Final report for LNE19-388R
Project Information
Pseudoperonospora cubensis is the oomycete pathogen responsible for cucurbit downy mildew (DM). Many cucurbits such as cantaloupe, pumpkin, watermelon, and squash are susceptible to P. cubensis, but it is particularly devastating to cucumber where it can cause losses of $1000 or more per acre. The use of resistant cucurbit varieties was an effective means of control of downy mildew in the United States until 2004. Presently, the use of chemical fungicides in addition to planting resistant varieties has been the only viable option to achieve effective control.
Both visible light and ultraviolet (UV) radiation have been reported to reduce the viability of fungal spores. UV-C, which is blocked by the atmosphere but can be generated by electrical sources, is well known for its germicidal properties. UV radiation has been demonstrated to be especially effective for controlling powdery mildew. The goal of this research project was to investigate the variables which would affect the viability of using UV-C as a treatment for DM: UV dose, application frequency, mulch type, phytotoxicity of treatments, and treatment cost.
Field trials were undertaken during the 2020 and 2021 growing seasons on a commercial fruit and vegetable farm in eastern Massachusetts. During the first year, controlled doses of UV-C between 120 and 480 J·m-2 were applied either once or twice weekly using a tractor and a three-point hitch mounted array of sources. The efficacy of UV treatments was compared to conventional fungicide treatments as well as to untreated controls. All UV treatments, as well as the untreated controls, were applied on black and reflective plastic mulches. The conventional treatment was only applied to beds made with black mulch. Visual assessments of foliar disease severity in the trial plots were made several times from planting through the end of productive life. The conventional fungicide treatment was reliably better (p<0.05) than the UV and untreated conditions. Conditions applied on reflective mulch were reliably better (p<0.05) than those applied on black mulch.
The second-year field trail compared the efficacy of a UV-only condition, weekly fungicide treatments, UV paired with weekly fungicide treatments, UV paired with every other week (EOW) fungicide treatments, and an untreated control. All treatments were applied to plants on black and reflective mulch except the untreated control, which was on black mulch only. A UV dose of 480 J·m-2 applied twice weekly was used for all treatments. The most to least efficacious treatments were UV + fungicide, fungicide, UV + EOW fungicide, and UV only. Mulch was not a significant variable (p<0.05).
None of the UV-C treatments affected the overall progression rate of downy mildew once the disease became apparent, although disease onset was delayed slightly compared to untreated controls. This delay may have been due to UV-C induced resistance to infection by the host. Unlike powdery mildews, downy mildew spores are darkly pigmented, possibly decreasing the efficacy of the UV-C treatments. The use of reflective mulch appeared to delay disease onset relative to black mulch in fields with significant sunlight exposure, perhaps due to lowering plant stress by maintaining a lower soil temperature. In contrast to the successful control of powdery mildew, none of the UV-C only treatments were as effective for controlling downy mildew as conventional fungicides.
Outreach activities included presentation of the research at conferences, publishing online video and podcast content, a grower trade magazine article, hosting a twilight meeting at the cooperating farm, sharing research with producers and Extension specialists and educators. Producers are interested in new pest management tools; 3/4 producers attending the twilight meeting would consider using UV for pest management on their farms.
The project goal wasto control cucumber downy mildew in the field using nighttime applications of UV-C. The team conducted a laboratory study to determine optimum pre- and post-infection doses of UV-C. A trailer suitable for cucumber fields housing UV-C lamps in Northeast was designed and built to demonstrate the efficacy of pre- and post-infection UV-C applications in the field.. The project team educated Northeast farmers on the practical application of UV-C treatments.
PROBLEM, NOVEL APPROACH AND JUSTIFICATION
Fifteen percent of the US fresh-market cucumber production, across almost 5800 acres, occurs in 3 Northeast States: NJ, NY and MD, with a production value of about 35 million dollars (USDA 2016 Vegetable Summary). Cucumber downy mildew (DM), a foliar disease, is potentially devastating for farmers in the Northeastern US. DM produces asexual spores that are wind-born and as a result, the disease becomes widespread. Infected leaves die prematurely resulting in fewer or lower-quality fruits (McGrath, 2017). The pathogen itself develops resistance to fungicides, and resistant cucumber crops are only effective for current pathogen strains. Presently, the use of chemical fungicides in addition to planting resistant varieties has been the only viable option to achieve effective control (Savory, et al., 2011).
Both visible light and ultraviolet (UV) radiation have been reported to reduce the viability of fungal spores (Rotem et al., 1985; Kanetis et al., 2010). Research on the mitigation of powdery mildew (PM) has shown that application of narrow-band, UV-C (254 nm) at night breaks down its DNA, thereby killing the pathogen (Blaustein and Sengsavanh, 2000). Nighttime applications of UV-C are more lethal to this pathogen than daytime applications because short-wavelength visible light (360-420 nm) inherent in daylight stimulates DNA repair mechanisms in this pathogen (Suthaparan et al. 2014).
