Home Posts Tagged "diseases"


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Researchers have found three species of predatory bug in the Miridae family that can limit the establishment of the harmful predatory bug Nesidiocoris tenuis in tomato crops. “Omnivorous predatory bugs could potentially be a great help in biological pest control,” researcher Gerben Messelink says.

In recent years the bug Nesidiocoris tenuis has become a serious problem in tomato growing. Although it controls whitefly and tomato leaf miner, the insect causes so much damage to the crop that growers tend to view it as a pest rather than a predator. These Mediterranean plunderers puncture fruits and flowers and cause necrotic rings which lead to stems breaking off. They inhibit plant growth, resulting in deformed plants. Tips fall off and production stagnates.

“Tomato growers in southern Europe have been using Nesi to control Tuta absoluta for years,” says researcher Gerben Messelink of Wageningen University & Research’s greenhouse horticulture business unit in Bleiswijk, the Netherlands. “Various other predators such as nematodes and parasitic wasps have been tried as well, but Nesi appears to be the most effective. Southern Spanish growers therefore mainly see this creature as a useful predator.”

But things are different hundreds of kilometres to the north, where Nesi is a serious problem for Dutch tomato growers. Crop damage is severe and the predatory bug is difficult to control selectively. Chemicals also have an adverse effect on Macrolophus, a relative of Nesi, giving pests such as whitefly free rein and disrupting the whole biological control system in the greenhouse. What’s more, this creature develops faster than related species. “It is a thermophilic insect, so when the temperature in the greenhouse is 20°C the population is already growing strongly,” Messelink says.

Omnivores in a spectrum

To tackle this problem, in 2015 WUR teamed up with the growers’ organisation LTO Glaskracht Nederland and submitted a project proposal, “Pest control with omnivorous predatory bugs”. This public-private partnership launched in 2016, financed half by the Dutch Ministry of Agriculture and half by the private sector. The private funding for this project comes from the Dutch tomato, gerbera and rose growers’ cooperatives, Stichting Programmafonds Glastuinbouw (the Dutch greenhouse horticulture programme fund foundation) and Koppert Biological Systems.

“Nesi is a predatory bug in the Miridae family, just like Macrolophus,” Messelink explains. “Miridae are omnivores that not only use plants as food but also serve as predators. There are many different species of Miridae and they are all located in different places in this spectrum. One species eats more plant, the other more prey. Macrolophus is a predatory bug that is very pest-oriented and only causes limited damage to plants. Nesi, on the other hand, quickly causes a lot of damage but is also a good pest controller.”

Messelink and his colleague Ada Leman ran a greenhouse trial last year in which they investigated whether establishment of this insect in tomato is limited if a population of other bugs is already established in the crop. To determine whether Nesi also affects the densities of its relatives, controls with these bugs without the notorious predator were also set up.

Insect cages

The greenhouse trial was carried out in large insect cages with one tomato plant per cage: grafted Brioso plants with two stems. The effect was assessed using three types of new predatory bug, which the researchers brought in from southern Europe. “We investigated the effect these three species had on pest control and we looked at the development and establishment of these insects in tomato and their secondary effects on Nesi. The result was quite spectacular,” the researcher says. “Nesi was able to establish in all treatments, but what we saw was that where we had built up a population with the new species first before introducing Nesi, establishment was reduced by an average of 90%.”

The final population density of the predatory bug among its three southern European cousins averaged 85%, 92% and 95% lower than the controls respectively. There was no significant effect the other way round.

Whitefly and cotton whitefly

“We confirmed in the laboratory that the adults of the three new species feed on the young Nesi nymphs, but it is not yet clear what effect Macrolophus has on the insect,” Messelink says. “We have observed that Macrolophus is often squeezed out in the greenhouse, and in the laboratory too we have noticed that this bug doesn’t feed on young Nesi nymphs. So using these new species could offer an advantage over Macrolophus. But it’s important to find out whether they are just as effective in controlling the main pests.”

Now that the researchers have established that the three southern European bugs control tomato leafminer, this year they are looking into the effect these species have on greenhouse whitefly and tobacco whitefly.

Standing army

Messelink is enthusiastic about the initial result. “You have to view the predatory bugs as a standing army. If you can deploy the new species preventively, if they can control different pests, tackle Nesi and don’t damage the crops, then we will be taking a big step forward in biological control.”

