Management of Powdery Mildew in Greenhouse and High Tunnel Tomatoes by Using Ultraviolet C Light

Final report for ONE20-377

Project Type: Partnership
Funds awarded in 2020: $29,171.00
Projected End Date: 11/30/2023
Grant Recipient: West Virginia University
Region: Northeast
State: West Virginia
Project Leader:
Dr. Mahfuz Rahman
West Virginia University
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Project Information


Production of vegetables such as tomatoes, cucumbers and peppers in protected environments, like greenhouses and high tunnels, has gained popularity in West Virginia due to proven advantages in extending the growing season and potential of intensive use of scarce flat land. However, due to less wind movement and limited escape of plant-transpired water, these structures are prone to high humidity. This leads to the development of destructive diseases, such as powdery mildew and leaf mold, which are less common in open field production system. Due to the limited fungicidal options for managing diseases in protected culture, plant pathologists have developed alternative management options consisted of non-chemical techniques such as lowering humidity and use of ultraviolet radiation that can make disease management sustainable and reduce cost. This project primarily focused on controlling powdery mildew by exposing tomato plants to the required amount of UV-C radiation. Project personnel built a UV-C cart with 5 UV-C lights (254 nm)  at each side of the cart. Powdery mildew-infected plants were exposed to UV-C light at various times. We found that 20 S exposure time controlled powdery mildew without any phytotoxicity or leaf burn of plants at pre-bloom stage. Efficacy of UV-C exposure was also determined in greenhouse with 9 rows of tomatoes, where three rows received optimized dose of UV-C once, another three rows twice and another three rows were left unexposed. We obtained adequate powdery mildew control (75% clean leaf area compared to non-treated).  We demonstrated the result to 15 tomato growers and reached out to 100 clientele to describe the potential of the technology. Results, including the method, were presented at the master gardeners meeting, annual meeting of county agents, pesticide safety education meeting, and WVU Extension Small Farms Meeting. 

Project Objectives:

This project sought to develop a sustainable powdery mildew management method on greenhouse and high tunnel tomato production system by using UV-C light. These UV units could be used repeatedly as and when needed to make it most cost-effective, free from environmental pollution and health hazards to workers. This system would make disease management sustainable as the probability of resistance development would be very low compared with fungicidal control of powdery mildew. Our objectives were to introduce a new system to the growers and provide them with tips on where to get technical cooperation in case they wanted to get their own UV-C unit assembled and run. Specific objectives were:

  1. Assess the usefulness of UV-C light in managing powdery mildew disease in greenhouse and high tunnel;


  1. Optimize the doses, exposure time to attain highest efficacy without any phytotoxicity/avoid damage to plants from light;


  1. Disseminate the technology to organic growers and small farmers through individual communication, grower meetings and conferences, annual field day events, newsletters and cooperative county extension programming.




Powdery mildew is one of the most common diseases of vegetables grown in greenhouses and high tunnels all over the world. Production of vegetables such as tomatoes, cucumbers and peppers in protected environments has gained popularity in West Virginia due to proven advantages in extending the growing season and opportunity of intensive use of scarce flat land in the mountain valley. However, due to less wind movement and limited escape of plant-transpired water, these structures are prone to high humidity. This had led to the development of destructive diseases, such as leaf mold and powdery mildew. In commercially grown tomatoes in the USA, yield losses due to powdery mildew may exceed 50%, depending on the age of the crops when the disease occurs, environmental conditions and effectiveness of fungicides (Grahame and Eric, 2016). Chemical control is often discouraged in protected environments due to the risk of health hazard. Due to the limited fungicidal options for managing diseases in protected environment, plant pathologists have explored alternative management options consisted of multiple non-chemical techniques such as lowering humidity and use of ultraviolet radiation lamps that can make management sustainable. These options can also reduce cost and health hazards, while attaining sustainable disease management. UV is a short wavelength part of electromagnetic radiation, which can be divided further into UV-A, UV-B, UV-C and vacuum UV radiation, with the potential to interrupt the life cycle of fungal pathogens. However, results from multiple studies indicate that UV tolerance of different crops widely vary to avoid phytotoxicity in terms of type of UV and dose (exposure time). For example, UV-B treatments applied on strawberry once or twice weekly during the night for 10 minutes were as effective as the best available fungicides applied on similar schedules for controlling powdery mildew. However, same UV-B dose although controlled powdery mildew on tomato but caused significant phytotoxicity on tomato plants. Thus, UV-C was successfully tested for tomato powdery mildew control. Survey conducted at the high tunnel vegetable production session during WVU small farm conference in 2018 and 2019 revealed that small insects such as spider mites, broad mites, white flies, aphids and a few diseases (leaf mold, powdery mildew) were the major pest issues on high tunnel tomatoes that would need sustainable management options.