Unlike PM which populates the surface of the host plant and is thereby directly available to UV-C applications, DM resides within the host. For asexual reproduction, however, DM conidia must be formed on the plant exterior. DM pathogens sporulate only during night (Yarwood, 1937). We hypothesized that nighttime application of UV-C will reduce spore formation, thereby minimizing the spread of the DM pathogen after infection. UV-C exposures prior to infection will induce resistance in plants to DM (Kunz et al. 2008; Patel et al. 2017; Wargent et al. 2006) and will inhibit sporulation (Brook, 1979). We further hypothesized that nighttime applications of an effective dose of UV-C pre- and/or post-infection will limit crop devastation by DM without affecting the marketability of the crop.
In conjunction with our partners from the University of Florida Gulf Coast Research and Education Center and Cornell University, we developed a UV-C dose-response function for strawberry PM that was implemented in a novel, tractor-pulled, night-operated UV-C lighting system. This cost-effective system reduced PM by 99% in field-grown commercial strawberries compared to non-treated controls. The device reduced PM severity as well as or better than biweekly fungicide treatments (Onofre, 2018). This research sought to leverage our knowledge and capabilities to develop UV-C dose, cost-effective apparatus and practical procedures that would demonstrably mitigate DM in field-grown cucumber.
KNOWLEDGE GAP AND NEW KNOWLEDGE
Previous studies have shown that prophylactic UV-B and UV-C exposures induce DM resistance in lettuce (Wargent et al. 2006) and Arabidopsis (Kunz et al. 2008). Another study demonstrated that UV-A (310-400 nm) at 6.2 Wm-2 or greater inhibits sporulation of grape DM (Brook, 1979). However, none of the previous studies evaluated pre- and post-infection effects of nighttime UV-C light for control of cucumber DM. In addition, the optimum UV-C dose for cucumber DM is not known, so growers cannot confidently apply UV-C treatments in the field.
Growers need to know how to select UV-C lamps, how many to use, the range of tractor speeds to use to provide the prescribed UV-C dose. More specifically, growers need to know the materials and specifications, including safety procedures, necessary to design and build a system that fits their field layout and operations.
INTEREST OF FARMERS AND SERVICE PROVIDERS
In 2018, we were funded by NYFVI to build a UV-C system to treat powdery mildew in summer squash. We have assembled an advisory board of researchers, farmers, extension personnel and lighting manufacturers for this application. The NY farmers and extension personnel on our board have indicated to us that cucumber DM is a significant annual problem for them across NY. They have indicated their pressing interest in having an effective cost-efficient UV-C system for control of cucumber DM in their fields.
Another experienced organic cucumber grower from PA also indicated a considerate interest in the UV-C trailer as they have difficulty controlling cucumber DM every year since 2011. As organic growers, they indicate there aren’t any effective organically certified chemicals to control DM. Several extension service providers from NJ, MA and NY confirmed that cucumber DM is an endemic problem to the region. According to the cucumber DM forecast program (http://cdm.ipmpipe.org/), DM spreads annually to most Northeast States. Northeastern extension programs alert farmers to continuously scout for DM in cucumber annually and typically recommend protecting their crop by applying chemicals from June to October.
RESEARCH PURSUED
Laboratory work was undertaken to determine the dose-response functions for pre- and post-infection UV-C treatment. However, it quickly became evident that the maintenance and propagation of Pseudoperonospora cubensis (the causal pathogen for cucurbit DM) was impractical in our lab facilities and the decision to investigate the dose response and dose application frequency combinations in the field was made.
A laboratory UV-C tolerance study was performed on cucumber seedlings in the lab to determine the maximum weekly dose rate which could be safely applied without negatively impacting yield. This dose was determined to be 1000 J·m-2 weekly, a limit used later when designing the field trials. To accomplish the field application of UV-C, a three-point hitch mounted attachment with an array of UV-C sources was designed by the research team and fabricated onsite at the cooperating farm site. In the first-year field trial, various combinations of UV dose (120, 240, and 480 J·m-2), application frequency (once and twice weekly), and plastic mulch (black and reflective) were evaluated and compared to a non-treated control and conventional fungicide treatment.
In the second-year field trial, the efficacy of various treatments was compared and benchmarked against no treatment: UV only (480 J·m-2, twice weekly), UV with weekly fungicide, UV with every-other-week fungicide, and weekly fungicide. In an effort to maximize its effect, the UV dose selected was the maximum deemed safe in the laboratory UV-C tolerance study. All treatments were replicated on both black and reflective mulch, whereas the non-treated control was limited to black mulch only.
Research results and new knowledge were disseminated through various print formats (trade magazine, peer reviewed journal article currently submitted for consideration). In person dissemination activities included presentation at conferences, a twilight meeting, and personal communication/visits with producers. Additionally, electronic means (podcast and YouTube video content) were also utilized to disseminate project information. Educational materials such as a research fact sheet, UV applicator attachment construction plans and operation information, along with UV safety documents were made publicly available on the internet.
