results for bacterial pathogensfrom this experiment would meet the FDA’s requirements forjuice. No such standard exists for oomycetes. Log reduction valuesfor Ph. capsici zoospores were less than those for all bacterialpathogens, because the highest concentration of zoospores that wecould obtain was 5 � 105/liter. Thus, it would be impossible tohave a log reduction of 6 or greater as was observed with thebacterial pathogens. The zoospores were clearly highly susceptibleto UV treatment, as there was 100% inactivation in pond water atthe lower pathogen concentration (5 � 104/liter). The only otherpathogen with 100% inactivation in pond water was E. coli atthe lower concentration. Clavibacter michiganensis, in all watersources, was the least susceptible to UV treatment, followed by L.monocytogenes in creek and pond water, among the bacterialpathogens, with the highest CFU/liter after treatment. Typically,Gram-positive bacteria are more recalcitrant to UV radiation thanGram-negative bacteria (30).

UV may not be applicable to all irrigation situations, particu-larly those applications requiring very high volumes. The pathlength through which UV can penetrate water is very small andlimits the volume of water that can be treated at one time. Highlyturbid waters may not be good candidates for UV treatment with-out prefiltering to remove some of the UV-blocking and -absorb-ing components; however, even at relatively high turbidity levels(20 NTU), we saw 99.9% inactivation. Turbidity is not the onlywater quality parameter that can affect the efficacy of UV treat-ment; dissolved solids such as iron can also absorb UV light anddecrease the UV transmittance.

More than half of the irrigation water in the United States isfrom surface water sources, and a recent survey of New York veg-etable growers found that 57% use surface water (31). With recentFDA regulations for human pathogens in irrigation water, moreoptions will be needed to treat these water sources. The data fromthis study suggest that UV light is a feasible treatment method togreatly reduce bacterial and oomycete pathogen populations insurface irrigation water without pretreatment. Additional re-search is needed to investigate the feasibility of this disinfectionmethod, utilizing additional water sources and on a larger scalethat would be in line with irrigation water volumes currently usedby a range of produce growers. UV light treatment systems havethe potential to significantly lower the risk of both plant andhuman pathogen contamination of crops from surface waterthrough irrigation or other agricultural applications and could beintegrated into effective food safety and plant health programs.

ACKNOWLEDGMENTS

This study was funded by the National Research Initiative CompetitiveGrants Program, grant number 2009-55605-05184, from the NationalInstitute of Food and Agriculture. Additional funding was provided byfederal formula funds through the New York State Agricultural Experi-ment Station.

We thank John Churey for growing the human pathogen isolates andfor his assistance with the UV processing unit. We also thank Holly Langeand Megan Buckley for their support in the laboratory.

REFERENCES1. Centers for Disease Control and Prevention. 2013. Water-related emer-

gencies & outbreaks. http://www.cdc.gov/healthywater/emergency/.2. Hong C, Moorman G. 2005. Plant pathogens in irrigation water: chal-

lenges and opportunities. CRC Crit. Rev. Plant Sci. 24:189 –208. http://dx.doi.org/10.1080/07352680591005838.

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