Webinar #4 - How do plant pathogens enter and move around irrigation systems? (January 2014)

By: Gary Moorman

Webinar Recording

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I'm Gary Moorman, a professor of plant pathology at Penn State University.

This is one of a series of webinars organized by the research team on the Specialty Crop Research Initiative Project titled, Integrated Management of Zoosporic Pathogens and Irrigation Water Quality for a Sustainable Green Industry. The project is funded by the USDA National Institute for Food and Agriculture and the research team is from several universities and the USDA-ARS as you can see from the various logos displayed here.

We would like to thank the panel of growers, the Society of American Florists, and the American Nursery and Landscape Association for their advice and assistance on this project. They are key partners in this project.

A major goal of this project is to gain a better understanding of the biology of plant pathogens in irrigation water and educate people about plant pathogens in irrigation water. One product of the project is going to be the publication of a book on the subject written by 49 authors from around the world. The book is to be published by the American Phytopathological Society in April 2014. Just Google 'Shop APS Press' in and the book will be advertised. The book has chapters on water sources and irrigation systems and the risk to plant pathogens posed by each is assessed.

In a previous webinar, I discussed what plant pathogens have been found in irrigation water.
►Today's webinar will address how plant pathogens might get into water and once there, how they might continue to pose a threat to the crop.

►You have to step back and look at the entire production system.
Each specific system is different.
In fact, each farm or greenhouse is different.
Let’s take it apart.

►The main sources are:

Many plant pathogens can be found in soil including species of Cylindrocladium, Pythium, Phytophthora, Fusarium, Rhizoctonia, and Thielaviopsis and most plant pathogenic nematodes.
We can assume that any soil used in crop production or in or near a farm or greenhouse may harbor plant pathogens and are a threat to crops if they get into water.
So, you have to think about where soil is located in relation to the irrigation water.

►Of course in nursery, shade tree, vegetable and orchard production, the crop is in field soil.
In containerized nurseries, soil can be part of the potting mix or the ground near the containers.
Even in supposedly closed production systems, like hydroponics in greenhouses, you have soil in walkways within the facility, parking areas, etc.
Any place you have soil, you may have one or more plant pathogens present and that is a possible harbor of pathogens for irrigation water, whether it is moved there by wind or on worker's feet, pet's feet or on contaminated tools or equipment.

►One example I will never forget is the case of a grower I worked with who was using a dibble board to make holes in potting mix in flats. A dibble board is a piece of wood with dowels or pegs in it. He would fill a flat with soil and press the dibble board into the potting mix so that holes for unrooted cuttings would be nice and evenly spaced. The problem was that when the dibble board was not in use, it was put on a dirt floor. I isolated Pythium.

from the dirt picked up by the pegs. He was putting Pythium from the floor into the holes he was creating. The potential was there for that Pythium to be picked up in water that was being recycled for irrigation.

►Also note that outdoors, most irrigation ponds, and collection ditches are soil lined. Or if they are lined with things other than soil or plumbed, it isn't too long before they accumulate soil from the crop and surrounding area. That soil is a source of plant pathogens.

I assume that soil is the #1 harbor for plant pathogens that eventually end up in irrigation water.

CROP DEBRIS - previous or current crop
Most plant pathogens have a stage in their life histories that can rest in a dormant state and survive periods of time when temperatures are extreme or moisture is not sufficient for growth.  Many plant pathogens have evolved a strategy of becoming dormant in the dead leaves, stems, branches, and roots where they previously caused disease.
Inside those tissues they are protected from the hostile environments of the soil and air and are away from competition with other organisms in the soil and air. They have at hand a ready supply of nutrients for when conditions become favorable for growth again. Erwinia, Xanthomonas, and Pseudomonas, and Ralstonia bacteria; Botrytis, Thielaviopsis, Rhizoctonia, Verticillium fungi, foliar nematodes, and tobacco mosaic virus, survive for months and in some cases for years in plant debris.

►I also include in debris, the material in pots and flats that are reused. This is an actual case.
I isolated Pythium from debris left in a pot that had been washed and disinfested at a commercial greenhouse.

►The debris can also be from a current crop. Most greenhouses that recycle water have some sort of filter to remove the coarsest particles. This is an excellent place to look for plant pathogens... in that debris.
In my research, we often find Pythium associated with the debris on these filters. To date, we have not found the highly pathogenic species of Pythium. Rather, we find the non-pathogenic or very weakly pathogenic species that seem to be aquatic in nature and are residents of water.

As noted in my earlier webinar, in our work at Penn State, it appears that Pythium aphanidermaum, a serious plant pathogen, is not a resident in the water of the commercial greenhouses where we do our work. Those are pot plant systems and not hydroponic systems.

Whether it is plant or soil debris from a previous crop, that is a good source of plant pathogens to contaminate water.


►Plants already being grown may be infected and new plants brought into a production system may be infected.

►This is a particularly good way for vascular wilt pathogens such as Fusarium oxysporum, Verticillium, and certain xanthomonads to be introduced into a production area and then move into a recycling irrigation system.
And in fact, those pathogens have been found in water and found to be able to move from the water into new plants.