REFERENCES
Blaustein, A. R., and Sengsavanh, N. 2000. Ultraviolet radiation. Pages 723-732 in: Encyclopedia of Biodiversity. S. A. Levin, ed. Academic Press, Massachusetts Brook, P. J. 1979. Effect of light on sporulation of Plasmopara viticola. New Zealand J Bot. 17: 135-138
Brook, P.J. (1979) Effect of light on sporulation of Plasmopara viticola. New Zealand Journal of Botany 17: 135–138.
Govindasamy, R., Arumugam, S., Gao,Q., Hausbeck, M., Wyenandt, C.A., Simon, J.E. Downy Mildew Impacts and Control Measures on Cucurbits in the United States. J. American Society of Farm Managers and Rural Appraisers. 2021: 78-88.
Kanetis, L., Holmes, G. J., and Ojiambo, P. S. 2010. Survival of Pseudoperonospora cubensis sporangia exposed to solar radiation. Plant Pathol. 59:313-323
Kunz, B. A., Dando, P. K., Grice, D. M., Mohr, P. G., Schenk, P. M. and Cahill, D. M. 2008. UV-induced DNA damage promotes resistance to the biotrophic pathogen Hyaloperonospora parasitica in Arabidopsis. Plant Physiol. 148: 1021-1031
McGrath, M. T. 2017. Effectively managing cucurbit downy mildew in NY in 2017. Cornell University Vegetable MD online: http://vegetablemdonline.ppath.cornell.edu/NewsArticles/Cucurbit%20Downy%20Mildew%20MGT-NY-2017-McGrath.pdf
Onofre, R. B. 2018. Design, operation, and efficacy of an apparatus using ultraviolet light to suppress powdery mildew of strawberry in open field production systems. (APS Webinar) https://www.apsnet.org/publications/Webinars/Pages/2018Abstracts.aspx?LID=917 Phytopathology 107:S5.49
Patel, J. S., Radetsky, L., Plummer, T., Bierman, A., Gadoury, D. and Rea, and M. S. 2017. Pre-inoculation treatment of basil plants with ultraviolet-B radiation induces resistance to downy mildew (Abstract) Phytopathology 107: S5.52
Rotem, J., Wooding, B., Aylor, D.E. 1985. The role of solar radiation, especially ultraviolet, in the mortality of fungal spores. Phytopathology 75(5): 510-514.
Savory EA, Granke LL, Quesada-Ocampo LM, Varbanova M, Hausbeck MK, Day B. 2011. The cucurbit downy mildew pathogen Pseudoperonospora cubensis. Molecular Plant Pathology 12(3): 217-226.
Suthaparan, A., Stensvand, A., Solhaug, K. A., Torre, S., Telfer, K. H., Ruud, A. K., Mortensen, L. M., Gadoury, D. M., Seem, R. C., and Gislerød, H. R. 2014. Suppression of cucumber powdery mildew by supplemental UV-B radiation in greenhouses can be augmented or reduced by background radiation quality. Plant Dis. 98:1349-1357
United States Department of Agriculture. “Cucumbers for Fresh Market and Processing Area Planted and Harvested, Yield, Production, Price, and Value – States and United States: 2014-2016.” In Vegetables 2016 summary. Page 46-50. https://www.nass.usda.gov/Publications/Todays_Reports/reports/vegean17.pdf (Last accessed October 17, 2018)
Wargent, J. J., Taylor, A., and Paul, N. D. 2006. UV supplementation for growth regulation and disease control. Acta Hort. 711, 333-338
Yarwood, C. E. 1937. The relation of light to the diurnal cycle of sporulation of certain downy mildews. J. Agricultural Sci. 54: 365-373
Cooperators
- (Educator)
Research
Question 1: For reducing disease severity from cucumber DM, what dosing regime is the best combination of (a) dose (amount x duration), (b) time of application (pre- or post-inoculation), and (c) interval between dosing (1 per week vs. 2 per week) for nighttime applications of UV-C? (laboratory).
Question 2: Does the efficacy of the best dosing regime increase with UV-reflective material? (laboratory).
Question 3: Is the best dosing regime as effective as conventional fungicide spraying? (field)
Question 4: Is the life-cycle benefit/cost of the UV-C system and dosing regime equal to or less than conventional fungicide spraying equipment and methods?
Objective 1 (Years 1, 2): Determine dose-response functions for pre- and post-infection nighttime UV-C treatments and application intervals for control of cucumber DM compared to untreated controls.
An investigation into UV-C tolerance was completed in the laboratory. Cucumber plants with two fully developed leaves were exposed to a single dose of UV-C (0, 5, 10, 25, and 50 kJ·m-2) and leaf area was tracked for 13 days post exposure.
