PQS

Stress Level of Pepper Plants Determines Whether Fusarium Infection Leads to Damage

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Excess Hydrogen Peroxide Is a Major Stress Factor

Interviewed by: Ank Van Lier. For: Onder Glas nr 1, January 2026

Fusarium frequently appears in pepper cultivation in drain water, the root system, or the substrate. Recent research shows that the presence of fungal spores does not always lead to problems or plant loss. Whether damage occurs largely depends on the stress level of the crop. One important culprit appears to be the use of stabilized hydrogen peroxide.

Fusarium has caused problems in pepper cultivation for several years. “This fungus leads to significant yield losses in production,” says Petra van der Goes of Plant Quality Solutions. She describes herself as a “forensic expert” in plant health and focuses on uncovering the underlying causes of crop problems.

In the case of Fusarium in peppers, Van der Goes observed something unusual last year. At one production site, she detected Fusarium spores in the drain water, root system, and substrate without the pepper plants showing any disease symptoms. “At another company where spores were found, wilting did occur, but not in the way typical for Fusarium. The roots were evenly light brown, while the central part of the roots was clean. In a Fusarium infection, this central tissue is also discolored. In addition, Fusarium can cause vascular bundles to become blocked, disrupting water transport. That was not the case here. Moreover, the plants were able to recover after adjusting the disinfection method, which is not possible with Fusarium. All of this raised many questions, and I wanted to find answers.”

Due to Fusarium, the vascular bundles become blocked and water transport is disrupted. As a result, the crop becomes limp and significant yield losses occur.

Fungal Spores, No Problems

At the beginning of this year, the plant expert therefore set up her own research. She was able to use the laboratory facilities of Valto Biocontrol, and propagation companies and growers also contributed financially. “My research showed, among other things, that Fusarium spores were regularly present in seed, but that young plants did not become diseased as a result.”

Even after the young plants were placed in a substrate, problems did not occur in most cases. Van der Goes conducted trials with pepper plants on rockwool, coconut, and a peat-based mix. “Although no disease symptoms appeared, I did find Fusarium spores in the drain water, the roots, and the substrate in all these cases.”

ABA and DKPs

At a certain point, however, things did go wrong with the pepper plants grown on rockwool. After a sudden change in weather, a typical Fusarium wilt developed. To explain this, the researcher had a drain water sample analyzed. “The results were striking. The sample contained high concentrations of abscisic acid (ABA). This stress hormone slows growth and regulates water balance. Plants produce this hormone when they experience stress, in this case due to the weather change. As a result, the plant becomes less resilient and sheds tissue, which explains the brown discoloration of the roots.”

In addition to the high levels of stress hormones, there was also a high concentration of diketopiperazines (DKPs). “These secondary metabolites disrupt the hormonal balance in the root zone and are therefore negative for plant growth. The plant itself produces a small amount of DKPs, but most of these metabolites are produced by microorganisms. The Fusarium fungus, among others, produces DKPs, and their production increases when the plant is under stress. These secondary metabolites, in turn, stimulate the production of the ABA stress hormone.”

Stress as a Trigger

The conclusion Van der Goes drew from this was that the presence of Fusarium does not always have major consequences for plant vitality. In other words, plants do not necessarily collapse when fungal spores are detected. “The fungus is often merely a passive inhabitant of the root zone and not always biologically relevant, as we call it. That only becomes the case when the crop is exposed to stress situations. Stress is the trigger for the development of problems and damage caused by Fusarium.”

According to Van der Goes, this creates a vicious circle. The higher the stress level, the more sugars the plant exudes through its roots. “These so-called ‘leaking’ sugars then form a nutrient source for microorganisms in the root system, such as Fusarium. This stimulates the fungus to become more active. If, during the fruiting phase, a high fruit load is added on top of this, it is often the final straw, and the crop loses its vitality and wilts.”

Too Much Hydrogen Peroxide

Why, then, have there been so many Fusarium problems in pepper cultivation in recent years? Which stress factors are responsible? “One major cause is that many pepper growers have significantly increased the use of stabilized hydrogen peroxide for disinfection, due to the loss of crop protection products and in an effort to keep Fusarium under control. However, hydrogen peroxide is an aggressive oxidant that causes considerable stress to the crop, especially on rockwool. With organic substrates, the problem is less severe because they can break down the peroxide more effectively. This partly explains why Fusarium problems are more pronounced on rockwool.”

