How to test your irrigation system for the presence of Phythophthora
By Neelam R. Redekar and Jennifer L. Parke
Phytophthora is a fungus-like organism — called a water mold — that can infect a wide variety of nursery plant species. One of the ways it can spread is through irrigation water.
Depending on size, a nursery can demand between several hundred to several hundred thousand gallons of irrigation water per day. Several sources of water may be used to meet these irrigation demands, including surface water (rivers, streams, canals, lakes, ponds, reservoirs), well water, rainwater, municipal drinking water, and recycled runoff water.
Of these, only rainwater, well water, and municipal water are free of pathogens. Growers should assume that all other sources are contaminated and disinfest them before use.
Alternatively, people can test their water sources at frequent intervals and treat only if necessary.
Once pathogens gain entry into the nursery, they can spread through the irrigation system, infect plants, accumulate in the runoff water, and establish in the water storage reservoirs. Disinfesting contaminated water before it is used for irrigation is essential for breaking this cycle.
There are more than 100 Phytophthora species; some are host-specific, while others can cause disease on hundreds of plant species. All require water to complete their life cycle.
Phytophthora species cause foliar blight, stem canker, shoot dieback and root rot on ornamentals, native plants, forest trees and agricultural crops worldwide. Many nursery plant species are highly susceptible, such as azalea, rhododendron, boxwood, and many conifer species. A host list is available on tinyurl.com/y6k25dah.
The plant nursery environment is optimal for growth and proliferation of Phytophthora. Oospores and chlamydospores are capable of long-term survival in soil or infected plant debris.
Under wet conditions, Phytophthora sporangia release numerous small, swimming spores called zoospores, which can infect plants and cause disease (Fig. 1). The zoospores not only survive and propagate in surface water but also can be delivered throughout the nursery in irrigation water.
Testing with bait
Baiting is a relatively simple technique that uses susceptible plant parts as “baits” to attract Phytophthora spp. This method selects for live, active zoospores, which can swim toward and infect the bait.
Growers can use rhododendron leaves or hard, unripe pears (such as Bartlett or d’ Anjou) as bait to detect Phytophthora, as they are susceptible to many species of Phytophthora. These baits can also capture several Pythium species, which are closely related to Phytophthora and cause damping-off of seedlings and stem cutting rots. There are two basic ways to bait: in a resealable bag (indoors), or in the water body itself (outdoors).
Collect a quart of water in a clean, soap-free container from near the surface of the water that will be tested. Pour half the water into each of two one-gallon resealable bags labeled with the date and sample name. Place a rhododendron leaf or a unripe pear in each bag. Be sure to choose leaves or fruits without spots, blemishes or bruises; they should be free from pesticides that could interfere with the test.
It is always a good idea to place a few leaves in bags with tap water or distilled water as a negative control.
Allow the bags to sit for seven days at 65–75 F, out of direct sunlight.
Then, remove the leaves or the pears and examine them for chocolate-brown spots, known as lesions, on rhododendron leaves or brownish red circular lesions on pears. If the pear has been sitting upright, you can sometimes see a “bathtub ring” of lesions around the pear made by zoospores (Fig. 2).
Spots made by Phytophthora are firm to the touch; if they are soft and mushy, they are likely caused by a soft rot organism; not Phytophthora. For more information, watch our tutorial on baiting for Phytophthora at tinyurl.com/yx8t8y5m.
To bait outdoors, make a mesh bag to hold the leaf baits. Plastic window screen material works well for this purpose. Some mesh bags are made with sleeves for individual leaves, as shown (Fig. 3). Attach the mesh bag to an air-filled, plastic milk jug to act as a float, as the greatest number of zoospores will be near the water surface. Secure the mesh bag with a nylon rope, so you can cast the bag out into the water and then retrieve it seven days later.
Remove the leaves from the bag and examine them for dark, chocolate-brown lesions. The outdoor baiting works well when the water temperature is between 48 F to 71 F.
If Phytophthora is present in water, zoospores will swim toward the surface of water surrounding the bait, colonize it and develop dark brown lesions. If the tested water is contaminated with a high amount of the pathogen, then brown lesions may appear within two to three days after baits are suspended in water.
The appearance of lesions on the bait is a good indication that Phytophthora species are present, but this should be confirmed.
Detecting Phytophthora after baiting
To confirm the presence of Phytophthora, remove a small piece of the bait lesion and test it with a commercially available diagnostic kit for Phytophthora species (Fig. 4).
Phytophthora diagnostic kits are based on polyclonal antibodies for detection of multiple Phytophthora species. The kits are rapid, easy to use and relatively inexpensive. For example, the Pocket Diagnostics® Phytophthora kit costs about $8 each when purchased in boxes of 50 kits. It takes less than 10 minutes to conduct the test. For details visit www.pocketdiagnostic.com/onlineshop or www.potadaho.com/test-kits.
These kits are designed for genus-level detection of Phytophthora species, but will not identify the particular species. To watch our online tutorial on how to use rapid test kits for Phytophthora, visit tinyurl.com/yy8s5vfh.
If the Phytophthora test is positive and you need to know what species is present, send the bait for additional testing by a university plant diagnostic lab (see list at end of article). (Please note that it is much easier to send leaf baits rather than pears through the mail.) Be sure to contact the lab in advance to make sure they can handle the sample.