Attempts to maintain and propagate downy mildew (DM) inoculum in the lab proved to be very challenging in year 1. This made exploring various combinations of UV dose, frequency, and mulch type in the lab difficult and impractical. In consultation with advisory committee members, Jim Ward (Ward’s Berry Farm) and Sue Schuefele (UMass Extension) the decision to explore these variables in a field study at Ward’s Berry Farm in lieu of laboratory work.
Objective 2 (Year 2): The optimum UV-C dose and the optimum interval for pre-infection and post-infection treatment (obtained from objective 1) will be tested along with a reflective material that increases the light reflecting upwards towards the underside of the leaves.
A field layout was developed that consisted of twenty-one 70-foot rows. Doses of 120, 240, and 480 J·m-2 were applied both once and twice per week to plants on standard black mulch and reflective mulch. Plots were also included that received no fungicide and conventional fungicide. The layout was created in such a manner to make applications of the various treatments as simple as possible for the farm staff. A document attached contains a field map of the conditions.
Trial Layout - Wards Berry Farm
Project staff worked with Jim Ward to design a tractor mounted UV applicator that was suited for use on the trial plot. The unit was designed to utilize 6 four-lamp UV fixtures mounted in a semi-circular shape and to be mounted directly to a tractor’s 3-point hitch (category 2). The electrical power to operate the lamps was provided by an on-board gasoline generator. Detailed construction drawings and assembly instructions were developed by the team and supplied to Jim and his staff. The construction documents and photos of the completed unit are linked below. The unit was constructed successfully on the farm during the late spring/early summer 2020 from materials supplied by the team.
Wards Berry Farm - Complete Drawings (3_24_20)
Instructions for assembling UV-C unit
Disease ratings were taken weekly by Sue Schuefele from July 27th through September 3rd, when all plants except those in the fungicide treated plots were severely affected by DM. Ratings were made by visually assessing the percentage of leaf area covered in DM lesions (yellowing, necrosis, sporulation) on 10 leaves per plot, then on the percentage of DM within the whole plot. In order to facilitate visual estimations, a walk-through of the whole experimental area was performed first, in order to get a sense for the overall extent of downy mildew severity and any other crop diseases or issues including bacterial wilt or fertility issues. The individual leaf inspections were then conducted using a 10x handlens and inspecting both the upper and lower leaf surfaces, to ensure that the researcher was looking carefully at leaf symptoms and not attributing leaf yellowing to DM without evidence of sporulation. Those ten random leaf samples further helped the researcher calibrate their eye and get a more detailed picture of disease severity. Finally, the researcher stepped back and evaluated the whole plot for percent leaf area affected by downy mildew.
Objective 3 (Year 3): Compare the efficacy of optimum UV-C doses to conventional fungicides for control of cucumber DM in the field.
All images and tables referenced in the following narrative may be found: Tables and Figures (Methods) 2022
The focus of the 2021 field trial was to compare the DM control efficacy of the best UV-C only condition from the 2020 field study, the grower’s conventional fungicide program, and a combination of both treatment types. Further analysis of the 2020 field trial data showed that although the 120 J·m⁻² dose applied once weekly (with reflective mulch) was empirically the most effective treatment, the same treatment was not as effective with black mulch. Collapsing across mulch type, 480 J·m⁻² had slightly (albeit not statistically significantly) higher effectiveness than the other doses, and application frequency of twice weekly was slightly more effective than once weekly. Accordingly, this dose/frequency combination was selected.
The treatments used were: (1) UV-C only (480 J·m-2) twice weekly, (2) weekly conventional fungicide, (3) fungicide weekly plus UV-C twice weekly, and (4) fungicide every other week plus UV-C twice weekly. The third treatment was included to determine if the addition of UV-C to the conventional weekly fungicide program would offer additional control beyond the conventional fungicide program alone. The fourth condition, added at the suggestion of Sue Schuefele (project cooperator), was included to determine if DM control could be maintained by adding UV-C treatments while reducing the number of conventional fungicide applications. The conventional fungicide treatments for DM applied during the 2021 field trial are summarized in Table 1. Each condition was replicated twice on both black and UV reflective mulch, for a total of four replications for each treatment. The last 4.5 m (15 feet) of one row was devoted to a control condition with no fungicide or UV-C treatment. The layout for this field study is shown in Figure 1. The cucumber variety Raider F1 (Harris Seeds) was used again for the second year of the trial.
Each row was divided into ten sections (as shown in Figure 1) and assessments of percentage foliar DM severity were made within each of the 10 sections to increase the sample size within each row. Assessments were performed visually within a square quadrant with 61 cm (24 inch) sides, placed randomly within each of the ten-foot row sections, using the same methodology used in the first year. The top and bottom sides of leaves within the quadrat were inspected to verify that the symptoms were consistent with DM in the same manner as 2020
Assessments were performed by a farm staff member trained to positively identify cucurbit DM by cooperative extension agents. A total of 5 disease assessments were performed between August 3, 2021 and September 9, 2021.