Thus, under this project we will utilize a combination of reducing humidity inside the high tunnel and exposing tomato plants to required amount of UV-C to control powdery mildew. Currently there is no data on the efficacy of UV-C on managing leaf mold is available. Although our primary target is powdery mildew, we will collect both PM and leaf mold data in grower cooperators’ greenhouse in the first year and high tunnel in the second year. We have documented severe powdery mildew in grower cooperator’s greenhouse (Appendix 1) and obtained only limited success in controlling it by lowering relative humidity. So, our interest and goal through this project is to demonstrate the efficacy of UV-C exposure to the greenhouse and high tunnel growers.


Click linked name(s) to expand/collapse or show everyone's info
  • Dr. Lewis Jett (Educator)
  • Paul Mock - Producer
  • Joyce Shafer - Producer


Materials and methods:

Materials and Methods

As shown in the 2022 report above, we assembled a UV-C cart by procuring all the parts. Tomato plants were grown up to pre-bloom stage in the greenhouse and were exposed to UV-C light (254 nm) for different time lengths viz. 10, 15, and 20 seconds, followed by 4 hours of dark for two consecutive nights in three groups to assess phytotoxicity.  Programming of exposure time was done with a TG-16 mechanical programmable timer plugged into the light source.  Lights were positioned 30 cm from plants at a 30° angle to center light penetration into the upper and lower canopies of the plants. Powdery mildew-infected tomato plants were maintained in a humid chamber for the whole period of the project and were used for UV-C exposure as needed. Infected plants were exposed to 20 S time as were found safe in the previous experiment.

The first greenhouse trial was set at Mock Farms near Berkeley Springs, WV. Mr. Paul Mock had greenhouses that were specifically built for heirloom tomato production. The tomato variety included in the study was an heirloom with very little powdery mildew resistance. These rows were 150’ long, of which 50' was used for the trial. We selected 3 rows subjected to UV-C radiation at night once, 3 rows exposed twice, and 3 rows left unexposed that worked as non-treated checks. Planting distance, trellising, and other cultural operations were done according to the grower cooperator’s commercial production system. No chemical or biological products were applied for disease control. Project personnel operated the cart for the trial as we could not assemble the autonomous ground vehicle. Irradiance was measured with a calibrated spectrometer (StellarNet Inc. EPP2000, Tampa, FL) to ensure light intensity didn’t exceed the level used for the lab experiment. Treated plants were evaluated within 72 hours of treatment for potential phytotoxicity. Powdery mildew data were recorded 5 days after light exposure from both adaxial and abaxial sides of the middle 5 leaves of 10 randomly selected plants of each treatment based on the %leaf area covered with white fungal growth and averaged. Disease severity(%) was determined from all three rows (5 replicate plant/row) 5 times starting from 10 days after UV exposure followed by every 10 days interval. The area under the disease progress curve (AUDPC) was calculated as described by Campbell and Madden (1990) using the following formula:

AUDPC = Σ    [(xi + xi-1)/2][ti - ti-1],

where n was the total number of evaluation times, xi is the proportion of the tomato leaves covered with fungal growth or disease severity (%) at ith observation; t, and (ti – ti–1) is the duration between each assessment. The linear mixed model, MIXED procedure of SAS software (version 9.3, SAS Institute Inc., Cary, NC) was used to perform the analysis of variance for AUDPC data on powdery mildew severity. Treatment was considered a fixed effect, whereas replicates and exposure time were considered a random effect. Treatment means were compared using Tukey's HSD test at α=0.05. This trial was repeated twice. 

Research results and discussion:

Final report We built a UV-C cart suitable for initial lab and greenhouse exposure time optimization (Fig. 1) trials and generated data on the efficacy of managing powdery mildew during 2021. Optimization of exposure time indicated that any exposure time above 20 sec can substantially damage plants, especially smaller ones. However, exposure time below 20 sec was not effective in killing powdery mildew spores. Although 20 sec exposure time caused some phytotoxicity (Fig. 2), plants could recover within 5 days. Our results indicated that 20-sec exposure to UV-C effectively killed powdery mildew spores as the disease did not come back even after incubating for 72 h post-exposure in a humid chamber. 

In the grower collaborator’s greenhouse in 2022, further optimization of exposure time produced consistent results, with 20 S being the best for pre-bloom or smaller plants to suppress powdery mildew with the lowest phytotoxicity. UV-C exposure of tomato plants affected with powdery mildew could significantly reduce AUDPC value. However, results from the 2022 trial indicated that the double exposure could provide higher suppression of disease (lower AUDPC value) than single exposure (Fig. 3). More work is needed to determine the exposure time to obtain maximum disease suppression without any phytotoxicity.

