Webinar #3 - What plant pathogens could be in my irrigation water? (December 2013)

By: Gary Moorman

Webinar Recording

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Hello everyone. Welcome to today's webinar

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


At the end of the Webinar, you can activate your phone and ask questions.
You can also pose questions using the chat box... and I'll get to those too.


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 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.

Major goals of this project include

We recognize, in developing this project, that many plant pathogens can be found in water. And that recognition has resulted in our coordinating the writing of a book on the subject. There are 49 authors from around the world. Chuan Hong at Virginia tech is heading the editorial team of Carmen Buettner at Humboldt - Universität zu Berlin, Germany
Walter Wohanka at Geisenheim Research Center, Germany and I. The book is to be published by the American Phytopathological Society in April 2014. Just Google shop APS Press next spring and the book will be advertised.

That book will have separate chapters on the various pathogen groups as well as chapters on the threat of plant pathogens in irrigation water in greenhouse crops, nursery crops, vegetables, and deciduous fruit and nut trees. There are chapters on the detection and mitigation of plant pathogens in water as well.

Today's webinar will address the issue of plant pathogens in general that could be found in irrigation water, regardless of the types of plants you grow or the irrigation system you currently use or are thinking of using.

I will present information on the wide variety of plant pathogens that have been found in water and are suspected of causing crop losses following spread via water.

I will also discuss some of the findings of our research project related to Phytophthora and Pythium species we have found in nursery and greenhouse water.

Plant pathogens in irrigation water are particularly of concern in high value crops such as woody ornamentals, floricultural crops, vegetables, and fruits where irrigation is intensive.

Numerous published studies, beginning almost 100 years ago, have documented the presence of plant pathogens in irrigation water
1921 = one of the earliest

Since that time, there have been numerous studies reporting plant pathognes from almost every major pathogen group being found in irrigation water: Bacteria, Viruses, Nematodes, Fungi, Fungus-like Oomycetes.

We have provided a handout with information from the upcoming book listing plant pathogens that have been found in various water sources... whether irrigation water or other water and the organisms I will be referring to throughout this webinar are listed in that handout.

NOTE that plant pathogens may enter a cropping system from several sources:

Water is just one of many possible source of plant pathogens entering a crop.

Today, let's consider WHAT could be in water.

Let's start with the BACTERIA

Bacteria are very simple living organisms, only a few of which are plant pathogens. None of the plant pathogenic bacteria form a resistant spore. They live as single celled micro organisms with a cell wall. They reproduce by one cell dividing into two... a very simple life history.

Here is a list of 12 different plant pathogenic bacteria that have been found in irrigation water and implicated in causing losses in the crops indicated.

Regardless of how they got into the water, it was shown that the the same genus and species found in the watger were found infecting plants.

I should point out that most of the published reports are that type of circumstantial evidence. A particular organism  is found causing disease. That particular organism is found in the water. And it is assumed that disease in the plants is coming from the pathogen dispersed in the water.

In fact, very few studies prove beyond a shadow of a doubt that the organism causing the disease actually came to the crop via water.
But the evidence is strong.

A key bacterium, Ralstonia solanacerum, has been known to cause disease when it is dispered via water.

In the older literature, this bacterium was called Pseudomonas solanacearum. The common name for the diseases it causes is bacterial wilt or in N. Am., called southern wilt

Ralstonia  is well known to be dispersed long distances in infected plants shipped from place to place. It is spread short distances on tools, equipment, clothing that becomes contaminated. Movement of soil that harbors the bacteria AND IN IRRIGATION WATER. Race 3, Biovar 2 of Ralstonia solanacerum is not endemic to North America. It is on the USDA-APHIS select agent list because it can infect potato

Race 1 is native to North America but does not go to potato.

Both can be spread via irrigation water.

Race 3 has been accidentally introduced into North America at least 3 separate times on geranium cuttings. Each time, it was caught and destroyed

If Race 3 of Ralstonia solanacerum is found– all plants sharing recycled water, whether recognized host or not, must be destroyed.

If that same water were being recycled to arborvitae, the arborvitae would also have to be destroyed if Race 3, Biovar 2 were found.

The reason is that Ralstonia can survive in the water and in soil. Infested soil would be sold with the plant and Ralstonia Race 3, Biovar 2 would end up in North America.

Erwinia carotovora is another important bacterium that has been found in irrigation water. That bacterium is a common soft rotter of vegetables and cuttings of herbaceous plants.

The great difficulty with bacteria is that there are very few chemicals that give a high level of plant protection against the diseases they cause. You need to try to exclude them from the production system rather than deal with them in the system.


True fungi generally considered terrestrial in habitat. Many have been found in water, including these. And here are more:

Fungi are so widespread, in so many habitats, it should not be surprising that some are found in water. Some probably just fall in.

Key ones = Fusarium and Verticillium

Once Fusarium or Verticillium is in the water, plants are repeatedly inoculated every time water is applied, the irrigation system collects and concentrates inoculum, and then disperses it

Very importantly, if inoculum is introduced into fields on a few transplants, the irrigation system then disperses Fusarium or Verticillium to irrigated areas that previously never had those plant pathogens.

They are two examples of true fungi that have been found in irrigation water.

Another group of organisms found in water are...

NEMATODES - Recognized as essentially aquatic – in thin films of water.

All plant parasitic nematodes have a hollow spear-like mouthpart called a style.

They have a well developed digestive system and reproductive system.

Many have been found in irrigation water...and here is a list of some:

In the background is Xiphenema, the dagger nematode. You can clearly see the stylet. This one is getting ready to molt and has a replacement stylet ready to put into position when the old one is shed.