Objective 4 (Year 3): Compare the life-cycle benefit/cost of the UV-C treatments to conventional fungicide treatments.
The per-acre cost of the following treatments was calculated for various acreages up to 25 acres: UV only, weekly fungicide, UV + weekly fungicide, UV + weekly fungicide (DM targeted only), and UV + every-other-week fungicide. While not field tested, UV + weekly fungicide (DM targeted only) was included in the financial analysis because UV-C has been shown in other crop/pathogen systems to effectively control powdery mildew (PM). This coupled with the observation that PM was well controlled in all treated plots during the 2021 field trial, we included this combination which removed the PM targeting materials from the fungicide program.
A total of twenty-five acres was deemed to be the practical limit of a two-row UV treatment attachment of a 2-row UV treatment attachment built to the specifications defined in financial analysis. Labor and material costs were included in the analysis, as was an initial equipment cost comparison and a life estimation for a UV applicator.
All images and tables referenced in the following narrative may be found: Tables and Figures (Results) - Final Report
Objective 1 (Year 1, 2): Determine dose-response functions for pre- and post-infection nighttime UV-C treatments and application intervals for control of cucumber DM compared to untreated controls.
AND
Objective 2 (Year 2): The optimum UV-C dose and the optimum interval for pre-infection and post-infection treatment (obtained from objective 1) will be tested along with a reflective material that increases the light reflecting upwards towards the underside of the leaves.
To determine UV-C tolerance, post-dose leaf area measurements were taken daily for 13 days post exposure. Noticeable phytotoxicity began at 5 kJ·m-2 every two weeks and became severe at higher levels (see Figure 2). The leaf area data were then modeled (Figure 3), which predicted that the point of no net plant growth was 5 kJ·m-2 every two weeks (2.5 kJ·m-2 weekly). Accordingly, to reduce plant damage and limit likelihood of yield loss, weekly doses were limited to 1 kJ·m-2. The later in-field studies confirmed that there was no discernable difference in plant size or productivity as a result of the UV-C application.
As noted in the Materials and Methods section, the maintenance and propagation of the DM pathogen was much more difficult than expected and lead to inconclusive results. Accordingly, the lab approach was abandoned in favor of field work.
The sets of data from the 2020 and 2021 field trials were analyzed in two ways. First, analysis of variance (ANOVA) was performed using the foliar disease severity ratings to determine which independent variables had a reliable effect on disease severity. Second, the area under the disease progress stairs (AUDPS) method (Simko and Piepho, 2012) was used to provide a composite index of the relative impact of each treatment and control condition on disease progression throughout the assessment period in each year.
Figure 4 shows the observed foliar disease severity values for each treatment and control condition, when black mulch was used, and Figure 5 shows the corresponding data for reflective mulch. Each point in Figures 4 and 5 is a single observation for the once-weekly doses or the average of two observations for the twice-weekly doses. The conventional fungicide program in 2020 was only applied with the Black mulch, so that condition only appears in Figure 4.
A four-way ANOVA was performed on the foliar disease severity data comprising a balanced experimental design with the type of mulch, the UV-C dose, the dosing frequency, and the date of assessment as independent factors. The mulch type had a statistically significant effect (F1,45=15.3, p<0.05) on disease severity, as did the date of assessment (F5,45=1184, p<0.05). There was also a statistically significant interaction (F5,45=8.89, p<0.05) between the mulch type and the date of assessment on disease severity. This can be observed by the fact that the disease severity values for the two mulch types were similar for the earliest and latest assessment dates but differed around day 20.
Qualitatively, the curves in Figure 4 also illustrate the large difference found in 2020 between the conventional fungicide treatment conditions and the control and UV-C treatment conditions. Disease severity remained under 20% under the fungicide condition for all observation periods, whereas it approached 90%-100% for all other conditions by the last observation period. Generally, the differences among the control and UV-C treatment conditions were small, although the untreated control condition tended to have greater disease severity values than the UV-C conditions.
AUDPS values (Simko and Piepho, 2012) were calculated for each condition representing each treatment type (or control), the frequency of application (for the UV-C treatment conditions) and type of mulch. These values are shown in Figure 6. Qualitatively, Figure 6 shows the much lower AUDPS value for the conventional fungicide condition than for all other conditions. It can also be seen that the AUDPS values are usually (with one exception for 120 J·m⁻² applied twice weekly) lower for the reflective than for the black mulch.
A one-way ANOVA for each treatment condition in Figure 6 was performed to identify whether there were statistically significant differences among the treatment conditions, and there were (F14,10=16.2, p<0.05). Tukey’s post hoc tests were carried out among each treatment to identify which conditions differed from the others. It was found that the conventional fungicide treatment (with black mulch) was statistically significantly (t=5.07 to 13.2, p<0.05) different from all other conditions. No other conditions differed from one another after adjustment for multiple pairwise comparisons.
