"Are You Sure You're Clean?" What a Greenhouse Clean-Out Removes, and What It Can Leave Behind

"Are You Sure You're Clean?" What a Greenhouse Clean-Out Removes, and What It Can Leave Behind

Crop changeover is the one time in the year when the whole house is empty and every surface, line and tank can be cleaned and disinfected. Done well, it lowers the disease carried into the next crop. It can't promise a perfectly clean slate, though. Some pathogens stay infectious for months on greenhouse surfaces, and Pythium can persist in plumbing even after disinfection.

It's also expensive to get wrong. At an industry session reported by Hortidaily in September 2026, the president of an Ontario greenhouse company described an outbreak of tomato brown rugose fruit virus (ToBRFV) across its 28-hectare facility. He put the clean-up cost close to $3 million, before counting the production lost while the greenhouse was replanted. When the bill is that size, "we cleaned" and "we're clean" need to mean the same thing.

Where pathogens hide after a clean-out

A good clean-out removes a lot. Even so, some pathogens can stay in places cleaning does not reach, or survive until the next crop.

Hard surfaces

Researchers at Fera Science in the UK (Skelton et al., 2023) found that, in their lab tests, ToBRFV stayed infectious for at least 7 days on every common greenhouse surface they tested: steel, aluminium, hard plastic, polythene, glass and concrete. On glass, hard plastic and polythene it was still infectious after 6 months. (They used a heavy coat of infected plant sap, so real surfaces may carry less.)

The same team found that contact time (how long the disinfectant stays wet on the surface) and the surface itself both matter. None of the first four disinfectants they tried worked with only one minute of contact. With longer contact, some products worked on every surface, but several did not work reliably on concrete. Heat also helped: soaking plastic trays in 90 °C water for 5 minutes stopped the virus, but 70 °C did not.

Not every disinfectant works on these viruses. US Department of Agriculture researchers (Chanda et al., 2021) tested 16 products against ToBRFV, using the kind of quick dip used for hands and tools. A few stopped nearly all spread, while others did little. An earlier study by the same group (Li et al., 2015) found the same spread of results for three other tomato viruses.

Check: walls, floors and concrete walkways, gutters, trolley wheels, harvest carts, and any crates and clips that get reused. Give each disinfectant the full contact time on its label.

Irrigation systems

Some pathogens spread through the water lines, tanks, drippers and filters that feed the crop. BC's agriculture ministry (Sabaratnam, 2024) describes Pythium as a fungus-like organism, commonly called a water mould, that releases "hundreds of swimming zoospores" that move through water.

Closer to home, a review by Sutton and colleagues at the University of Guelph (2006) found that pipes, tubing, tanks and other plumbing in hydroponic systems are "often potential sources" of Pythium in the next crop, even when the systems are treated with disinfectants. From their own observations, the authors also report Pythium resting spores in the slimy film that builds up inside plumbing.

Check: source water, stock tanks, the lines that bring drain water back for reuse, and the far ends of drip lines.

Substrate and plant debris

Root bits, leaf litter in corners and reused substrate can all carry disease into the next crop. Pythium forms tough resting spores on decaying roots. BC's agriculture ministry (Sabaratnam, 2024) says these can survive "prolonged adverse conditions in soil, greenhouse growing media and water." The Guelph review (Sutton et al., 2006) adds that reusing rockwool or coconut fibre slabs without treating them lets pathogens carry over to the next crop.

Viruses linger in debris too. Researchers at the Volcani Center, an agricultural research institute (Dombrovsky et al., 2022) note that ToBRFV and its close relatives (the tobamovirus group) can stay infectious for months in soil and plant debris. Ontario researchers (Fougere et al., 2026) give growers plain advice: "Do not replant into infected growing media."

Check: any substrate you plan to reuse, debris under benches and slabs, and drain channels.

What comes in the door

A clean greenhouse can pick up a pathogen again on the first day. BC's agriculture ministry (2024) says ToBRFV is seed-borne, so "both seeds and transplants are the major pathways" for bringing it in. Researchers at the University of Palermo (Davino et al., 2020) found the virus sits on the seed coat. Only about 2 or 3 in every 100 seedlings from infected seed picked it up. But, citing earlier work, the authors note that the virus spreads so easily from plant to plant that a few infected plants can be enough to infect a whole house within months.

People carry it too. The Fera team (Skelton et al., 2023) found ToBRFV stayed infectious on bare hands and on gloves for at least 2 hours, and a 30-second hand wash did not reliably remove it. They recommend gloves, changed often, alongside hand washing. The Canadian Food Inspection Agency (CFIA, 2019) tells growers to use "seeds and seedlings from trusted sources."

Check: incoming transplants and seed lots, plus glove, hand-washing and visitor rules for the first weeks of the new crop.

For Propagators

If you raise transplants for other growers, your clean start is also your customers' clean start. Seed and transplants are a major way ToBRFV moves into a new greenhouse, so hygiene in the propagation house carries forward into every greenhouse you ship to.

Ontario researchers (Fougere et al., 2026) put it plainly: "Thorough clean outs between crops are vital." They also advise growers to "use as much resistant seed as possible," and report that seed companies have become better at screening their seed sources.

Water moulds matter in propagation too. BC's ministry (Sabaratnam, 2024) notes that Pythium can cause damping-off (young seedlings collapsing at the base), and that it can come into a greenhouse in plug transplants, growing media and irrigation water. Its advice includes a thorough year-end clean-up of surfaces, tanks and water lines, and "sterile propagating media." The Guelph review (Sutton et al., 2006) adds, from the authors' own observations, that infected transplants often look healthy when they ship.