►Another example of introducing a pathogen into water is Pelargonium flower break virus.

►That was, at one time, a very common virus in geraniums... highly contagious. It has been shown that when an infected plant is placed in a geranium production system, the virus can exit roots, enter water, and be distributed to other geraniums in the system.

►A very new publication reports that potato virus Y, pepino mosaic virus, and potato spindle tuber viroid are all able to exit plant roots, enter water and infect roots of plants irrigated with that water.

►Greenhouse systems using recycled water are known to have been contaminated with Xanthomonas that goes systemic in begonia and spread to other begonias.

►as does Ralstonia, a vascular pathogen that spreads among plants sharing water.

There are numerous studies of Pythium-infected plants placed in greenhouse irrigation systems that then act as a source of inoculum for other plants sharing the water.

►Outdoors in nursery production, Chuan Hong's group at Virginia Tech has shown that Phytophthora species are found in irrigation water captured from nurseries and survive in containment ponds.

So an infected crop can be a source of pathogens that get into the water.

►You would expect that non-crop plants in or near the production area that are susceptible to the same pathogens as the crop can be sources of plant pathogens.
A good example is Phytophthora ramorum in the Pacific northwest nurseries.
Apparently it was brought in on plants. Sporangia and zoospores produced on those plants then entered water and non-crop plants downstream became infected.  Now non-crop plants are a continuing source of Phytophthora for any plants irrigated with that water.


►The spores of fungi and sporangia of some species of Phytophthora can be carried by air currents for long distances outdoors. Infected plants in nearby gardens and forests can supply enormous numbers of spores.
Thus, even if great care is taken to eliminate other sources of pathogens, the air we breathe may carry certain disease-causing organisms into water. 

►Insects can move plant pathogens. I think it is well established that adult shoreflies can move Pythium and Fusarium within greenhouses.

The water itself at its source or along its path to the crop can be a harbor of plant pathogens.
This is well known to occur with Phytophthora and Pythium can be spread via water.

That is the main topic of interest in our Specialty Crops Research Initiative project: the aquatic ecology of Pythium and Phytophthora.

►Surface water supplies such as lakes, ponds, rivers, and streams contain Pythium and Phytophthora in the sediment as well as free in the water. 

            Ground water
            Ponds, lakes, and other surface impoundments
            River, stream, or other free-flowing source
            Water collected from roofs or paved surfaces
            Municipal water supply

…all can be used as irrigation water. Let’s consider each and the risk they pose for supplying plant pathogens.


Ground water
In general, ground water and natural springs tend to not harbor plant pathogens unless the water table is close to the soil surface.  Conventional wisdom is that the soil and the soil microbial community above the water table acts like a filter that physically or biologically removes most plant pathogens, unless the pathogen is particularly adapted to the soil environment.  As plant root systems penetrate the soil, soilborne organisms including certain plant pathogens colonize the root surface and may come in contact with relatively shallow ground water.
When ground water is extracted by using wells, there is the potential for contamination by debris or surface runoff unless the well is properly cased. 

Ponds, lakes, or other surface impoundments
Such sources are at high risk to contamination by plant pathogens carried by wind currents, storm water runoff, and debris carried to them. Plant pathogen content may vary with the size of impoundment, particularly its depth and whether the impoundment water is being withdrawn from the top or bottom of the water column. The status of plant pathogenic viruses at various depths has not been assessed.

Rivers, streams
Rivers, streams offer another water source widely considered to be at high risk for harboring plant pathogens. Even in forested areas with little or no human activity, plant pathogenic viruses that occur in a wide variety of crops have been detected in streams.

Water collected from roofs or paved surfaces
Natural precipitation can be collected from greenhouse, storage building and other roofs as well as from other impervious surfaces and channeled to cisterns or impoundments.  In theory, those surfaces should harbor very few plant pathogens and pose a low risk of being a source of plant pathogens. But, it is possible that those surfaces could collect plant pathogens blown onto those surfaces from surrounding areas. Then precipitation would wash the pathogens into an impoundment.

Municipal water supply
Potable water is normally very low in or free of plant pathogens and is considered to pose a very low risk.
Generally, it is expensive and there is an increasing threat of restrictions on its use for agriculture during periods of drought. That threat is becoming more serious as the frequency of droughts increases. 

Water collected from fields and non-paved areas = tailwater
Natural precipitation and excess irrigation water applied to fields (termed, tailwater) is collected by channels and directed to ponds or other impoundments and used for irrigation. This water poses a high risk for harboring plant pathogens from all the sources already mentioned: soil, plant debris, crop plants and the vegetation surrounding the crop, and the air.

Once a plant pathogen is in the water, then it has to get to the crop.

Water distribution to a crop can be by        
►Flooded furrow, Flooded field...  or greenhouse equivalents: Trough, Ebb and flood floors or benches
Overhead sprinkler, Trickle or drip
Hydroponic, nutrient film
Sand bed or capillary mats

►For capturing and recycling water we have
            Indoor tanks
            Outdoor reservoirs
            Soil lined channels

Each distribution and recycling network must be examined on a case by case basis to assess the potential for exposing crops to plant pathogens.


