to reduce powdery mildew, downy mildew and potentially other pestsMobile UV Arrays (“Dragons”) UV light is an emerging agricultural tool that can work with, or as an alternative to, conventional pesticides to manage disease. UV light can reduce disease severity through a germicidal effect on leaf pathogens and in some cases stimulates disease resistance. Currently the Dragon is an implement designed for a tractor’s 3-point hitch.

Dragon Fast Facts UV light modes of action

• UV-C germicidal effect, used for more than 100 yearsto sterilize surfaces and water

• May stimulate disease resistance for certain crop andpathogen combinations

Benefits • Applicable in rain, wind, wet or dry, zero re-entry time• No mixing, water or chemistry, just electricity

Dose and speed • Each crop requires testing to find the optimal dose.

A dose of 200 J m-2 once per week is effective forstrawberry powdery mildew and is a decent startingpoint to optimize management. Length of applicatorand dose determine the tractor speed

Pests • Powdery mildew, downy mildew, expected to be

effective against other fungi, bacterial and viraldiseases on surfaces, possibly certain insects

Do-it-Yourself (DIY) plans • For farmers to build on the farm (see download)

Cost • About $5K–6K plus 40–60 hours build time for a single

row unit (with 2 kW generator) that can be used atspeeds of 1–3 mph depending on dose requirements

Safety • Follow instructions for operation and know crop limits

Background Many studies have examined the use of light to reduce foliar disease in crops, including UV-B and UV-C and even visible light.1–6 UV-C light has been used for more than 100 years to sterilize surfaces and water. The doses, measured in joules per square meter (J m-2) to kill many types of bacteria, viruses and spores are well known. For example, the National Sanitation Foundation standard for

Class A sterilization of drinking water requires a UV-C dose of at least 400 J m-2 that typically reduces pathogen concentrations by well over 1000-fold when designed properly.7 More recently, UV-C light has been shown to have a high efficacy in reducing crop diseases.

Mechanics and Dosing The Dragon delivery system is mounted to a tractor and pulled at a precise speed through the field to deliver the required dose (Figure 1). The optimal dose and schedule will vary for different crop and pathogen systems. When designing for a new crop-pathogen system, the optimal procedure is determined through a combination of laboratory and field studies to ensure that disease severity is reduced while minimizing damage to the crop.

Experience suggests a once per week schedule of 200 J m-2 is a good starting point to find an optimal UV-C pest management strategy, whether the effect is direct, or prophylactic. To minimize damage to the crop, our experience suggests 1000 J m-2 per week of UV-C for cucurbits should not be exceeded, though this value may be lower or higher depending on the crop system.

Modes of ActionResearch indicates that UV-C may work through two modes of action. In some cases, such as powdery mildew, the disease can be managed post emergence through a direct germicidal effect. With downy mildew the UV-C is most effective if delivered prior to infection as a prophylactic which may stimulate the plant’s defense system. Preliminary lab data suggests that disease resistance can persist for as long as 6 days after UV exposure.

Research has also shown that for powdery mildew management, application after sundown (with at least a 4-hour dark period before dawn) is more effective than daytime application because the blue-violet portion of sunlight stimulates the pathogen’s DNA repair mechanisms.

Results from Field TrialsField trials for strawberry powdery mildew have shown great promise (Figure 2 shows an example data set from Florida). Doses in the range of 85 – 240 J m-2 applied once to twice per week achieve reductions in strawberry powdery mildew equal to or better than the best commercially available chemical fungicides.

Efforts have been extended to squash powdery mildew and cucumber downy mildew. In the first field trial of squash powdery mildew (Figure 3), the data were not as favorable as for strawberries. This may be due to the canopy structure of squash, as the leaves begin to self-shade. Our field sensors verified that as time went on, lower

Figure 2. Example data from a field trial of UV-C treatment of strawberry powdery mildew (black) compared to fungicide (Quintec/Torino; orange) and untreated (blue).

Figure 1. Tractor mounted “Dragon” UV array used on summer squash at Kinderhook Creek Farm in Stephentown, New York, in July 2019. More than 90% of the lamp optical output is UV-C light, but some visible light is emitted that can be seen as a blue glow.

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Figure 3. Comparison of disease severity (solid lines) and yield (dashed lines) between UV-C treated (gray), fungicide (orange) and untreated (blue) in summer squash, 2019.

and lower doses were recorded at the same plant height. An early starting point and higher dose at the outset may also increase efficacy. In this trial, even aggressive fungicide treatments had a relatively high level of disease severity and yield loss later in the season.