Considering only the UV treatment groups, the AUDPS values could be analyzed using a three-way ANOVA with the UV-C dose, the dosing frequency and the type of mulch as independent variables. This ANOVA revealed a statistically significant main effect of mulch type (F1,7=9.80, p<0.05), but no other main effects nor interactions among the variables. Because the AUDPS values collapse across the date of assessment, this analysis result is consistent with the ANOVA on the disease severity values.
Objective 3 (Year 3): Compare the efficacy of optimum UV-C doses to conventional fungicides for control of cucumber DM in the field.
Figures 7 and 8 show the progression of disease for each of the control and/or treatment conditions as a function of time (day after assumed infection as described previously). There are two primary qualitative differences between the data in these figures for 2021 and the corresponding data in Figures 4 and 5 for 2020. First, there appears to be a greater separation among the conditions in terms of the days that the disease begins to take hold in the plants, especially between the untreated control condition (which exhibited greater than 50% foliar disease severity by day 21, and the other conditions which exhibited less than 20% disease severity on the same day. Second, the disease severity for the fungicide treatment conditions approached 80% by the end of data collection whereas in 2020, disease severity was held to less than 20% with the application of fungicide. (Possibly, disease severity in 2020 for the fungicide treatment condition would have eventually increased to nearly 100%.)
For the four treatment conditions (i.e., fungicide, UV, UV plus fungicide, and UV plus EOW fungicide) for which both types of mulch were used, a three-way ANOVA was performed on the disease severity values, with treatment, mulch type and date of assessment as independent factors. The section number of each row was included in the analysis as a covariate factor to identify whether there were any systematic differences within each row; there were not. The treatment (F3,761=118, p<0.05) and the date of assessment (F4,761=1958, p<0.05) had statistically significant main effects on disease severity, and there was also a statistically significant interaction between treatment and assessment date (F12,761=52.5, p<0.05). This can be observed in Figures 7 and 8 where the disease severity was similar across all treatments for the first and last treatment dates, with the most variation among treatments for the intermediate dates. Unlike 2020, the type of mulch did not have a statistically significant (F1,761=0.98, p>0.05) effect on disease progression.
Mean AUDPS values (Simko and Piepho, 2012) for each treatment and mulch condition were calculated and are shown in Figure 9. A one-way ANOVA was performed to assess differences among the conditions, which were statistically significant (F8,149=45.2, p<0.05), with Tukey’s tests to assess pairwise comparisons (summarized in Table 4). In general, there were no significant differences (p>0.05) in AUDPS between mulch types for the same condition. All conditions except for the UV-only conditions differed significantly (p<0.05) from the untreated control condition (which only used black mulch). The combination of fungicide and UV-C treatment with the black mulch was statistically significantly different (p<0.05) from the fungicide-only treatment with the same mulch type, suggesting a small impact of UV-C treatment in conjunction with fungicide.
Excluding the untreated control condition, a two-way ANOVA was performed to assess how the treatment condition and mulch type, and the interaction between them, affected AUDPS. The section from 1 to 10 was included in this analysis as a covariate to identify whether there were any systematic differences across each of the treatment rows; there was not. There was a statistically significant (F3,149=110, p<0.05) main effect of treatment, but the mulch type did not exhibit a statistically significant main effect (p>0.05). There was a significant interaction (F3,149=2.72, p<0.05) between treatment condition and mulch type on AUDPS; this is seen in Figure 9 where the black mulch resulted in somewhat higher AUDPS for the fungicide treatment condition, but lower for the UV-only treatment. Aside from the two-way interaction between the treatment and mulch type, this analysis of the AUDPS values was consistent with the ANOVA on the disease severity values in identifying significant differences among the treatments but not between the two types of mulch in 2021.
Objective 4 (Year 3): Compare the life-cycle benefit/cost of the UV-C treatments to conventional fungicide treatments.
The cost analysis summary is below. Please refer to the attached document Cost Analysis of UV and Conventional Treatments for the complete analysis.
With respect to cost of operation, for 10 acres or less, the least to most expensive treatments on a per acre basis were as follows: UV only, UV + weekly fungicide (DM targeted only), UV + every-other-week fungicide, weekly fungicide, and UV + weekly fungicide. At 25 acres, the cost of weekly fungicide application becomes about the same as UV + every-other-week fungicide due to economies of scale.
As technological improvements are made that allow UV sources to be more compact, integration into spraying equipment will become possible. Concurrent application of UV and fungicide eliminates the additional labor costs of UV application, making it much more economical to apply.
The cost of a two row UV treatment attachment was determined to be $11,205 and the median publicly advertised price of a sprayer was found to be $6,100. With routine maintenance, and when treating 25 acres, the UV treatment attachment would be expected to last 11 growing seasons before the generator would likely need replacement. The rest of the attachment would be expected to a total of 38 growing seasons due to lack of “wear parts”.