Flooded field - water applied for a specified duration and then drained. The entire field is diked to retain the water and the field is graded so that excess water is channeled to waste, to retention ponds for reuse, or back to the source such as a stream, river or impoundment.

Flooded floor or ebb & flood
The greenhouse equivalent would be Flooded floor or bench
Ebb and flow or flood and drain sub-irrigation systems apply water to impervious, diked or slightly sunken floors
►or large tray-like benches.
►The water is taken up by the potting mix and plants in containers. The risk of plant pathogens in this system is determined by the amount of plant pathogens in water used to charge and maintain the system, the health of the plants put into the system, and measures taken to prevent the introduction of pathogen-containing stray soil or plant debris into the system. Once a plant pathogen is introduced, however, the risk of spread within the system is very high. This poses a high risk of dispersing plant as the water flows.

Flooded furrow
In this distribution system furrows are created in a field, parallel to rows of crops and graded so that water applied at one end of the furrow flows to the opposite end. Excess water is channeled to waste, to retention ponds for reuse.
This system poses a high risk of dispersing many different plant pathogens it can accumulate plant pathogens and disperse them as the water flows.

Trough system
The greenhouse equivalent of the flooded furrow would be the trough or nutrient film system where water flows down the row among plants.

The pathogens, for the most part, are carried passively attached to soil or debris or free in the water.
Size and weight of the pathogen or what it is attached to will determine how fast it settles out along the path.

Some probably do not settle out. For example, nematode cysts are very light and float or tumble along very efficiently. Virus particles probably stay suspended.

Hydroponic or float systems
These are the greenhouse equivalent of the flooded field.  In the float system, seedlings are grown in trays of soilless potting mix or trays that hold bare rooted plants. Multiple trays are floated in a common bath, usually, enough trays to completely cover the water surface. The risk of plant pathogens in this system is determined by the amount of plant pathogens in water used to charge the system and the health of the plants put into the system. Once a plant pathogen is introduced however, the risk of spread within the system is very high.

Sand beds or capillary mats
Water is applied as needed by the plant, not continuously.  Sand and mats must have particle size that allows the capillary movement of water to plants but be coarse enough to allow excess water to drain away. Generally, there is little or no runoff from sand or capillary beds and excess water is not recycled. The main risk posed in this system is whether plant pathogens are in the water source used, in the plants put into the system, and any contamination of the beds later.

I have never seen a report of large crop losses in a capillary mat system.


It is important to note that if excess water from any of the irrigation systems is collected and reused during subsequent irrigations, that is at high risk for also recycling plant pathogens.

Indoor tanks
When water in greenhouses is recycled, it is usually impounded in a tank of some sort. Tanks may be above ground or may be in-ground. If tanks are not covered, they become subject to contamination with plant pathogen-containing stray soil and plant debris. In addition, they can accumulate plant pathogens from crops.

Outdoor reservoir
Outdoor reservoirs used as a source of water in recycling systems are subject to contamination with plant pathogen-containing stray soil and plant debris and they too accumulate plant pathogens. Thus, they have a high potential for dispersing plant pathogens.

Soil lined channels
Soil lined channels that return excess water from the irrigated crop to a holding tank or other impoundment are subject to contamination.

Unlike open channels, pipes used to return water to holding tanks or impoundments are not readily accessible for cleaning or decontamination.  Seams and acute bends in the plumbing are points where debris and plant pathogen-harboring biofilms can accumulate. Those biofilms may be ongoing sources of plant pathogens.

Let's talk about the findings of Dr. Hong's group at Virginia Tech concerning the recycling of water from ornamental nurseries. His research is a major part of our Specialty Crops Research project.

►His group has clearly shown that water coming off the nursery during irrigation can have a high concentration of Phytophthora propagules. The amount of Phytophthora is highest at the point where the effluent enters the retention pond. The number of propagules declines with increasing distance from the entry point.
►Baiting was done to detect the Phytophthora.

A future webinar will address that and how you can take advantage of that knowledge.

Points to ponder:
Water is one of many sources of plant pathogens. Other sources include
plants...the crop itself and plants in and around the production area
debris from previous or current crops
airborne or arthropod borne inoculum

Plant pathogens…
            can be in the initial water…
            can enter the water at any point in the system
            are moved passively in the system
Each water source and production system is unique
Verify that WATER is the source of the pathogen

It is common for farmers to use multiple water sources and irrigations systems to produce crops. Each source and distribution and recycling system must be assessed on a case by case basis for the potential for exposing the crop to plant pathogens carried in irrigation water.

If there is a particular plant pathogen currently causing crop losses, then you need to look at the ecology and epidemiology of that pathogen to assess possible harbors. Water is one possible source but that needs to be confirmed.

Of crucial important is to VERIFY that the water is actually the source of the plant pathogen involved BEFORE you do anything.

►That is the subject of the next webinar.
As a tease, I will tell you that to my knowledge, there are no commercial labs that will routinely test water for pathogens. But there are ways of getting around that.

►We’ll stop here and I can try to answer questions.

Any questions?