by Sally Colby

Root and other crop damage caused by Pythium, Phytophthora or other organisms is an ongoing concern for growers, and it’s often difficult to nail down precisely which genus and species is the source of the problem. In some cases, irrigation water is the source of pathogens.

Dr. Gary Moorman, Penn State department of Plant Pathology and Environmental Microbiology, explains the Specialty Crop Research Initiative Project titled ‘Integrated Management of Zoosporic Pathogens and Irrigation Water Quality for a Sustainable Green Industry’, funded by USDA NIFA (National Institute for Food and Agriculture).

The goal of the project is to gain a better understanding of the biology of plant pathogens in irrigation water and to provide education on the topic. Moorman says the crucial question for growers is to determine whether irrigation water is clean or contaminated. He noted the issue is usually brought up when the grower experiences crop losses.

Plant pathogens from almost every major group of living organisms have been found in water, and a variety of plant pathogens can infect a crop. Moorman says because some plant pathogens are not likely to be waterborne, growers should consider other sources of contamination.

“There are certain other plant pathogens well-known to be associated with water, and those need to be examined,” he said. “If you are losing plants because of one of those pathogens, then it is certainly worth considering water as a possible source. The reality is that we might be looking for a needle in a haystack, and it’s helpful to know at least what the needle looks like.”

Moorman says it’s impossible to look for every possible pathogen in water. “It’s important to determine which plant pathogen is of most concern,” he said. “To do that, any plant disease clinic can help determine which pathogen is causing losses.”

The grower who suspects irrigation as the pathogen source has a good start in knowing what to look for and how to mitigate the problem. Moorman says water testing labs routinely test for alkalinity, hardness, pH, coliform bacteria and other human pathogens, but not plant pathogens. He suggests growers contact state university labs or the appropriate state department of agriculture regarding pathogen testing of water.

Since Pythium or Phytophthora are the among most common causes of problems in irrigation water, Moorman outlines the initial steps a grower can take to determine the presence of those pathogens. Prior to sampling, the grower should contact the person who will be doing the test to make sure that person knows the sample is coming, when it is coming, and that they’ve had the opportunity to suggest to the grower how to collect a good sample.

Moorman describes two main methods of sampling water for Pythium, Phytophthora and most other pathogens: grab sample and baiting. A grab sample is relatively simple and involves collecting water for testing at certain points in the irrigation system.

“Usually, the sample is collected at each major point in the irrigation system,” said Moorman. “Many farms have more than one source of water, more than one holding impoundment and more than one distribution system, so samples should be collected at each of those key points.”

Water samples should be collected during the growing season. In outdoor production facilities, avoid collecting samples in winter, and avoid collecting samples from greenhouses when there is little or no production occurring. “If a farm or greenhouse is recycling irrigation water, it’s a good idea to collect the sample at a central point where the water is coming off the crop and going back into an impoundment,” said Moorman. “Another place to consider sampling is near the intake point, before it enters the pump for irrigation.”

Moorman says two liters of water at each collection point is enough for a sample. Water from each collection point should be tested separately and not mixed to create a homogenous sample. Moorman also suggests not collecting samples from the bottom of the water column. Samples for Pythium or Phytophthora testing can be collected in clean containers, such as a clean milk jug or a clean two-liter soda bottle. Water samples for bacteria or virus testing should be collected in clean, unused containers.

“For grab samples, you can attach a rope to a bucket or other wide-mouthed container,” said Moorman. “Hang onto the rope, throw the container into the water, and transfer the water to the milk jug or bottle.” Ideally, samples should be collected the same day they will be shipped, but samples can be refrigerated if there will be a delay in shipping.

Label each container (date and location) and place in a plastic bag. Samples should be stored in a box or cooler to prevent contact with direct sunlight until shipping. Cold packs will help keep the water at a desirable temperature, with cardboard or bubble wrap to prevent direct contact with the sample. For shipping, pack samples in a sturdy box with some absorbent material and include a note about each bottle (where it was collected, date collected) and place the note in a sealed bag.

Seal the shipping box well – packages won’t be delivered if there are signs of leakage. Moorman suggests using overnight express delivery, and make sure the samples will be delivered on a day the lab is open.

On-farm testing is another option for growers, and while testing can confirm the presence of Pythium or Phytophthora, it won’t identify the species. “There are over 200 different species of Pythium and there may be that many of Phytophthora,” said Moorman. “In most cases, water can contain many species of Pythium, but most are not serious plant pathogens. Some are not plant pathogenic at all, a few are very weak pathogens, and a relatively small number are serious plant pathogens. The test kit will give you a positive regardless of which Pythium species is present. In the case of Phytophthora, most species are plant pathogens but some are weakly plant pathogenic.”

Moorman says it’s important to identify the species because of the host range of plants that species affects. “You may have a particular Phytophthora in your water, but it isn’t a threat to the crops you’re growing,” he said. “A test kit will only tell you that it’s present. It won’t give you enough information to further evaluate whether that’s important or not.” If testing yields consistently negative results, it’s possible that the target pathogen might not be present in the body of water being sampled.

Another sampling method is baiting, which involves putting organic matter in the water for a specific length of time to allow the target organism to colonize it, then testing the colonized bait. A variety of baits, including rhododendron leaves, can be used for Phytophthora. Moorman says it’s best to use whole leaves because cut surfaces can be quickly colonized by Pythium.

Bait leaves are placed in an onion bag, attached to a float and put in the water that’s going to be tested. “When you recover the baits, you can cut them up, plate them out and identify which organism you have,” said Moorman, adding that he uses creeping bentgrass to bait for Pythium. “You can also use fruits for bait. It’s well-known that apples and hard pears work well for Phytophthora. But you can use the leaf or fruit of whatever crop is having the problem.” Moorman added that it’s important to place the bait in a container that will prevent animals from accessing it.

The advantage of baiting is that the bait is submersed in the water and exposed to the pathogen for long enough that the pathogen is attracted to it, and is exposed to a larger volume of water than could be collected with a grab sample.

If bacterial plant pathogen is suspected, a filtering technique can be used to concentrate the bacteria, but requires a filter with a finer pore size than is used for Phytophthora. Moorman suggests using 0.22 micron filter for bacteria, then testing the filter paper with a commercially available kit that can identify a particular genus and species of that bacteria.

For growers who suspect a viral pathogen, one technique is to grow a plant known to be susceptible to that virus in a manner that would guarantee the plant would never be exposed to that virus while the plant is growing, then irrigate that plant using water from the source to be tested. Repeat irrigation over several days and wait for symptoms of the virus to appear. “There are kits for specific viruses, so once the plant shows symptoms, you can test the plant using a kit,” said Moorman. “Or you can send the infected test plant to a lab to verify that it is infected with that target virus.”

Moorman says it’s important to recognize that water is only one of many sources of plant pathogens and to know which pathogens may be causing problems. It’s also essential to know which tests need to be run, understand the results, and know how to use the results to solve the problem.

“The science of detecting and identifying plant pathogens in water is progressing but there are some big gaps in our techniques, especially gaps in techniques that can be used easily in the field by trained people and by farmers,” said Moorman. “There are some large gaps in getting from the sample to knowing exactly what species is or is not in that water.”