How to Check You're Clean

A greenhouse can look clean while pathogens remain. Testing after the clean-out checks what your eyes cannot:

  1. Swab the high-risk surfaces after cleaning and drying, before planting. Focus on the places listed above. (For viruses such as ToBRFV, read the next section first.)
  2. Sample the water from the source, the tank and the far end of the lines.
  3. Test incoming plant material before it goes into the clean house.
  4. Re-check early in the new crop, once plants have been in for a few weeks. This matters most in houses with past problems.

One Catch: "Detected" Doesn't Always Mean "Alive"

Some tests can pick up traces left by pathogens that are already dead. So a positive swab doesn't automatically mean the clean-out failed.

The Fera team (Skelton et al., 2023) showed this for ToBRFV. After disinfection, swabs still tested positive, even when the virus could no longer infect plants. Seed researchers in Italy (Davino et al., 2020) saw the same thing on treated seed.

For bacteria, fungi and water moulds, there are tests that can tell live organisms apart from dead remnants.

These live-versus-dead tests are not offered for viruses, including ToBRFV. If a virus shows up after cleaning, the result needs careful reading. Look at what was sampled, where it was taken, and how soon after disinfection.

Before You Replant: a Short Checklist

Ontario's agriculture ministry (LeBoeuf et al., 2022), in its guidance on bacterial diseases of tomato, recommends removing all plant material before starting a new crop. It also recommends disinfecting racks, tools, equipment and greenhouse surfaces. The checklist below adds testing, checks on incoming plants and hygiene rules to those cleaning steps:

  • ☐ Plant material and debris fully removed, including under benches and in drains
  • ☐ Surfaces cleaned, then disinfected with a product suited to each surface type, for the full label contact time
  • ☐ Irrigation lines, tanks and filters flushed and disinfected
  • ☐ High-risk surfaces and water sampled after disinfection (for viruses, see "detected doesn't always mean alive")
  • ☐ Results back, and any positives understood before planting
  • ☐ Propagating media new or sterile
  • ☐ Incoming transplants and seed checked
  • ☐ Glove, hand-washing and visitor rules set for the first weeks of the new crop

References

British Columbia Ministry of Agriculture and Food. (2024, June). Emerging virus diseases of greenhouse vegetables [Factsheet]. https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/agriculture-and-seafood/animal-and-crops/plant-health/disease-factsheets/greenhouse-vegetables/emerging_virus_diseases_of_greenhouse_vegetables.pdf

Canadian Food Inspection Agency. (2019, November 29). Tomato brown rugose fruit virus. https://inspection.canada.ca/en/plant-health/invasive-species/plant-diseases/tobrfv

Chanda, B., Shamimuzzaman, M., Gilliard, A., & Ling, K.-S. (2021). Effectiveness of disinfectants against the spread of tobamoviruses: Tomato brown rugose fruit virus and cucumber green mottle mosaic virus. Virology Journal, 18, Article 7. https://doi.org/10.1186/s12985-020-01479-8

Davino, S., Caruso, A. G., Bertacca, S., Barone, S., & Panno, S. (2020). Tomato brown rugose fruit virus: Seed transmission rate and efficacy of different seed disinfection treatments. Plants, 9(11), Article 1615. https://doi.org/10.3390/plants9111615

Dombrovsky, A., Mor, N., Gantz, S., Lachman, O., & Smith, E. (2022). Disinfection efficacy of tobamovirus-contaminated soil in greenhouse-grown crops. Horticulturae, 8(7), Article 563. https://doi.org/10.3390/horticulturae8070563

Fougere, G., Griffiths, J., & McCreary, C. (2026, January 21). PART 2. Genomic diversity of ToBRFV: Is it changing in Canada? ONgreenhouse vegetables. https://ongreenhousevegetables.ca/2026/01/21/part-2-genomic-diversity-of-tobrfv-is-it-changing-in-canada/

Hortidaily. (2026, September 17). Practical takeaways from Canada's greenhouse sector. https://www.hortidaily.com/article/9873077/practical-takeaways-from-canada-s-greenhouse-sector/

LeBoeuf, J., Celetti, M., Cuppels, D., Dick, J., Pitblado, R., & Loewen, S. (2022, January 18). Bacterial diseases of tomato: Bacterial spot, bacterial speck and bacterial canker. Ontario Ministry of Agriculture, Food and Agribusiness. https://www.ontario.ca/page/bacterial-diseases-tomato-bacterial-spot-bacterial-speck-and-bacterial-canker

Li, R., Baysal-Gurel, F., Abdo, Z., Miller, S. A., & Ling, K.-S. (2015). Evaluation of disinfectants to prevent mechanical transmission of viruses and a viroid in greenhouse tomato production. Virology Journal, 12, Article 5. https://doi.org/10.1186/s12985-014-0237-5

Sabaratnam, S. (2024, June). Pythium diseases of greenhouse vegetables [Factsheet]. BC Ministry of Agriculture and Food. https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/agriculture-and-seafood/animal-and-crops/plant-health/disease-factsheets/greenhouse-vegetables/pythium_diseases_of_greenhouse_vegetables.pdf

Skelton, A., Frew, L., Ward, R., Hodgson, R., Forde, S., McDonough, S., Webster, G., Chisnall, K., Mynett, M., Buxton-Kirk, A., Fowkes, A. R., Weekes, R., & Fox, A. (2023). Tomato brown rugose fruit virus: Survival and disinfection efficacy on common glasshouse surfaces. Viruses, 15(10), 2076. https://doi.org/10.3390/v15102076

Sutton, J. C., Sopher, C. R., Owen-Going, T. N., Liu, W., Grodzinski, B., Hall, J. C., & Benchimol, R. L. (2006). Etiology and epidemiology of Pythium root rot in hydroponic crops: Current knowledge and perspectives. Summa Phytopathologica, 32(4), 307-321. https://doi.org/10.1590/S0100-54052006000400001

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