Irrigation water can be a source of contamination: Irrigation water demands vary from several hundred to several hundred thousand gallons of water per day, depending on the size of the nursery. 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. You should assume that all other sources are contaminated and disinfest them before use. Alternatively, you can test your water sources at frequent intervals and treat only if necessary. Once pathogens enter 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.
Phytophthora: Phytophthora is a fungus-like organism called a water mold, meaning that it requires water to complete its life cycle. Phytophthora spp. 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 other conifer species.
There are more than 100 Phytophthora species; some are host-specific, while others can cause disease on hundreds of plant species. 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. The zoospores not only survive and propagate in surface water, but also can be delivered throughout the nursery in irrigation water.
Baiting can be used to test irrigation water for Phytophthora: 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.
You can use rhododendron leaves or hard (unripe) green pears (Bartlett, 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).
For baiting indoors, collect a quart of water in a clean, soap-free container from near the surface of the water you wish to test. Pour half the water into each of two 1-gallon resealable bags labeled with the date and sample name. Place a rhododendron leaf or a hard green 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 to 75°F out of direct sunlight. Then remove the leaves or the pears and examine them for chocolate-brown spots (“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. 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 the tutorial on baiting for Phytophthora at the Clean Water3 website: http://cleanwater3.org/decontaminate.asp#decontaminatetab2.
For baiting outdoors, you will need to 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. 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 spp. are present, but this should be confirmed.
Detecting Phytophthora after baiting: To confirm the presence of Phytophthora, you can remove a small piece of the bait lesion and test it with a commercially available diagnostic kit for Phytophthora species. Phytophthora diagnostic kits are based on polyclonal antibodies for detection of multiple Phytophthora spp. 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, and it takes less than 10 minutes to conduct the test. These kits are designed for genus-level detection of Phytophthora species, but will not identify the particular species.
If the Phytophthora test is positive and you would like to know what species is present, you can send the bait for additional testing by a university plant diagnostic lab (see sidebar). 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 your 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.
Take steps to disinfest your 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. You can 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. (These methods will be described in a subsequent article in this series.) In the meantime, visit the waterborne solutions tool that summarizes water treatment technologies used to control plant pathogens at the Clean Water3 website: http://cleanwater3.org/growertools.asp.
A success story in managing irrigation water to reduce Phytophthora: 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 (above) 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/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, you can test your irrigation water along its flow path at regular (monthly) time intervals. By routinely testing the water with baiting, you can assess your risk for waterborne Phytophthora, and can implement preventive measures, if necessary.
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.
Inclusion of a commercial product in the article reported here implies no endorsement by the authors or Oregon State University.