Webinar #11 - Building Pathogen Mitigation Features into Water Recycling Systems (August 2014)

By: Chuan Hong

Webinar Recording

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►Hello, everyone, welcome to the webinar! I'm Chuan Hong, a professor of plant pathology at Virginia Tech. I will be leading today’s webinar: Building pathogen mitigation features into water recycling systems.

►This is the eleventh webinar in a series, organized by the multi-institutional research team on the Specialty Crop Research Initiative Project: Integrated Management of Zoosporic Pathogens and Irrigation Water Quality for a Sustainable Green Industry. This project is sponsored by the USDA National Institute of Food and Agriculture. We greatly appreciate the Society of American Florists and the AmericanHort, the advisory panel and many farmers for their continuing support. They have played key roles in every stage of this project from proposal development to field studies and educational programs.

►As discussed in the previous webinars, agricultural runoff capture and reuse is crucial to sustainability of the horticultural industry. However, runoff containment ponds potentially are plant pathogen accumulators and redistribution centers. At least 57 species of Phytophthora, 29 species of Pythium, and many other pathogens have been found in water. These pathogens could wipe out entire crops within weeks if irrigation water is not decontaminated before reaching crops.

The questions are where and when to act and how?

► A recycling irrigation system typically consists of three major components: production beds, runoff water containment pond, and pump house.

These components also are three critical control points for pathogens in water.

Poll Question 11.1:  What are two most cost effective water treatments for killing pathogens in irrigation water? (Please select TWO)
  • Biological control agents
  • Chlorine
  • Chlorine dioxide
  • Heat treatment
  • Slow sand filtration
  • Ultraviolet irradiance

►Here is another way to look at our lines of defense against waterborne pathogens in association with the stage of horticultural business development.

11.2 Phytophthoras are commonly regarded as “plant destroyers” and “water molds”. Which of the following statements is true? (Pease select ONE)
  • All Phytophthora species are plant destroyers.
  • All Phytophthora species are adapted to water environments.
  • All Phytophthora species are adapted to water environments and plant destroyers.
  • Most plant pathogenic species of Phytophthora are adapted to water environments.
  • Most plant pathogenic species of Phytophthora are not adapted to water environments.

► Overall, there are three types of water recycling systems:

Following I will show you the 15 years of research data by a national consortium in collaboration with many forward-thinking growers.
Some of these results and concepts have already been put into practices by collaborating growers to modify their water recycling systems for pathogen mitigation. These include:

These modifications are one-time investment but they will have ever-lasting pathogen mitigation benefits.
We hope you can use the same data to examine your existing water recycling systems and determine whether a modification(s) is worthwhile investment for long-term pathogen mitigation purposes.

We certainly encourage you to use these data to design and set up water recycling systems for new production facilities, so you can capture and reuse runoff water without recycling plant pathogens. In this case, there may NOT be any extra cost at all.

►Let us look at two examples of one-pond water recycling system. You will clearly see the difference and reach your own conclusion, which layout has less pathogen risk.

►This is a Google map showing one section of a 400-acre nursery in eastern Virginia. Production area on the left is about 15 acres. Runoff from this area is channeled into a containment pond via a single entrance as indicated by a blue arrow. A pump house is located against the runoff entrance. Water path from the runoff entrance to the pump house is about 140 meters.

►Among the first questions we asked were: whether pathogen populations change along the water path? If so, do they increase or decrease?

►To answer these questions, Dr. Ghimire baited the pathogens along the water path at 20 meter intervals beginning at the runoff entrance.  

►After 7 days, we retrieved the baits, cut leaves into pieces and plated them on selective media to let the pathogens of interest grow. We then used a DNA fingerprinting technique to determine what pathogens were recovered and how abundant each is at each baiting station.

►This figure show average pathogen recovery data. X-axis is the distance from the entrance and Y-axis is % of leaf piece from which Phytophthora pathogen was recovered.

As you can see, pathogen recovery decreased with increasing distance from the entrance. The pathogen recovery at the entrance was 12 times of that at the 100-meter baiting station. This set of data tells us that pathogen mitigation can be achieved simply locating pump house away from the runoff entrance.  In this case, the control efficacy is 92%!