Pitfall and future work plan: Our research technician moved UV-C cart in the greenhouse (Fig. 4) with appropriate personal protective equipment (shield). However, minor exposure with this PPE can’t be ruled out completely, and there is a health risk from prolonged exposure to UV-C. We are in the process of assembling an autonomous robot with the light bulbs fixed in both arms capable of running in between rows. It will also have the capability to self-charge when it runs out of battery charges by docking to a charging station. Thus, it should gain the capability of treating a large commercial greenhouse.

Research conclusions:

Results from this project indicated that UV-C light could be used to control the powdery mildew of tomatoes in the greenhouse. The trials conducted in the greenhouse with the UV-C cart were only suitable for a small area. Although we initially planned to assemble an autonomous robot to perform the job, it was not completed due to the lack of logistics. However, future projects should get enough instructions from the methods and results mentioned above to assemble and run those in the greenhouse and high tunnels. Grower cooperator for the greenhouse trial had a good impression about the technology to provide sustainable management of powdery mildew if improved version of the tool could be made available. We could reach out to more than 100 growers to show the possibility of using this UV-C based management tool in the future. Current UV-C technology based tools are available from companies for large greenhouses only. There is a huge potential to get custom-built smaller scale autonomous robot to be used in small greenhouses and high tunnels.  

Participation Summary
1 Farmers participating in research

Education & Outreach Activities and Participation Summary

10 Consultations
1 On-farm demonstrations
4 Webinars / talks / presentations
1 Workshop field days
1 Other educational activities: WVU extension Ag and Natural Resources research highlights

Participation Summary:

15 Farmers participated
2 Number of agricultural educator or service providers reached through education and outreach activities
Education/outreach description:

We conducted outreach through field day that has accelerated adoption of technology. We sent the results to WV greenhouse and high tunnel growers association through our listserve  <[email protected]>. We observed that UV-C was welcomed by the growers as sustainable means of powdery mildew control and crop yield and quality improvement. However, they wanted to have a more user-friendly delivery option such as unmanned ground vehicle. Results were presented at WVU-certified organic farm field day, American Phytopathological Society (APS)-Potomac Division, and WVU EXtension Small Farms Meeting.  

Some objectives related to high tunnels were not achieved due to the inability of the UV-C movement on uneven surfaces. That also affected some outreach plans for high tunnel growers. 

Learning Outcomes

2 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key areas in which farmers reported changes in knowledge, attitude, skills and/or awareness:
  1. Germicidal activity of UV-C
  2. Importance of maintaining exposure time to avoid phytotoxicity to tomato plants
  3. Justification behind exposing plants to UV-C at night rather than daytime
  4. Justification of reducing relative humidity and use of UV-C simultaneously to achieve better disease control
  5. Probability of using autonomous tools in agriculture

Project Outcomes

2 Farmers changed or adopted a practice
1 Grant applied for that built upon this project
2 New working collaborations
Project outcomes:

UV-CReport-2022 Low cost UV-C unit was assembled with two rows of UV-C bulbs (10 total) at both sides of the cart (Fig. 1). Due to COVID related restrictions, optimization of  UV-C exposure time that  can effectively suppress powdery mildew (PM) was done in the lab condition. Powdery mildew affected tomato plants were collected from a greenhouse located at Taylor County, WV. These plants were kept in a humid chamber built on the lab bench  where new pot grown tomato plants were brought on a regular basis for PM infection. These plants were used for exposure time optimization(Fig. 2) . Limited greenhouse trials were also conducted for evaluating the efficacy of UV-C on larger plants. Results indicated that UV-C exposure time for suppressing PM varied with plant age. Similarly, UV-C tolerance without any phytotoxic effect also varied with plant age. Cooperating producers were invited to witness the effect and were provided training on the operation of the unit. As the optimization of exposure time for suppression of PM and avoiding phytotoxicity for tomatoes of different growth stages has been completed, cooperators are expected to use the unit in the greenhouse and high tunnels in 2022 and 2023, respectively. As the tool was difficult to move on uneven surfaces and may pose health risks if moved manually, we are assembling an unmanned ground vehicle that will remove these limitations. 

Assessment of Project Approach and Areas of Further Study:

Powdery mildew is caused by the obligate fungal parasite. This fungal pathogen can't be grown or saved on synthetic media. Naturally infected plants need to be maintained in humid chamber and new plants are needed to be brought in to maintain the inoculum year round for conducting experiment. The utilization of UV-C for tomato disease control is a relatively new technology. Utilization of this tool requires growers understanding and demonstration of results to them. This project is designed to make progress on those gradually. We achieved our initial objectives by assembling a cost-effective UV-C cart. We also optimized the exposure time to kill powdery mildew spores without causing any major phytotoxicity. Next step is to let grower cooperators use it in the greenhouse/high tunnel to determine the efficacy of this tool in managing powdery mildew on tomatoes.

As the tool was difficult to move on uneven surfaces and may pose health risks if moved manually, we are assembling an unmanned ground vehicle that will remove these limitations.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.