In addition, the previously mentioned DKPs, which are also negative for plant growth, reach the plant more quickly in rockwool than in organic substrates. “This is because rockwool is an inert substrate, meaning DKPs bind less readily to other microorganisms.”

Closed cultivation systems also cause stress for the plant, Van der Goes notes. “Through recirculation, residues and negative elements such as DKPs are continuously circulated. You cannot get rid of them, and their concentrations build up. Pepper plants do not thrive under those conditions either.”

Preventing Stress

To prevent problems, it is therefore essential to keep the stress level of pepper plants as low as possible. But how can growers do this? “Limit the use of hydrogen peroxide as much as possible. It does more harm than good. We also need to move away from the idea that water must be 100 percent clean.”

She also advises enriching the substrate microbiome with biostimulants and biofungicides, which increases the crop’s resilience against the fungus. “It is especially important to have a high diversity of microorganisms present in the microbiome.”

Caution is also advised when reusing substrate. “Not only with rockwool, but also with, for example, coconut and peat-based substrates. Fusarium spores are present there as well and can build up with reuse.”

Fusarium-Free Seed as the Ultimate Solution

Finally, Van der Goes advises ensuring optimal crop management. “By that I mean supporting the plant when needed. If, for example, a major weather change is coming, provide extra amino acids and plant extracts to help the plant cope. This improves stress tolerance and reduces the likelihood that Fusarium spores will lead to problems.”

According to the researcher, the ultimate solution lies in seed that is free of Fusarium spores. “But that is difficult, because seed suppliers do not feel a sense of urgency. After all, the spores are not biologically relevant in young plants, as my trials also showed. As a result, seed companies do not see the need to address this issue.”

Summary

Fusarium spores are common in pepper cultivation, but research shows that damage only occurs when plants experience stress. In particular, high doses of stabilized hydrogen peroxide are a major stress factor, and recirculation also has a negative effect. Limiting the use of hydrogen peroxide, maintaining a diverse substrate microbiome, careful handling of substrate reuse, and good crop management can help prevent and reduce problems.

When Biostimulants Tip the Balance: Lessons from Callus Formation in Sensitive Crops

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Biostimulants are valuable tools in horticulture. They can improve rooting, enhance resilience, and support stress recovery. But just like with fertilizers or crop protection products, the right dose and the right combination are crucial.

In a recent case, callus tissue developed in young pot roses. Callus is disorganized, undifferentiated plant tissue — essentially the plant “switching on” regeneration mode. While this may sound beneficial, in practice it can lead to poor root development, growth abnormalities, and weaker plants.

After ruling out pathogens such as Agrobacterium tumefaciens, the focus turned to cultural practices and inputs. The analysis showed:

  • Electrical Conductivity (EC) was relatively high for rooting cuttings, limiting proper root initiation.
  • Recirculated irrigation water inadvertently carried multiple rounds of biostimulants to young plants.
  • Hormone-rich inputs such as certain seaweed extracts (rich in cytokinins) posed a moderate-to-high risk of callus formation, especially when combined with rooting powders or other stimulants.

Other biostimulants like vermicompost or amino acid mixes had low risk, but the synergy of multiple stimulants together created “too much of a good thing.”

Key takeaways:

  • Seaweed extracts are powerful, but overdosing or doubling up with similar products can unbalance auxin–cytokinin ratios, triggering callus instead of healthy roots.
  • Combining stimulants without clear strategy can cause antagonistic or unpredictable effects.
  • Safer, complementary combinations include worm compost + amino acids (for soil and nutrition) or chitosan + amino acids (for stress management).
  • In propagation, choose either a rooting powder or a hormonal biostimulant — rarely both.

The lesson? Biostimulants are not “the more, the better.” Used thoughtfully, they build plant strength. Used excessively or in the wrong mix, they can compromise the very growth they are meant to support.

Case Study: When Growing Conditions Tip the Balance in Xanthomonas Infections

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A few years ago, strawberry plants of a well-known cultivar were propagated under controlled conditions. Inspections during the season found nothing unusual — clean mother plants, a new tray field, no water recirculation, and routine checks all gave the green light.

Yet once plants were distributed, a puzzling pattern emerged. Some growers reported heavy outbreaks of Xanthomonas fragariae, while others with plants from the very same source saw little to no issue.

Why such a difference? The answer lies not only in the presence of the bacterium, but in the growing environment that favors its development.