References:
Lee DJ, Lee JS, Choi YJ. 2021. Co-occurrence of two phylogenetic clades of Pseudoperonospora cubensis, the causal agent of downy mildew disease, on oriental pickling melon. Mycobiology 49(2): 188-195.
Simko I, Piepho HP. 2012. The area under the disease progress stairs: Calculation, advantage, and application. Phytopathology 102(4): 381-389.
Question 1: For reducing disease severity from cucumber DM, what dosing regime is the best combination of (a) dose (amount x duration), (b) time of application (pre- or post-inoculation), and (c) interval between dosing (1 per week vs. 2 per week) for nighttime applications of UV-C? (field)
Based on our results and on the results of previous studies, a nighttime UV-C dose of (a) 480 J m-2, applied (b) prior to DM becoming established, at an interval of (c) twice weekly appears to be the best combination of UV-C treatment factors to reduce severity of downy mildew in cucumber plants. It should be noted, however, that differences among the doses used in this study (ranging from 120 to 480 J m-2) and among the treatment intervals (once or twice weekly) were relatively small and none of the UV-C-only dose/frequency combinations explored provided effective control of downy mildew.
The similar progression of all the disease severity ratings regardless of dose or interval suggests that the timing of the application is most critical in order to expose spores to UV-C before they are embedded into the leaves. For this reason, an interval of twice weekly may be more effective because it doubles the chances that spores will be intercepted and irradiated before they can land and infect the host leaves.
Question 2: Does the efficacy of the best dosing regime increase with UV-reflective material? (field)
In our 2020 field trial there was a beneficial effect of the UV-reflective mulch material, but this was not observed in the 2021 trial.
We speculate that the impact of reflective mulch was greater in 2020 because the field for the test crops had more sunlight exposure potentially resulting in a larger increase in soil temperature with black mulch relative to the reflective mulch. Higher soil temperatures could have induced stress in the host plants, thus reducing resistance to downy mildew infection. In 2021, the field used for the tests was in a field receiving fewer hours of direct sunlight due to surrounding trees, possibly reducing differences in soil temperature between the dark and reflective mulch types. Based on the findings of the field trials, the use of reflective mulch does not make a consistent improvement to the efficacy of UV-C treatment of downy mildew.
Question 3: Is the best dosing regime as effective as conventional fungicide spraying? (field)
Our results from both 2020 and 2021 field trials indicated that the UV-C only dosing regimes tested in this study were not as effective as conventional fungicide treatment. Disease onset occurred sooner for the UV-C only treatments than for fungicide, however the rate of disease progression once established was very similar for all treatments.
We did find evidence that the combination of UV-C and fungicide slightly outperformed fungicide alone in 2021, which suggests that fewer fungicide applications could be used in combination with UV-C treatment to achieve the same control of downy mildew as conventional fungicide treatment alone.
One possible reason that UV-C treatment, with the doses and frequencies applied in this study, has limited efficacy is related to the manner in which the pathogen propagates. When infected leaves produce spores, they are dispersed by air currents and rain. When viable sporangia land on a host leaf, they germinate in moisture on the leaf’s surface producing biflagellate zoospores that encyst in stoma where a germ tube is formed that penetrates the leaf’s surface through the stoma. Hyphae form in the mesophyll layer and produce clavate-branched haustoria in the host’s cells. Once this process occurs, the pathogen is very well protected from UV-C and other environmental factors.
Additionally, the presence of melanin pigment in cucurbit downy mildew spores (Lee et al., 2021) provides them with increased resistance to UV-C. The class of fungicides known as melanin biosynthesis inhibitors (MBIs), sometimes used for control of rice blast, may be helpful in reducing the melanin protection of the spores and could possibly be an effective treatment in conjunction with UV-C.
Question 4: Is the life-cycle benefit/cost of the UV-C system and dosing regime equal to or less than conventional fungicide spraying equipment and methods?
UV alone, at the dosages and frequencies of application investigated, was not an effective treatment to control cucumber DM. UV + weekly fungicide performed as well, or better than weekly fungicide alone but was substantially more costly. There may be some circumstances when the additional control offered by this combination approach would warrant the extra cost; however, under most circumstances this combination treatment would not be cost effective.
While not included in the field trials, the combination approach of UV treatment plus weekly application of downy mildew targeted fungicides is more cost effective than the full weekly fungicide program. UV has been shown to effectively control powdery mildew in other crops, consistent with observations from the 2020 and 2021 field trials showing effective control of PM in the UV treated cucumber plots. By removing the fungicide materials intended to control PM, the fungicide costs can be lowered substantially while still maintaining control of PM through the UV applications. This approach has the potential to save approximately $35 per acre, while also reducing the amount of fungicide materials used.