At the end of the day, the researchers want to be able to offer growers a total package. “We are looking for a predatory bug that controls an infestation as effectively as possible and doesn’t damage the crop,” Messelink says. “I have high hopes. I think these insects will be a big help in biological control in the future.”

Experience in rose and gerbera

As part of the same study, the researchers also investigated the use of omnivorous predatory bugs in gerbera and rose. “We think that this insect can offer a solution in these crops too,” researcher Gerben Messelink says.

“Whitefly, caterpillar, Echinothrips – all these pests can be controlled with predatory bugs, so it is possible but we still need to do more research. We have observed that this insect has difficulty establishing in gerbera, mainly because of the mildew control technique that’s currently used. Growers spray the crop, and it is not so much the toxic value that eliminates the predator; research has shown that spraying under high pressure ‘blows away’ the predator. So if we can find another way of controlling mildew, we can also build up a population of predatory bugs in the crop and control pests that way.”

The researchers are also trialling building up a population of these predators in rose. “Rose is a woody plant, which makes it difficult for this insect to establish. Bugs prefer hairy, herbaceous plants. But we might be able to keep these predators in the crop by using a host plant, for example. We have already demonstrated that we can control Echinothrips in rose effectively this way.”


Three species of predatory bugs in the Miridae family can limit the establishment of their harmful cousin Nesidiocoris tenuis in tomato growing. Researchers looked at the effect they have on pest control. They investigated the development and establishment of these three southern European predators in tomato and their secondary effects on Nesi. The final population density of Nesi with the three predatory bugs tested was around 90% lower than in the control. It is likely that these omnivorous insects could ultimately also offer a solution in rose and gerbera.

Text and images: Marjolein van Woerkom.

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Dutch chrysanthemum growers are feeling quite optimistic about thrips control. After years of high pest pressure, growers are getting more adept at integrated pest control, thanks to meticulous scouting, a good predatory mite, supplementary feeding and biological crop protection products. Of course, there isn’t a one-size-fits-all approach that works for every nursery. Chemicals are still an indispensable part of the mix as a backup. Growers and suppliers outline the latest developments.

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Viruses are among the smallest pathogens infecting other living organisms. Plants are also susceptible to a large number of viruses that can cause serious diseases.

Viruses consist solely of a piece of genetic material (RNA or DNA) and a protein coat, or capsid. They therefore cannot survive independently and need other organisms to multiply and spread. Plant viruses replicate in plant cells and use insects (especially aphids and whitefly), mites, nematodes, soil fungi and even humans and our tools to move from one host to another. There are specific interactions between viruses and their vectors. Cucumber mosaic virus (CMV) is transmitted by aphids, for example, and tomato spotted wilt virus (TSWV) by thrips.

Therefore, controlling viral diseases not only entails starting with clean and certified plant material but also involves monitoring and controlling the transmitters (vectors) of plant viruses. Vigilance is essential, because new viral diseases can appear in plants when changes in populations of viruses and their vectors give rise to new situations.

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Crop protection products are sometimes unfairly labelled as losing their efficacy against pests or diseases. On closer investigation it becomes clear that these products don’t always end up in the right place or are not being taken up properly. In that case, adjuvants can be indispensable if you use them in the right combinations. Uptake can sometimes increase by as much as six to eight times.

Substances that support the use of crop protection products and enhance their effect are on the rise. A year ago, no fewer than 101 different ones were registered with the Dutch Board for the Authorisation of Plant Protection Products (CTGB). Thirty companies are involved in the development and distribution of these products.

One of these is the Dutch manufacturer SurfaPLUS, which is actively promoting their correct use in a series of events for consultants and users. Director Hans de Ruiter sees it as his mission to do this. That’s hardly surprising, since in his previous job he was project leader at Wageningen University & Research, where he was intensively involved in research into these substances. But this research rarely if ever takes place in the public domain these days. Instead, he has it carried out by private research institutions. After all, there’s no question that this extremely useful work must go on.

Variety of effects

“Adjuvants” is actually a collective term for products that work in a variety of ways. An important function that is particularly relevant to open-field cultivation is reducing spray drift. This keeps the active ingredients where they need to be to do their job. Another function is reducing volatilisation during spraying or after contact.

