Is Your Water a Source of Plant Disease? How to Sample and Find Out.

Old rustic water pipe with running drinking water and plastic bottle being filled up, aid concept.

After reading the previous posts in this series, you’re now aware of WHICH pathogens can be spread through your irrigation water, WHERE they can accumulate, and WHY that’s important.  We’re now moving on to posts covering WHAT you can do about it!

This post will walk you through how to sample water sources on your farm, and which tests you can run to determine if your water is helping – or hurting – your crop.

This is the third article in a series about water sanitation. The goal of this series is to get you reflecting on your own irrigation system before you are faced with a problem.  The first post covered where problems are likely to occur in your greenhouse ; the second  covered the types of pathogens found in water . Posts over the next few weeks will cover interpreting lab tests, on-farm disease monitoring, and water treatment options.  These will be good refresher resources, so make sure to bookmark them.

Water Testing Labs

Several labs in Ontario will process water samples, and check for everything from nutrient levels, to pesticide residues, to fungal and bacterial loads. Most sample for pathogens using a “DNA multi-scan”, which just means they are looking bits of DNA from a wide range of known pathogenic species.  Labs currently accepting grower water samples include:

Lab Services, University of Guelph (

SGS-Agri Food Labs (

A&L Canada Laboratories (

But part of getting useful results from these labs is knowing WHERE and HOW to properly sample.  We’ve compiled a list of do’s and don’ts to help you get started.

Water Sampling – DO’s

Checking your water for pathogens isn’t something you should be doing ONLY when you think you have a disease problem. Just like regular monitoring for insect pests in your IPM program, checking your water should be part of your regular integrated disease management (IDM).

You’ll also want to check different points along your irrigation system to determine which water sources are potentially contaminated, and how this is flowing through your greenhouse.

When it comes to WHEN, WHERE and HOW to sample, consider the following DO’s.

1. DO monitor at frequencies that relate to the level of risk.  These include:
Different points in the production cycle (especially before a new crop goes in); when changes in source water quality occur (e.g. switching from roof water to water stored in a cistern), when sensitive crops are present (based crop type and/or stage – this is why knowing your crop’s susceptibility to diseases BEFORE you plant is so important); when labour is available (we know NO ONE is going to sample water in the middle of the Mother’s Day rush).

A long sampling pole can help you grab samples from ponds and cisterns that are difficult to access.

2. DO Sample from ALL sources of water, and from various points along your irrigation system. This can reveal potential sources of contamination, and gives you a picture of how pathogens are  spreading throughout your farm. Testing along your irrigation system can also reveal issues with water pressure and distribution which may be causing wet or dry spots in the crop which can also lead to disease problems. Try to sample from locations that are close to where the water enters the production area (e.g. in the first few rows or benches) and those that are near the end of the line to get a better idea of the problems you might be facing.

3. DO sample recirculating water BEFORE and AFTER water treatment. This tells you if your chosen water treatment strategy is actually working like it’s supposed to. NEVER assume that it’s functioning properly!  All systems need maintenance, and some need proper levels of active ingredient(s) to work, which can change frequently depending on the amount of organic matter in the system.  Be safe not sorry, and follow a regular testing schedule to maintain your investment.

water sampling infographic
Various points along your irrigation system you should sample regularly. To truly understand your water quality throughout the farm, it’s better to sample from multiple points at once than one point frequently.  Diagram by C. Dayboll.

4. DO follow the guidelines suggested by your chosen lab. They’ve outlined these to make sure get you the BEST results.  For example, the University of Guelph Lab services has specific guidelines for collecting water samples as well as general shipping/handling guidelines.  Make sure to read over these thoroughly BEFORE collecting and sending samples. In a pinch, you can use disposable water bottles to collect samples, but ensure they rinsed with your source water well before taking a sample. Never use vessels that were used for another purpose (e.g. scoops for mixing fertilizer, your morning coffee cup of choice) to grab samples as they may leave residue in your sample that can affect your results.

Make sure your water samples are well labeled so you can interpret the results.  Include range numbers or locations to help you remember what was sampled.

5. DO have a consultant or specialist help you interpret the results of any tests. They can help you figure out which pathogens are pests of concern, and what to do about them. More on the interpretation of various tests in coming up in the next blog post!

Water Sampling – DON’Ts

What NOT to do can be as important as what to do.  Consider these don’ts and make sure to think of others that may apply to your own farm.

1.DON’T leave sampling until the last minute. Sampling should be a proactive activity, not a reactive one. But if you’re like many of us, a lack of time and too much on your plate can mean that your water samples are taken after crops have gone in, or you spot something that it’s right. Schedule regular water sampling into your calendar (or a trusted employee’s calendar!) and make sure it gets done ahead of sensitive crop cycles or your big spring season.

2.DON’T be afraid to send in several samples at once, despite the cost. Whether you like to do a more through check of your irrigation systems twice a year, or you are facing a problem and don’t know where to start, more samples can help you pinpoint a problem more accurately in the greenhouse, saving you time and headaches.

3. DON’T forget to factor in costs of regular sampling into your pest management costs.  DNA multiscan tests (that sample for all potential pathogens down to species) cost around $250/sample; a supply of 50 Petri films (see more on this below; used for general counts of unspecified fungi in water) cost $135.  This is a drop in the bucket compared to the 10-50% crop losses I’ve seen due to pathogens in recirculating water, so make room for them in your pest control budget.  If you’re experiencing an active pathogen issue, and aren’t sure of the source or how to treat, OMAFRA specialists may be able to cover some of the testing costs to help you figure out the problem.

On-Farm Water Testing

As a compliment to lab testing, on-farm testing protocols have also been developed by the Soil Research Group, as part of a Canadian Agriculture Partnership project.  These methods use adapted Petri dishes, called “Petri-films” to grow fungi that may be in your water. Although more labour intensive and less specific than lab testing, the Petri-film method is relatively cheap and allows for rapid results and repeated testing.  More on this method and it’s advantages (and disadvantages) will be discussed in an upcoming post in this series.


A 3M Yeast and Mold Petri Film growing various fungi. These films can be used to help quantify the amount of fungal pathogens in your irrigation water, and are a useful component of regular water testing.





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