Education & Outreach Activities and Participation Summary
Educational activities:
Participation Summary:
During summer of 2020, a website with detailed information on construction and operation of UV treatment attachments was launched as part of another research project. These resources, however, were distributed to individuals interested in this research project as well.
In late 2020, research team members authored an article featuring the research involving use of UV to combat plant disease. The article mentions this research project and contains a quote from Jim Ward, the cooperating farmer, and was published in January 2021 in Country Folks Grower magazine. A copy of this article is attached here: Country Folks Grower UV Article - 2021
In March 2021, Nick Skinner was a panel member in a one-hour podcast “Shining UV Light on Plant Pathogen Management.” Other panel members included a grower and two extension agents. Many topics including the history of germicidal UV use, modes of action, pathogens and crops that have been researched, UV safety, and ongoing research, including this project, were covered. Support of the NE SARE was acknowledged. The podcast's audience is individuals involved in commercial horticulture and has reached 130 listeners at the time of this report, and can be found at the following site: https://enych.cce.cornell.edu/podcasts.php
A field-day/twilight meeting highlighting the research project and providing attendees with information about the use of UV as a plant pathogen control technique was held on August 18, 2021, at Ward’s Berry Farm in Sharon, MA. A total of 14 stakeholders (not including the organizers) attended. A summary of the field day event may be found in the attached files: Field Day Announcement, Field Day Summary
The research results from this project and another project focused on UV powdery mildew control were be presented at the 2022 Eastern NY Fruit & Vegetable Conference organized by Cornell Cooperative Extension on February 16, 2022. The section of the presentation given pertaining to this research project may be seen here: 2022 ENY Comm Hort Conference Presentation
A presentation disseminating the present research and its findings was recorded and posted to YouTube. The presentation included UV treatment background, as summary of the field work, the results from the field studies, combination approaches, safety topics, and technological advancements and their implications. This video can be found here: https://www.youtube.com/watch?v=6g_S2T49VfU
An abstract featuring this research project was submitted and selected to be included in the American Phytopathological Society (APS) annual meeting in August, 2022. The project will be included as an “ePoster”, which includes both a poster and short video.
The team prepared a manuscript entitled “Effectiveness of the Field Application of UV-C for Cucumber Downy Mildew Control” and submitted it for consideration to the Journal of Horticultural Sciences. At the time of this report’s submittal, review is still pending.
Learning Outcomes
Outreach activities included project outcome dissemination as well as discussions with various producers, educators, extension personnel, and lighting industry professionals about the use of UV on farms. The key areas focused on during the outreach activities were: history of germicidal UV use, modes of action, pathogens and crops that have been researched, UV safety, ongoing research, how to construct a UV applicator attachment, and the print resources available to producers and extension staff/educators.
Project Outcomes
Based on the research conducted to date, a commercial producer from eastern Massachusetts said he thought "that a combination of varietal resistance and the UV-C could really work."
In reference to the UV control of downy and powdery mildews, a commercial producer from Massachusetts attending the field day said, "Wonderful and very informative. Love this potential for commercial organic growing."
The laboratory studies resulted in an assessment of UV-C tolerance by the host, but it was very difficult to maintain and propagate the cucurbit DM pathogen. The UV-C dose/pathogen response investigation work was much more successful in the field.
Based on the laboratory UV-C tolerance work, a maximum weekly dose under 1000 J·m-2 was used (480 J·m-2 applied twice weekly for a maximum weekly dose of 960 J·m-2). There was no evidence of a detrimental effect on the plants or yield at this level. This suggests that the “hardening off” process that occurs when seedlings are transplanted into the field may stimulate an additional tolerance to UV-C doses, which may be beneficial in combatting the difficult to manage DM pathogen.
The COVID-19 pandemic presented some challenges. Institutional limitations on travel and meetings were one such challenge. Another was the cancellation or shift to virtual conferences, which made some of our anticipated dissemination activities difficult. In response, an effort was made to pursue other digital means of dissemination (i.e., YouTube and podcast presentations).
There are several topics related to the present research which could be further investigated. One such topic is the investigation of higher UV-C doses and their effect on pathogen control and plant vigor and yield. Another opportunity is to investigate the use adjunct treatments, such as titanium dioxide or melanin biosynthesis inhibiting fungicides, to make the pathogen more susceptible to UV-C damage. Technological and engineering improvements to the UV-C application device are an additional opportunity for investigation. The recent and rapid development of solid-state UV-C sources will certainly make existing low-pressure discharge tubular lamps obsolete. Additionally, their relatively compact size promises to make more compact and powerful applicator attachments possible. These newer designs can make integration into other equipment (i.e., sprayers) possible, as well as making faster and more economical to operate equipment.
Information Products
- Research Spotlight: UV-C Treatment of Cucumber Downy Mildew (Fact Sheet)
- UV-C Tractor Attachment Construction Resources (Manual/Guide)