Ongoing and Future WorkMuch like other forms of pest management, a combination of better understanding modes of action, timing and practical in-field optimization continues. This technology is currently being evaluated against cucumber downy mildew, and trials to optimize the strawberry powdery mildew schedule continue. In 2020, studies on beet Cercospora leaf spot disease will begin. We are also planning to further optimize treatment for powdery mildew and angular leaf spot disease on cucurbits, including both improving the schedule and adding leaf agitation to get more uniform exposure.

Input NeededFarmer feedback is critical to mature this technology, because farms may have different requirements for costs, speed and application schedule. For example, if going faster is important, it may justify building a longer Dragon device that can deliver the required dose in less time. Also, multiple Dragon modules could be ganged together to treat several rows with each pass.

Building and Operation of Dragons on the FarmThe photos above show the Dragon design, along with a Building and Safety Session at Kinderhook Creek Farm in Stephentown, New York. (1) A model designed for a single row tractor pulled system, (2) a view inside the Dragon with UV-C lights energized that also emit a faint bluish visible light, (3) a safety session taught by

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Lighting Research CenterRensselaer Polytechnic Institute21 Union St.Troy, NY 12180 USA Phone: 518-276-7100Website: www.lrc.rpi.edu

1. Suthaparan, A. et al. Suppression of Cucumber Powdery Mildew by Supplemental UV-B Radiation in Greenhouses Can be Augmented or Reduced by Background Radiation Quality. Plant Disease 98, 1349–1357 (2014).

2. Suthaparan, A. et al. Suppression of Powdery Mildews by UV-B: Application Frequency and Timing, Dose, Reflectance, and Automation. Plant Disease 100, 1643–1650 (2016).

3. Wilcox, W. F. et al. Effects of Sunlight Exposure on the Development and Management of Powdery Mildew. 10.

4. Patel, J. S., Radetsky, L. C. & Rea, M. S. The value of red light at night for increasing basil yield. Can. J. Plant Sci. 98, 1321–1330 (2018).

5. Illumination for Plant Health (IPH) Program at the Lighting Research Center. https://www.lrc.rpi.edu/programs/plants/plants_home.html.

6. Patel, J., Radetsky, L., Nagare, R. & Rea, M. Nighttime Application of UV-C to Control Cucumber Powdery Mildew. Plant Health Progress, In Press (2020).

7. NSF Residential Drinking Water Treatment Standards - NSF International. http://www.nsf.org/services/by-industry/water-wastewater/residential-water-treatment/residential-drinking-water-treatment-standards.

Andrew Bierman from RPI’s Lighting Research Center provided to farmers to enable safe operation of the system, (4-6) photos from the building instructions, (7) Nick Skinner, who leads mechanical design. Once the building materials arrive on the farm, they can be assembled in about a week by a skilled technician.

References

AcknowledgementsThe work on summer squash powdery mildew and prophylactic treatment of cucumber downy mildew, as well as development of these outreach materials is supported by a New York Farm Viability Institute (NYFVI) grant to the Lighting Research Center at Rensselaer Polytechnic Institute. The work on strawberry powdery mildew is supported by USDA, nighttime treatment before and after cucumber downy mildew infection is supported by NE SARE. Special thanks to Larry Eckhardt and Andrew Eckhardt of Kinderhook Creek Farm for building and demonstrating the Dragon at their farm as well as providing in-kind contributions and advice for the field trial. This document was designed by Rebekah Mullaney.

Contact and More InformationFor general questions, please contact Mark Rea at ream@rpi.edu or 518-276-7100. For technical questions on building materials, contact Nick Skinner at skinnn@rpi.edu.

Download building instructions and more information at https://www.lrc.rpi.edu/programs/plants/plants_home.html or scan the code below with your smartphone.

Contributing Research TeamsLighting Research Center, Rensselaer (RPI)

Mark Rea, Leora Radetsky, Jaimin Patel, Andrew Bierman, Nick Skinner, Tim LaPlumm, Martin Overington, Howard Ohlhous, Ziggy Majumdar

Cornell Agritech

David Gadoury, Sarah Pethybridge

Gulf Coast Research and Education Center, University of Florida

Natalia Peres, Rodrigo Onofre, Paulo Pinto de Mello Neto

Driscoll’s Inc. Global Plant Health

Jenny Broome, Kelly Ivors