These products can also ensure that droplets of the solution stay on the plant or leaves and that the active ingredient is more evenly distributed. In other cases, the products can improve contact by “gluing” the active ingredients to the leaf surface. Lastly, they can boost uptake of active ingredients by making them soluble or making the plant’s waxy cuticle more permeable.

Every adjuvant therefore has its own characteristics, and some do several things simultaneously. “That’s important,” de Ruiter says. “because we know from the research how poor the uptake of active ingredients can sometimes be without these products.”

Resistance decreases

To start with a concrete example, de Ruiter cites the “Vertimec case”. This product, which is based on the active ingredient abamectin, is authorised for the control of spider mite, thrips and leaf miner in both ornamental and vegetable cultivation. In tomatoes, for example, it can be used three times per cultivation cycle, and more often in ornamentals. Vegetable growers tend to use it sparingly because it has an adverse effect on biological controls. But with the emergence of pests and diseases that are difficult to control, such as tomato russet mite, growers sometimes need to reach for the chemicals.

De Ruiter: “I hear complaints from growers that a product is becoming less effective at the correct dosage. The story goes that certain insects or mites have become resistant. But it doesn’t have to be that way. If you use the right adjuvant, a product generally does what it’s designed to do. In fact, an effective combination of the two products can actually reduce the risk of resistance.”

Abamectin is a product that is inadequately absorbed by the leaves when sprayed on its own. With the correct adjuvant, uptake can increase by six to eight times. It is poor uptake that increases the risk of resistance.

Combination works better

In 2014, research was carried out at the Westland Demo Nursery (Demokwekerij Westland) into the effect of abamectin and Hasten, an adjuvant based on an esterified canola oil, on an infestation of Californian thrips on sweet pepper. The treatments were as follows: untreated (water), 100% abamectin, 100% abamectin with Hasten, 50% abamectin and 50% abamectin with Hasten.

Thrips control with abamectin on its own was no higher than 15-25%. The combination with the adjuvant worked two to three times better. Previous research into spider mite control in cucumber yielded the same outcome.

Gentle products

In the summer of 2016, Botany BV carried out research into a combination of the adjuvant Elasto G5, a glycerol-based polymer, and XenTari, a biological agent based on Bacillus thuringiensis, against the golden twin-spot moth in sweet pepper. The treatments consisted of untreated (water), XenTari, XenTari with Elasto G5, and Elasto G5 on its own. Both pupae and adult moths were released into the crop and the researchers waited until various stages of caterpillars were present. A total of three treatments were carried out at weekly intervals.

The trial showed that the adjuvant improved the effect of the active ingredient. The product provided better coverage on the crop and improved distribution of the active ingredient (see graph).

These were mild substances that did not cause any damage and left no residues behind. Nonetheless, a warning would not be out of place, says De Ruiter. “Adjuvants can also boost the effect of products. They can make ‘hard’ products even harder.”

Fewer inhibitors

The use of Elasto G5 has also proven its worth in another way: in combination with inhibitors. In 2014, Delphy ran a trial with the adjuvant in combination with Alar (daminozide) in pelargonium which revealed that the use of inhibitors can be reduced by half. “That cuts costs for growers quite substantially, because inhibitors are expensive. It depends on the crop and the variety, but we have sometimes seen costs cut by as much as 44%.”

The days of pioneering with adjuvants are over, says de Ruiter. They have since found wide acceptance and the trial results are better than in the past, when some substances were too aggressive. The gentler products are gaining ground. “Of course, we have to keep on investigating new opportunities and we need to communicate the results we obtain with caution.”

Hence the events, which are held fairly regularly. Incidentally, SurfaPLUS is not the only company doing research into these substances. Crop protection product manufacturers such as Bayer Crop Science and Certis include them in their programmes, and the Dutch companies Modify and GreenA are also active players.


Adjuvants that enhance the effect of crop protection products are gaining ground. Not only do they get the active ingredients working better, they also help to avoid resistance. The right combination can halve the need to use inhibitors in some crops, delivering substantial savings. It’s important to know which combinations are the right ones because an adjuvant can also reinforce a product’s adverse effects.

Text: Pieternel van Velden.
Image: Botany.

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The fungus Mycosphaerella, also known as Didymella bryoniae, can cause serious problems, particularly in cucumber cultivation. It can infect leaves, stems, flowers and fruits. The fruits can be affected on the inside (internal fruit rot) and the outside (external fruit rot).