►So, the next questions we asked was What caused the pathogen decline along water path?

11.3 Which of followings may cause pathogen decline along water path in runoff containment ponds? (Please select ALL that apply)
  • Water quality
  • Surrounding microorganisms
  • Aquatic animals
  • Aquatic plants
  • Others, please specify in the CHAT box

►To answer these new questions, we monitored water quality in the same ponds 24 hours a day and 7 days a week. The top left photo shows a water quality meter that has probes measuring temperature, pH, dissolved oxygen (DO), oxidation-reduction potential (ORP), electrical conductivity (EC), salinity, total dissolved solids (TDS), chlorophyll a, and turbidity.

The instrument is mounted in the center of a pond and set to take surface water quality data every hour. Some of these instruments are connected my office computer through a telemetry system and satellite, so I can see what is going on in each pond real-time. On the bottom left is a computer screen shot showing water quality readings of different parameters in a pond.

►This table summarizes three-year data from one containment pond. As you can see, what pH fluctuated from 6.4 to 10.3, dissolved oxygen from 0.3 to 26.5 mg/L, and electrical conductivity from 48 to 614 mS/cm.

►What these fluctuations mean for pathogen survival? To answer this question, Dr. Kong did a number of studies on how zoospores of different Phytophthora species respond to water quality changes.

►First, she looked into the water pH impact. As shown in this table, water pH evaluated ranged from 3 to 11 for different periods of time from a few minutes (0 day) to 7 days.

►Next, she studied the EC impact on pathogen survival. The EC range examined were from 0 to 3.58 dS/m for periods of time from 0 to 14 days. 1 dS/m is equivalent to 1000 mS/cm. It was very interesting that zoospores of Phytophthora species survived better in dirty water (higher EC) than clean water. This is another piece of great news for farmers because most pond water has an EC range of 0.1 to 0.4 dS/m, at which zoospores do NOT do well.

►She also investigated the DO impact on pathogen survival. She looked into this from two different directions: One focused on DO elevation and the other focused on reduced DO.  As expected, zoospores are aerobic. They survived the best at 5 to 6 mg/L of dissolved oxygen. Their survival decreased at both higher and lower DO levels.

The above data indicate that pH, DO and EC ALL are limiting factors affect zoospore survival in pond water.

We have begun investigation into antagonistic bacteria against Phytophthora species in water but we have not found any silver bullets yet. keep looking…., so stay tuned of our progress.

►Here is another example of single-pond water recycling system with a pump house built by the runoff entrance. In this case, there is no pathogen decline process, whatever pathogen entering this pond is pumped out immediately, so pathogen is being recycled and spread in this system.

►An important message from these studies is:

If a farmer only has one pond on his property, his best bet is to channel runoff from production areas to enter the pond via ONE single entrance and locate pump house and inlet as far away from that entrance as possible. The purpose is to settle out pathogens along the water path before they reach the pump inlet.

►Now, let’s look at the two- and multiple-pond water recycling systems. We have three examples.

►This is an overview of another nursery in eastern Virginia. At this nursery, runoff water from production areas is channeled to be captured in this small pond. When it is full, water overflows to the next pond, then to the third pond. Water is pumped out from this third pond. This three-step process is another way to settle out pathogens along water path for risk mitigation.

►This nursery, located in central Virginia, also has three ponds in its water recycling system. Runoff from all production areas is captured in top pond, VA21. When this ponds is full, water overflow to the second pond, VA22, then to the third pond or bottom pond, VA23. Water is pumped out from this bottom pond for irrigation. We recovered nearly 20 species of Phytophthora including some very destructive pathogens such as P. nicotianae, P. palmivora, P. pini in the top ponds, but fewer in the middle pond, and only one major species, P. ganopodyides, from the bottom pond. P. gonapodyides is mostly saprophytic, nonpathogenic or very weak pathogen.

►This is the most sophisticated water recycling system I have ever seen with runoff water flowing through six ponds before reaching an irrigation pond. Thanks to Dr. Warren Copes at USDA ARS Horticultural Lab in Mississippi for identifying this nursery for the SCRI project. Warren did some baiting in this recycling system and his results show that pathogen populations declined with every pond step.