  • The bacterium enters leaves through stomata and thrives in high humidity with free moisture on foliage.
  • Temperatures of 18–24 °C accelerate its growth.
  • Young, tender leaves are especially vulnerable.
  • High nitrogen fertilization makes plants lush, but also more susceptible.
  • Water splash and crop handling can move the bacterium from plant to plant.

In some fields, these conditions lined up perfectly — turning a hidden, symptomless infection into a rapid epidemic. In others, the environment was less favorable, and the disease failed to establish.

Management lessons from this case:

  • Even when plant material appears “clean,” opportunistic pathogens can strike if conditions align.
  • Strict sanitation, careful nitrogen management, and minimizing leaf wetness are essential preventive steps.
  • Early detection through scouting and diagnostic testing helps prevent widespread spread.

The key insight: Xanthomonas infections are as much about the crop’s environment as about the pathogen itself. A clean start is important, but staying clean requires vigilance against the growing conditions that give the bacterium its opportunity.

How Risk Assessment Helped Uncover the Source of a Plant Health Crisis

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In a recent project, large-scale plant losses in asparagus production raised urgent questions. Symptoms included shortened and fibrous root crowns, cracked and discolored roots, unpleasant odors, and disrupted stem growth. Lab analysis revealed heavy infections with Fusarium oxysporum and Cylindrocarpon didymum. Although Phytophthora was tested, it was not detected. The strong odor suggested a secondary Erwinia infection.

A structured risk assessment was key to moving beyond symptoms and lab results. During the sessions, we traced the most likely origin of the widespread issue back to a practice a few years earlier: shredding crop residues on the windward side of the growing area. This led to contaminated dust entering the basins and irrigation system. While disinfection systems were in place, no method is 100% effective, especially when particle loads are high and water turbidity reduces efficacy. This provided a pathway for infection across the entire facility.

The assessment also highlighted contributing factors:

  • Prohibition of certain chemical seed treatments, leaving only water-dipping as a preventive step.
  • Movement of plants between plots, increasing cross-contamination risk.
  • An existing background level of infection, amplifying new outbreaks.

With this understanding, we designed a multi-step plan of action:

  • Knowledge transfer on pathogen lifecycles and symptom recognition.
  • Trials combining nutrition strategies with biological controls (“measure to manage”).
  • Revision of hygiene protocols, co-created with cultivation staff for ownership.
  • Optimized plant flow to protect young seedlings.
  • Field sanitation and soil reset trials.
  • Treatment strategies for new plants entering production.
  • Ongoing training in pathogen identification, supported by lab analysis.

Next to tackling the plant health issues, fertilization practices were also adjusted to better support root health and resilience. The results spoke for themselves: already in the first year after implementation, 80% fewer issues were recorded.

The outcome of this process was not only the identification of the likely cause but also a clear pathway to reduce risks and prevent recurrence. By combining scientific analysis with practical on-site risk assessment, it became possible to transform a widespread challenge into a structured recovery plan.

Part 1 Plants Don’t Whisper – They Broadcast

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We tend to think of plants as passive. In truth, they’re broadcasting signals all the time. We just have to tune in.

Brix as an early-warning signal

How sugar concentration reflects photosynthesis and plant energy.

Role of sugars in plants

Sugars in plants are formed through photosynthesis, a process where plants use light energy, water and carbon dioxide to create glucose and oxygen. Here is how it happens:

Formation of sugars in leaves: the process occurs in the chloroplasts of plant cells, primarily in leaves. Light is absorbed by chlorophyl, the green pigment, and converst carbon dioxide and water into glucose and oxygen. During daytime, the about of sugars formed is highest between 2 pm and 4 pm.
After glucose is produced in the leaves, it is often converted into sucrose (a disachharide sugar( for transport through the plant. Sucrose ios more stable and can be moved mopre efficiently over long distances.
Sucrose is tranpotered through the phloem vessels to different parts of the plant, including roots, stems, flowers, and growing parts. This movement is driven by pressure differences between source (leaves) and sink (growing tissues) areas.
Some of the sugars may be stored in roots or other storage tissues as starch, which can be converted back to sugar when needed.

Sugars are used for energy in respiration, a process that breaks down glucose into energy (ATP) that powers cellular activities, including growth.

Sugars also serve as building blocks for other compounds like cellulose, which makes up the cell wall, or amino acids, which are used to build proteins. This is crucial for the growth of new tissues, including leaves, stems, and flowers.

Brix is a scale used to measure the concentration of dissolved sugars in a liquid, particularly in plant sap or fruit juice. It represents the percentage of sucrose by weight in the solution.

