In response to the emergence of what has become known as ‘pepper wilting’ in high-tech greenhouses, Grodan initiated and contributed to extensive international research to better understand the complex interaction between pathogen presence, plant stress and hygiene practices. Findings from this research indicate that the presence of oxidising agents, which are primarily applied for the control of biofilm within irrigation lines, may act both as a trigger and as a modifier of symptom expression. This is especially the case if the oxidising agents enter the root zone in the presence of the opportunistic and aggressive pathogen Fusarium oxysporum f. sp. radicis-capsici (FORC) or others from within the family described as Fusarium Oxysporum Species Complex (FOSC). Based on these insights, Grodan has issued new hygiene-related advice to help growers avoid unintentionally making their crops more susceptible.
Pepper wilting is a symptom where pepper plants lose rigidity due to disrupted water uptake or damage to the plant’s vascular system. Pepper wilting is observed in plants grown on both stone wool and organic substrates, prompting the Grodan team to investigate what might be modifying symptom expression (Photo 1). Although the microbiomes of the two substrates differ, experts agree that neither microbiome is inherently inferior to the other. But the question remained: how and why was the pathogen taking hold so severely in some cases?
Photo 1. Symptoms of Fusarium oxysporum f. sp. radicis capsici (FOSC) in pepper plants grown in two substrates at Vineland (Canada): stone wool (left) and coco coir (right).
The lack of scientific insight into the exact cause of the pepper wilting situation was creating uncertainty and confusion within the industry. “As a company, we are committed to supporting growers by providing evidence-based insights to guide their decision-making. Therefore, we proactively embarked on an extensive international research programme aimed at filling the knowledge gaps based on a scientific approach,” says Andrew Lee, Head of Crop Advice, Europe and Asia at Grodan.
Independent research in search of evidence
Besides participating in several externally led consortia trials, Grodan initiated and funded its own independent research studies. These were set up in collaboration with respected organisations such as Wageningen University Research (WUR), Vertify and Botany BV in the Netherlands, and Vineland Innovation & Research Centre in Canada. “To link our theories to real-world practice, we also teamed up with growers who had experienced pepper wilting symptom expression, obtaining samples of slabs and plant materials from them for analysis and further investigation,” explains Frank Janssen, Grodan’s R&D Manager.
Growers’ survey
As part of the research programme, Grodan conducted a thorough survey, comprising around 70 questions, among growers in Benelux and Canada to gain detailed insight into their habits and behaviours. “Not all growers in the survey were experiencing wilting issues. However, we asked all growers the same questions related to their operations – from their climate and irrigation strategies to their use of plant protection products,” adds Janssen.
The survey also included specific questions about how they maintained water hygiene and control of biofilm within the irrigation system itself. “Interestingly, all growers who had witnessed pepper wilting since 2022 or were currently experiencing symptoms used oxidising agents at detectable concentrations at the dripper,” he continues. This strengthened the team’s suspicions about the role that oxidising agents were playing in pepper wilting.
Research trial setup
The researchers adopted the hypothesis that oxidising agents entering the root zone may both initiate and increase symptom expression. In their first trial, they applied a non-stabilised oxidising agent (hydrogen peroxide) to virgin stone wool (i.e. containing no plants and no roots) and to virgin organic substrates under identical controlled conditions.
The results demonstrated that substrate composition influences the rate at which hydrogen peroxide breaks down. In substrates with higher levels of organic matter, the oxidising agent degraded more rapidly, whereas in more inert substrates it remained chemically active for a longer period. These findings suggest that substrate characteristics can influence the duration of root tissue exposure to oxidising agents.
“Longer exposure to peroxide in stone wool increases the risk of harm to the root epidermal tissue and root hairs, and damaged roots provide a pathway for any pathogens that may be present in the root zone to take hold,” he continues.
“However, this does not mean that organic substrates are safer for peroxide treatment. Importantly, faster degradation in organic substrates does not eliminate the potential for root stress,” explains Lee. “The research makes clear that contact time only modifies symptom severity, not the fundamental toxicity of the oxidising agent. In other words, an oxidising agent is still damaging the roots, exposing them to risk of infection from disease.”
Lee also suggests that growers should think about why they are adding hydrogen peroxide or a similar treatment in the first place: “The aim is to remove organic material from within the water system, so adding it via your substrate choice is counterintuitive. And of course, an organic substrate offers no protection against the pathogen FORC or FOSC.”
Root damage even at low concentrations
Next, the team infected young plants with FORC and tested different concentrations of hydrogen peroxide, measuring the subsequent plant health based on the nitrogen balance index in the leaves. “After inoculation with FORC, we saw an immediate stress response in the plant. The stress response was heightened when dosing just a small amount of hydrogen peroxide,” states Janssen.