Plant diagnostic labs can attempt to identify the species by growing it in a petri dish containing an agar nutrient medium selective for Phytophthora. Identification can be based on microscopic features or DNA-based techniques. For most growers, genus-level identification is enough to demonstrate that their water is contaminated and requires treatment.
Taking steps to disinfest water
Irrigation water treatment can minimize the spread of Phytophthora and help reduce losses. There are several methods for water disinfestation with chemical, physical and biological modes of action. Choose which method to use based on the mode of action, the volume of water to be treated, installation and operational costs involved, space requirements, and safety and environmental concerns.
Refer to the “Waterborne solutions tool” that summarizes water treatment technologies used to control plant pathogens at the CleanWater3 website: http://cleanwater3.org/growertools.asp.
Successful reduction of Phytophthora in irrigation water
A large container nursery in Oregon recycles 90 percent of its irrigation water; it succeeds in preventing Phytophthora infestation of its irrigation water by treating it first with sodium hypochlorite or calcium hypochlorite.
We collected water samples from several different steps along the irrigation water pathway, and baited the water samples with rhododendron leaves to see if Phytophthora was present. We then used a DNA sequencing technique to identify the Phytophthora species that were on the bait leaves. The diagram (Fig. 5) illustrates results of baiting irrigation water in the month of September 2015.
The main source of water for the nursery was a creek. The creek was infested with four Phytophthora spp., including two pathogenic species complexes, P. parsiana and P. megasperma. (A complex is a group of closely related species that cannot be distinguished with our method of sequencing.)
The creek water was pumped to a retention reservoir where we detected two species complexes: the P. parsiana complex from the creek, and the P. citricola complex from another source, likely the runoff water.
From there, the water was filtered and chlorinated, with a target concentration of 2 ppm and a 10-minute contact time. No Phytophthora species were detected in the chlorinated water, which was then used for overhead irrigation in the growing areas of the nursery. Although the irrigation water was clean, and the plants did not show any symptoms of Phytophthora, we did detect P. citricola complex in the soil/crushed rock material underneath the containers.
Soil in nurseries is commonly infested by Phytophthora species, which can survive for years in bits of plant debris that infiltrate the soil and crushed rock. It is very difficult to disinfest contaminated soil.
Runoff water from these growing areas, after contacting the contaminated soil, was found to harbor P. parsiana complex, P. citrophthora, and the P. citricola complex. Pumping the runoff water back into the retention reservoir carried these Phytophthora species with it. Fortunately, the subsequent chlorination treatment prevented Phytophthora from entering the irrigation water, so the contamination cycle was broken. Following this example, test irrigation water along its flow path at regular (monthly) time intervals.
By routinely testing the water with baiting, a grower can assess the risk for waterborne Phytophthora, and can implement preventive measures, if necessary.
Acknowledgments: Funding for this material is based upon work that is supported by the National Institute of Food and Agriculture, U. S. Department of Agriculture, Specialty Crop Research Initiative competitive program under award number 2014-51181-22372.
Disclaimer: Inclusion of a commercial product in the article reported here implies no endorsement by the authors or Oregon State University.
Dr. Neelam R. Redekar is a Research Associate (Postdoc) in the Department of Crop and Soil Science at Oregon State University, Corvallis, Oregon; she can be reached at Neelam.Redekar@oregonstate.edu
Dr. Jennifer L. Parke is a Professor (Senior Research) in the Department of Crop and Soil Science and the Department of Botany and Plant Pathology at Oregon State University in Corvallis, Oregon; she can be reached at Jennifer.Parke@oregonstate.edu
Plant disease diagnostic labs
Oregon State University Plant Clinic — http://plant-clinic.bpp.oregonstate.edu
- Baiting water for Phytophthora $75
- ELISA assay for Phytophthora $45
- Testing water via filtration for Phytophthora or Pythium $110
The Pullman Plant Pest Diagnostic Clinic at Washington State University — www.plantpath.wsu.edu/diagnostics
- Baiting water for Phytophthora $40
Plant Diagnostics Laboratory at University of Massachusetts — www.ag.umass.edu/services/plant-diagnostics-laboratory
- Testing irrigation water for Pythium, Phytophthora, and Rhizoctonia $50
Florida Extension Plant Diagnostic Clinics at University of Florida — https://plantpath.ifas.ufl.edu/extension/plant-diagnostic-center
- Baiting for Pythium and Phytophthora $65
- Immunostrip test for Phytophthora $50
Plant Disease Diagnostic Clinic at Cornell University — www.plantclinic.cornell.edu
- ELISA assay for Phytophthora $70
- Immunostrip test for Phytophthora $60
- PCR for Phytophthora ramorum $50
Plant Diagnostic Lab at North Dakota State University — www.ag.ndsu.edu/pdl/services-and-fees
- Immunostrip test for Phytophthora $45
Plant Disease Diagnostic Clinic at University of Wisconsin-Madison — www.pddc.wisc.edu/services-fees
- ELISA assay for Phytophthora $35
- Dip stick serological test for Phytophthora $8
Plant Disease Diagnostic Lab at Texas A & M University — https://plantclinic.tamu.edu/fees
- ELISA assay for Phytophthora $20