VOCs – scent messages

Plants emit volatile organic compounds to warn neighbors or call in help.

Plants have developed sophisticated communication mechanisms to respond to environmental stimuli, interact with each other, and even defend against threats. Two major ways plants send messages are through volatile organic compounds (VOCs) released into the air and root exudates released into the soil.

How VOC’s work….

When a plant is under stress, such as from herbivore attack, mechanical damage, or environmental stress, it may release VOCs into the atmosphere. These compounds can be emitted by leaves, stems, or flowers. There are different types of VOC’s
Herbivore-Induced Volatiles: These are released when a plant is grazed or damaged by herbivores. They act as distress signals to nearby plants, which may then activate their own defense mechanisms (e.g., producing chemicals to deter herbivores or attract natural predators of herbivores).
Plant-Plant Communication: Plants can also release VOCs that alert nearby plants about environmental changes, like drought or temperature stress, allowing neighboring plants to initiate protective responses.
Attracting Pollinators: Some plants release pleasant-smelling volatiles to attract pollinators like bees, butterflies, or birds.

VOCs act as airborne signals that are detected by receptors in other plants or organisms (like insects). These signals can trigger defence responses and attract beneficial organisms, providing an indirect defence mechanism.

Root exudates

Plants use soil chemistry to communicate with microbes and other plants.

Root exudates are compounds released by plant roots into the surrounding soil. These exudates are a form of communication with the soil microbiome, other plants, and even herbivores.

How root exudates work….

Roots excrete a variety of substances, including sugars, amino acids, organic acids, phenolic compounds, and secondary metabolites. This process can occur under normal conditions or in response to specific stimuli (like pathogen attack or nutrient deficiency).
Nutrient-Rich Compounds: Plants excrete sugars, amino acids, and organic acids to nourish beneficial microbes like mycorrhizal fungi, which help with nutrient uptake (especially phosphorus).
Allelopathic Compounds: Some plants release chemicals into the soil that inhibit the growth of nearby plants, reducing competition for resources (this is known as allelopathy).
Signal Molecules: In response to pathogens or other stressors, plants may release signaling molecules (like jasmonic acid or salicylic acid), which influence the microbial community around their roots and help in activating defense pathways.

Plants don’t only use one form of communication – they often use both. An example: when plants release VOC’s in response to herbivory, these signals can also affect microbial populations in the soil around the roots, increasing the plant’s resistance to pathogens or pests.

Electrical signalling

Plants send action alerts from leaf to leaf, preparing tissues for defence.

While volatiles and root exudates are primarily chemical signals, they can be translated into electrical signals through the plant’s biochemical processes. The key translation happens through ion fluxes across cell membranes, which alters the plant’s electropotential and generates electrical currents that propagate through the plant. These electrical signals are crucial for the plant’s response to environmental changes, herbivory, and pathogen attacks, allowing it to activate defense mechanisms or adjust growth processes accordingly.

Practical takeaway for growers

Start tracking brix levels and noting changes in plant appearance before visible damage. Even small shifts can mean stress.

“A plant in distress is never silent — the skill is in learning its language.”

Wilting Strawberry Plants? Don’t Just Blame Neopestalotiopsis

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Positive PCR but no active growing Neopestalotiopsis

When strawberry plants start to wilt, it’s easy to point the finger at Neopestalotiopsis. Especially when PCR results come back positive. But let’s take a moment to understand what that actually means. PCR confirms the presence of DNA, not necessarily a living, active fungus. So yes, Neopestalotiopsis might be there – or it was there – but it’s not the whole story.

Plating Neopestalotiopsis

When we plate these plants, we often see a much more complex picture. Multiple pathogens show up: Fusarium spp., Colletotrichum, Cylindrocarpon, Phytophthora, and Neopestalotiopsis. In other words, this is not a single-pathogen issue. We’re dealing with a disease complex – and more importantly, a root rot – above ground disease complex.

Focusing only on leaf infections caused by Neopestalotiopsis could lead you to overlook the more serious threat lurking below ground. Root rot quietly wipes out your plants, and by the time it becomes visible, it’s often too late.

Going heavy on fungicides might seem like a quick fix, but be careful, this can backfire. Overapplication leads to extra plant stress, and stressed plants are more vulnerable to root pathogens. Plants can’t fight on two fronts at once. If all their energy goes into managing chemical stress and foliar infections, the roots become an easy target.

I have been analysing plants in the Netherlands as well as Canada. They tell the same story. We have a disease complex, not just Neo-P.

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