Moreover, by comparing the application of hydrogen peroxide in various concentrations, the team noticed that symptom expression was also being modified by dose concentration. The trial showed that the FORC-inoculated plants receiving 50 ppm doses of hydrogen peroxide were the first to exhibit wilting symptoms. Plants receiving 100 ppm of peroxide were the next to wilt. Those receiving a 200 ppm treatment expressed symptoms of wilting a week later, followed by FORC-inoculated plants that had not received any doses of hydrogen peroxide. “This non‑linear dose interaction was a pivotal insight and enabled us to further refine our hypothesis based on the evidence,” he continues.
“At 50 ppm, the concentration of oxidising agent was clearly not enough to kill the aggressive pathogen FORC. However, it was causing damage to the epidermal root tissue and root hairs, therefore weakening the plant’s natural defence system, leading to faster wilting and ultimately death as the aggressive fungal pathogen progressed rapidly,” says Lee.
Photo 2. Pepper plants showing wilting severity under increasing hydrogen peroxide dosages, displayed left to right: 0, 50, 100, and 200 ppm respectively. On the left plants in an early stage and on the right plants after 4 weeks.
“The roots were also damaged by a 200 ppm concentration of oxidising agent, but at this concentration it was also killing or at least suppressing the FORC. That’s why it took longer for those plants to die. At the other extreme, when no oxidising agent was applied, FORC was still attacking the plants but, since there was no compounding effect of the oxidising agent continuously damaging the roots, it took longer for them to die,” he explains.
Photo 3. Microscopic images (x100) of root tissue damage on pepper plants (Alzamora RZ) growing in stone wool blocks when dosing stabilized hydrogen peroxide at 50 ppm in an ebb-flood system.
Furthermore, the presence of oxidising agents in the root zone may significantly disrupt the root-zone microbiome, a system that is essential for overall plant resilience. When oxidising agents are applied repeatedly, they can also reduce beneficial microbial populations, destabilise the rhizosphere, and impair critical processes such as natural disease suppression.
Rethinking common hygiene practices
The dosing of oxidising agents is common practice as part of water hygiene routines in greenhouse cultivation. “They do indeed help prevent biofilm build-up in irrigation systems, providing that these products are used strictly for system disinfection, following legal regulations and the supplier’s instructions. In that context, hydrogen peroxide can be an appropriate sanitation tool because it breaks down into water and oxygen after completing its disinfecting action,” says Lee.
“However, this does not mean that dosing oxidising agents into the active root zone is safe or beneficial. In the substrate, these agents do not simply ‘add oxygen’; they oxidise and damage root tissue, weaken root hairs and make the plant more vulnerable to pathogen ingress. The same chemical reactivity that helps clean irrigation lines becomes harmful when it comes into direct contact with living roots. Therefore, we recommend avoiding measurable concentrations of any oxidising agent in the root zone during cultivation,” he emphasises.
Moreover, due to the uncertainty around pepper wilting, growers have been encouraged in recent years to increase the dosage level of oxidising agents as an extra preventive measure, just in case any pathogens are present. “Similarly, other studies conducted at WUR investigating the use of oxidising agents in the cultivation of gerbera show that they have no effect on plant pathogens (Fusarium) at concentrations below 160 ppm – levels that are far higher than those required to control biofilm or indeed cause root damage,” adds Lee.
Reduced efficacy of plant protection products
A fast-growing pathogen such as FORC or FOSC can soon cause plants to wilt and die, which is why growers experiencing pepper wilting often apply plant protection products (PPPs). “But if they continue to dose oxidising agents afterwards, they will create two contradictory actions. On the one hand, they are adding PPPs to combat the pathogen. But on the other, the oxidising agent will continue to damage the root epidermal tissue and root hairs, reducing PPP efficacy,” he adds.
Recommendations for growers
Overall, the findings suggest that certain hygiene practices, although well-intended, may have contributed to increased plant vulnerability under specific conditions in the root zone. Based on these insights, Grodan has further clarified its guidance regarding the appropriate use of oxidising agents within irrigation systems. “We are not saying that oxidising agents should not be used. Indeed, they can be very effective at controlling biofilm when applied correctly and in line with legal regulations and supplier instructions, although there are alternative technologies available,” states Janssen.
“However, based on the new findings, it is important to ensure that oxidising agents used for system disinfection do not result in measurable concentrations in the active root zone,” adds Lee. This means avoiding measurable concentrations of oxidising agents at the dripper. One practical solution would be the installation of a return loop in the irrigation system and then only dosing the oxidising agents at the end of the day.
“Growers have expressed their appreciation at gaining greater clarity on an important contributing factor. We are continuing to monitor the reactions from the market and, where necessary, we will further fine-tune our guidance to reduce the risk of pepper wilting and to support resilient crop development under high-tech growing conditions,” concludes Lee.
For further information or tailored guidance, please contact your Grodan advisor.