►The message from these studies in VA and MS is:

If a farmer has the luxury to build multiple ponds, his best bet is to arrange those ponds to have a stepwise water flow with runoff from ALL production areas captured in the top pond and water pumped out from the bottom pond for irrigation. Again, the purpose is to settle out pathogens along water path.

►Now, let’s look at examples of drainage and conveyance components as part of a water recycling system. Specifically, we will focus on:

►Here is a common scene at some production facilities.

What is good here? The production bed is elevated on the gravel pad.

What is the problem? As you can see, overhead irrigation is on with some plants sitting on the runoff path. If any of the plants in this production bed is infected by a pathogen, that pathogen is more than likely being washed off the plants and leached out the container, contaminating every plant and the soil on the runoff path. If anyone walks by, his/her shoes will carry pathogens wherever s/he goes. This minor problem can be easily corrected.

►This photo reminds me of an exciting moment during my first visit to a nursery in Maryland. Thanks to Drs. John Lea-Cox and David Ross for identifying this nursery with one the best drainage and conveyance systems I have ever seen.

At this production facility, surface water in production beds runs to their sides, then drains through underground pipes that traverse underneath production areas to a containment pond. Such a drainage system not only reduces the risk of runoff water picking up pathogens along its path but also provides a foundation for a clean and dry production environment.

►An underground drainage system also opens an additional possibility for effectively draining water within the production bed. At this Oregon nursery, runoff water is directed to the center of production beds connected to an underground drainage system, so that no runoff water leaves the production beds, thus, completely eliminating pathogen entry from contaminated sidewalks and roads.

►Conveyance system is an extension of the drainage system and could be of critical importance to pathogen mitigation. Here is the best conveyance system designs I have ever seen. This is a pump house, nearby is where runoff water comes out underground pipes. If runoff water is allowed to run straight to this containment pond, it is going to be pumped out right away and that would be disastrous. Instead, this nursery developed a J-shaped conveyance system to divert runoff water enter containment pond at the side opposite to the pump house. They also built a number of brilliant ideas into this system.

►First, they added these ‘speed bumps’ in the ditch to slow runoff movement.

►They also put rocks along the ditch to further slow water movement. And after water flow makes second 90o left turn, they have another type of ‘speed bump’ to slow down or even stop runoff water from moving forward.

►And that speed bump is to add well water.

►then water flow makes another 90o turn through rocks. If anyone has seen a better conveyance than this one, please let me know!

►The important message here is:

Design and use good drainage and conveyance systems to reduce direct contact of runoff with diseased plants, infested materials and areas, and to settle out pathogens to prevent them from reaching containment ponds.

►The overall take-home messages from this webinar is

11.4 What may a grower do to increase the runoff water turnover time? (Please select ALL that apply)
  • Channel runoff from all production areas into a containment pond via a single entrance and locate pump house and inlet away from that entrance
  • Arrange multiple ponds to have a stepwise water flow with runoff water from all production areas being captured in the top pond and water pumped out from the bottom pond for irrigation
  • Add sedimentation pond(s)/steps to a water recycling system
  • Set up a good drainage and conveyance system
  • Add “speed bumps” to conveyance systems
  • Others, please specify by phone or via Chat box

►Here are a couple of references if you like to learn more details about the results presented today.

►Before concluding the presentation I like to thank each and every member of my lab for their hard work.  Here is a team photo in the summer of 2013. Many have moved on to permanent positions and we also have several new postdocs in the lab.  I also like to thank all of my collaborators in particular of Dr. Gary Moorman at Penn State, Dr. John Lea-Cox at University of Maryland and Dr. Warren Copes at USDA ARS in Mississippi. They all have played a very major role in these important studies. I am very grateful to all the collaborating farmers who have provided every support I can ask for in this world. Lastly, I like to thank you all for your participation and attention! Now I am ready to answer any questions you may have. Please UNmute your phone and ask your questions.

►Our next webinar will shift focus to water quality. It will be on September 2. I look forward to seeing you again next month!