Here is the rest of the list. The nematode here has a small stylet... near the word peanut.

To make sure you've been paying attention to the webinar, there will be a spelling quiz at the end. Those of you who pass the spelling test will then be tested on your pronunciation of the nematode names.

Nematodes require water to survive and for locomotio. Short distances = centimeters or inches.

Early as 1923, it was shown that the wide dispersal of Ditylenchus dipsaci depended upon irrigation. More recently - foliar nematode was shown to spread is via irrigation water in alfalfa.

Cysts of Heterodera and Globodera can be carried in and dispersed by irrigation water. And each cyst contains many eggs.

That brings us to the VIRUSES.

There is always an academic debate around viruses... Are they alive or not.

A plant pathogenic virus is just a piece of nucleic acid, either RNA or DNA, surrounded by a protein coat. The RNA or DNA directs the plant biochemical machinery to reproduce the RNA or DNA and the protein coat. The virus itself can not reproduce itself.

Viruses can cause significant losses in forest trees, field crops, ornamentals, tree and small fruit crops…practically anything we grow is susceptible to several viruses.

Many viruses have been found in irrigation water.

There is one called Pepino Mosaic Virus that is commonly found in salsa because it infects tomato. That virus survives the cooking process, our digestive processes, and waste water treatment processes. It is used to trace the movement of waste water in the environment... it is so stable. But that is an unusual one that can be found in water.

Shown that tomato bushy stunt virus (TBSV) and cucumber green mottle mosaic virus (CGMMV) spread among plants in hydroponics.

Similarly, Tomato Mosaic Virus (ToMV) and Pelargonium flower break virus spread in nutrient film irrigation in greenhouses.

These viruses are very stable and tough. They are released from the roots of infected plants into the water and then spread during irrigation.

The last group of plant pathogens is near and dear to the project team and is probably the best known group of plant pathogens associated with water, OOMYCETES.

And many of the chemicals used against true fungi are not effective against the Oomycetes.

Plant pathogenic Oomycetes include the downy mildews, NOT TO BE CONFUSED WITH THE POWDERY MILDEWS which are true fungi. Phytophthora and Pythium are oomycetes and there are some others. Phytophthora and Pythium have very frequently been implicated in dispersal via irrigation water.

This is a partial list of Phytophthora species. You can examine the list in detail in the handout. Just notice the length of the list.

This is a partial list of Pythium species.

All of these that have been found in water. We tend to think of them as well suited to water because they are known for their zoospore formation.

Sporangia of Phytophthora release zoospores directly into water.

Pythium forms a vesicle, the zoospores develop in the vesicle, and then the vesicle breaks open to release the zoospores into the water. Not all species of Pythium form zoospores and not all isolates within a species of Pythium form zoospores.

In our project, we have isolated Phytophthora and Pythium from infected plants in greenhouses and nurseries and we have isolated them by baiting the water and by passing water through special filters which are then inverted onto agar in order to isolate them.

Our primary goal has been to document the presence of plant pathogenic Phytophthora and Pythium and follow their presence in time and space in water.

Several years ago, there were 120 known species of Phytophthora and it was estimated that there were probably an additional 200 waiting to be discovered.
We may want to increase that number

Dr. Hong's group has found more than 50 species of Phytophthora in nursery irrigation water.
Including several new to science.


Some of these are well known plant pathogens and have been found infecting plants in the nursery where they were isolated... like Phytophthora drechsleriPhytophthora cinnamomi and Phytophthora nicotianae.

The new species discovered are being studied now.

Dr. Hong's group is characterizing all the species they've found in water and working to categorize them according to the crop loss risk they pose if they are found in irrigation water. They are looking at pathogenicity, host plant range, frequency with which they are found in water, and other characteristics and trying to categorize them as posing a high, medium or low risk to crops when they are found in water.

In my part of the research project, we are looking at Pythium in greenhouse production.

In previous research, we have found 3 species responsible for most of the crop losses in the northeastern U.S.

We've looked for these intensively in irrigation water used in ebb and flood systems in commercial greenhouses producing potted plants. To date, we have not found these 3 species in the water.

We're looking elsewhere too... like in biofilms that form in the plumbing.

But we've found many other Pythium species... the most prevalent are:

And 4 completely unknown species that are similar to but not the same as some known species. So for now, we are just grouping them. Isolates with DNA sequences similar but not identical to these 4 groupings:

One characteristic all of these share is that they produce huge numbers of zoospores in water.

Students in my lab are characterizing these isolates. Including pathogenicity and fungicide resistance. Pythium helicoides is sometimes found causing crop losses... but fairly rarely.

We have not found the others causing crop losses under commercial conditions.

Some in each are plant pathogenic in our tests, others are not. Some are resistant to fungicide, others are not.

A big question in our lab is, since certain species are very frequently in water and the highly plant pathogenic P. aphanidermatum, P. irregulare, and P. cryptoirregulare are not, are the highly pathogenic ones being inhibited in water by the others commonly in water? That research is underway. We're doing inoculation tests for pathogenicity of each separately and interactions on plants and in water

Take away points:

From these brief examples, you can see that a wide variety of plant pathogens do indeed pose a threat to crops because they can enter irrigation systems, be accumulated there, and dispersed to crops repeatedly during a season.

They can also be dispersed via irrigation to production areas not previously infested with these plant pathogens.

What we have learnd in our project is that it is not sufficient to merely identify the GENUS of organism present.

In particular:

If you find a Phytophthora and Pythium in water, you must identify the exact species you find in water in order to assess the risk they pose.

The issues of detecting plant pathogens in water and then assessing the risk to a crop will be subjects of upcoming webinars