Generally speaking, the first infections in crops grown under glass are caused by ascospores (sexual spores) which enter the greenhouse mainly via the air. For infection to take place, there needs to be high relative humidity (RH >85%). Internal fruit rot is caused by the fungus penetrating the fruit via the flower. Stems, older leaves and the outside of fruits become infected via damage (wounds). The risk of infection can be somewhat reduced by aiming for lower RH in the greenhouse.

It is thought that mildew-tolerant varieties are slightly more susceptible to Mycosphaerella, perhaps because growers of these varieties use fewer mildew products which also help to control this fungus, such as Collis, Rocket and Fungaflor.

Tekst: BASF.
Images: BASF and Wageningen University & Research.

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As a cucumber grower you have to have iron discipline and nerves of steel. An apparently invisible enemy – CGMMV – is always lurking ready to attack. It’s a troublesome disease that can have profound consequences. If an infection strikes early in the year, the costs in terms of loss of production, early clearing, cleaning up and starting again quickly mount up. Shortfalls of €10 per square metre are no exception.

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A consortium of three companies, Ecoation, Metazet FormFlex and Micothon, has launched a new robot for scouting plants in greenhouses.

Revolutionary patented sensors register plant diseases and plant production factors before human eyes can discover them. The data generated is automatically displayed on a greenhouse map. This finally makes it possible to steer and protect the plants on an almost individual level, leading to minimal need for crop protection combined with maximum production of “always healthy plants”.

Saber sensor

The robot registers the diseases and development of the plants in greenhouses with technology based on the patented Saber sensor, which detects pests, diseases and deficiencies at an early stage. It can also be equipped with cameras, a gyro sensor and sensors for RH%, temperature, CO2, crop top temperature and a PAR sensor, resulting in almost total plant control.
Stand number: 12.107

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Erwinia is a bacterial disease that can cause damage in a range of crops. It often produces watery looking, rotten patches on the leaves, fruit, flower or stalk. The bacterium is quite common and can develop rapidly in the greenhouse, spreading from plant to plant through crop handling and splash dispersal. Hygiene measures are crucial for keeping an infection under control.

The rotten patch gives off a very specific odour similar to rotten eggs. The bacterium breaks down the cell walls of the plant with the aid of enzymes, enabling the infection to develop rapidly. Wilting and rotten patches cause the plant to die or damage the product to such an extent that it is unsaleable.

The general type is referred to as Erwinia spp. A crop can be affected by certain species that are particularly harmful to that particular plant. The species often found in pot plants include E. cypripedii, E. carotovora subsp. carotovora, E. carotovora subsp. atroseptica and E. chrysanthemi.
A new species which can cause a lot of damage in strawberries has recently been identified: Erwinia pyrifoliae. The symptoms are blackened fruits and bacterial slime.

Text and images: Groen Agro Control.

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Virtually nothing can be done about excessive root growth. However, this troublesome bacterial disease doesn’t always get the chance to develop. Other microorganisms that live in the root environment fight against its rapid spread. The search for underlying mechanisms is gradually progressing. It’s a brainteaser for the researchers.

Genetic changes have always occurred and they take place in nature all around us. Rhizobium rhizogenes for example, is a bacterium that very successfully infects plants with pieces of its own DNA. These pieces (T-DNA) merge with the plant’s DNA, which as a result behaves somewhat differently. For example, it can become more sensitive to the hormone auxin. This sets off explosive root growth. The roots then excrete substances that create a favourable environment for the bacteria to thrive, which perpetuates the situation further.
These ideal living conditions for the bacteria have large consequences for the host. Infected crops such as tomato, aubergine and cucumber show more vegetative growth and production lags behind. At least 400 plant types are susceptible to these bacteria and it can cause big problems for the greenhouse crops mentioned.

Difficult to grasp

Over the last decade a lot of research has been carried out into the mechanism that causes this abnormality and into methods (hygiene measures) to prevent the spread of the bacteria. Marta Streminska and Ineke Stijger, both researchers at Wageningen University & Research Greenhouse Horticulture, are researching a new strategy to prevent excessive root growth in collaboration with the Dutch growers’ association LTO Glaskracht Nederland, Rijk Zwaan and Koppert Biological Systems. This is within the project ‘Next Generation Plant Health’.
“It is still very difficult to get a grip on this disease,” says Streminska. “One year we see a lot of infection, another year hardly any, sometimes even on the same nursery and under exactly the same circumstances. We still don’t know why that happens.” There is no chemical cure. Once the plant is infected, the DNA changes forever. Since no solution can be found through chemical means, except the thorough disinfection of the watering system, a biological approach offers the best potential. In this respect the activity of microorganisms around the plant’s roots plays a central role.

Fathom out the system

Within the plant health project the two researchers are looking into all the fundamental principles that could set off or stop the excessive growth of roots. Until now this research has mostly been carried out within the four walls of their laboratory. The results and next steps are regularly discussed with the project’s committee of growers from the national committees for tomato and eggplant. “We are not looking for an antibiotic as this would increase the risk of resistance build-up. We are studying the entire microbial system,” explains Streminska.
Previous research showed that the symptoms of excessive root growth are not always observed in old slabs. These old slabs contain an established, stable microbial environment that it is not going to be pushed around by a wrongdoer.

Used substrate slabs

Therefore the researchers are now studying fungi and bacteria that were isolated from used substrate slabs to try and fathom out the microbial mechanisms involved. In addition they are testing a range of biological products that could offer solutions. In one tomato trial for example slabs were used that were treated with useful microorganisms. Here too excessive root growth reduced.
“The systematic assessment of all microorganisms is a huge undertaking,” says Stijger. “We want to know exactly what happens in the slabs. If there is a substance or microorganism that can slow down the explosive growth of this bacteria you could add this to new slabs.”

Disrupt communication

From research into human medicine we know that bacteria don’t strike without warning. Instead they wait until an ‘army’ of bacteria has built up and then launch a joint attack. Bacteria communicate with each other via the production of signal substances. Different bacterial types produce different signal substances. For example, Bacillus makes different signal substances than Dickeya or Rhizobium. Streminska and Stijger are looking at ways of disrupting this communication system to prevent a joint attack from occurring.
There are therefore several ways to approach the problem. The researchers are cautious about drawing conclusions too early. Stijger: “Even if we start to understand why excessive growth reduces in one crop, it does not automatically mean than it will be the same for another crop. Besides the root environment contains many organisms that work together.”
More than 90% of all the bacteria present live in symbiosis with the plant’s roots and actually stimulate growth. If you add organisms or substances that reduce the excessive root growth the rest still need to remain alive. If that doesn’t happen other undesirable changes might occur.

Disinfect and protect

In anticipation of the results the researchers sketch a scenario that could be used in greenhouse vegetable production. Firstly before the crop is planted the slabs and watering system need to be scrupulously clean. Opinions are divided on the cleaning effect of hydrogen peroxide and some people have preferences for certain types of brands or compositions. Stijger is not sure if this is a valid case. She does know that some strains of bacteria are more sensitive to peroxide than another. “The problem is that bacteria spread rapidly throughout the system if just a tiny little bit is left behind. The bacteria appear to protect themselves with biofilm that is very difficult to remove.”
Then, immediately after disinfection has taken place, the new substrate slabs have to be injected at the start of the cultivation with substances or microorganisms that will create a stable living environment for the plant’s roots to grow. As a result pathogenic bacteria have no chance to multiply and launch an attack.
Streminska and Stijger finished most of their laboratory research in 2016. They are now starting to test the substances on young plants.


Two years ago researchers in the Netherlands began a systematic approach to solving excessive root growth. Ineke Stijger and Marta Streminska virtuously studied the natural substances and organisms that could slow down the explosive development of Rhizobium bacteria. It is gradually becoming clear that a great number of different organisms are needed to develop a stable and resilient system.

Text and images: Pieternel van Velden

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The first symptoms of chrysanthemum white rust (Puccinia horiana) are visible on the upper side of the leaf. These are light green to yellow coloured spots of 2 to 5 mm in size. The centres turn brown and necrotic. The characteristic pustules or teliospores occur on the underside of the leaf. When there is a heavy infection the leaves wilt.

High humidity and a period of wet leaves are conditions during which this fungus is able to spread. Under optimal conditions new infections of white rust are possible within five hours. Climate changes in the greenhouse and watering at an unfavourable time increase the risk of contamination.
The fungus is mainly present on the leaves but stems can also be affected. Spores are spread via water, air, plant material, tools, hands and pets. White rust only occurs on chrysanthemums and has a quarantine status in several countries.

Text/photo: Syngenta