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.”
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.
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.”
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.
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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Advisor Piet van Adrichem of Delphy has mapped out how the virus has spread in the Netherlands. He believes that back in 1996, all cucumber nurseries in the province of Drenthe were infected with it. In 2002 the disease broke out in the south-east Netherlands – in just one road to begin with ¬– but by 2010 it was common throughout the area. The picture is no different worldwide: it’s not just a Dutch problem.
Fortunately, the picture is no longer so bleak. Some nurseries have stopped growing cucumbers altogether on account of the virus, but others have become much better at adhering to hygiene protocols. After all, prevention is better than cure. And curing a virus attack is extremely difficult.
The fact that the cucumber growing area of Drenthe is now virtually virus-free is thanks to a small advance post, led by Ron Peters, which has been courageously mounting resistance to the advancing virus army. Originally a cucumber grower himself, Peters moved into plant breeding and now runs an experimental nursery at which seed companies test their varieties. He spent many years studying the spread and control of the virus and came up with a method of getting nurseries virus-free.
“We always fought a fierce battle against CGMMV but it kept coming back,” he says. This was even more of a concern during his years as a plant breeder because his crates were going to and from nurseries. “The basis of a clean crop is hygiene. If you don’t have your hygiene under control, the virus will come back. And once the plant is infected it will never recover.”
The hygiene protocol Peters developed consists of strict rules to follow between cycles, when the old crop is cleared out and the greenhouse undergoes a major clean-up. Discipline in crop handling and harvesting is also emphasised. Cultivation advisors and researchers have always paid a lot of attention to preventing the spread of viruses, so there’s nothing new there. But there is one striking difference. Besides hydrogen peroxide, Peters also uses Hortiwash to clean between cycles, a cleaning product sold by the Dutch company Horticoop.
Once this new product came onto the market, it was possible to roll out the protocol in the form of a step-by-step plan. The method caught on. The first growers to use it took back control of their crops’ health. And as Peters had put a lot of energy into developing the method and advising and assisting these growers, it made sense to him to charge a fee for his services.
Finger on the pulse
Peters agreed with the manufacturers of the cleaning product that he would continue to provide the advice himself. “It’s quite an unusual situation,” says John Sonneveld of Horticoop. “We have our own in-house advisors who help growers change over to the next cycle. And now we’re outsourcing that to an external consultant. What’s more, we don’t even know exactly what the protocol entails.”
But Peters stuck to his guns. His customers pay him 14 eurocents per square metre at clean-up time, and for this one-off payment he is always available to provide assistance. If necessary, he will even hop on a plane to help growers abroad. Clients sign a form to confirm they won’t disclose the information. Does that work? “Of course,” he says with a smile. “After all, why would you pass on something you’ve paid for?”
To begin with, the cucumber community reacted to this price tag with annoyance, as this was something that hadn’t been done before. But as soon as it became clear that the specialist’s method worked, the mood changed. “Many nurseries I visit are in trouble because they have had a severe attack halfway through the year,” he says. “Having to clear out the greenhouse and lay new slabs impacts significantly on yields. A sick crop will have been producing less for some time, and it takes time to clear and replant the greenhouse. So you try and avoid this kind of situation.”
Cucumber grower Arjan de Gier of A&A Growers, which has sites in England, the Netherlands and southern Europe, was one of the first people Peters approached. The virus had struck at several of their sites. “We did everything we could to get the problem under control,” de Gier says. The symptoms appear between seven and twenty days after infection. Once you can see it in the canopy, it’s too late. “We did everything we could think of during and after the end of the cycle but it still came back the following year. But we applied the protocol in 2015 and we’ve been virus-free ever since.” After de Gier, more growers came on board. Peters: “It’s much easier to stay clean now because the pressure from infection has dropped.”
Lack of awareness
The specialist is struck by the lack of awareness he often encounters on his visits to growers in the Netherlands and abroad. For example, only using plastic overshoes and footbaths is not enough, he says. “That really doesn’t help. If you’re growing in gutters, the plants don’t touch the ground anyway. You have to think logically about how and where you come into contact with the plants. You might wrap yourself up like a supermarket cucumber before you step inside the greenhouse, but then you forget to clean the harvest crates.”
It’s also important to think about transfer from the outside in. “In an area in which the virus is prevalent, a flock of sparrows entering the greenhouse to nibble on the flowers will almost certainly cause an infection. But if the whole area is virus-free, those sparrows can’t really do much harm.”
Last season, Peters travelled to various countries to advise growers. Among them was a brand-new nursery in Volgograd, Russia, which had started having problems almost immediately. “Their first artificially lit crop had barely been going two months when it was hit by a massive virus attack,” he says. “The crop had to be cleared, but everyone pulled together and we managed to get the virus under control.”
This was largely thanks to the dedication of all the staff on the site, the consultant says. Around 40 people, from directors to workers, attended an intensive training course. With the strict protocol they put in place, the nursery is still virus-free after nine months. They still find the occasional sick plant but the disease is under control.
Peters is seeing an increase in problems with CGMMV worldwide. He is now getting requests for assistance from Bulgaria, Romania and Hungary. The virus also haunts growers in Canada, although they often prefer to handle it their own way, including by planting very short-cycle crops which are removed if the infection gets too serious. Not the wisest solution, he thinks, because it reduces yields and pushes up costs. “It’s difficult to keep the virus out, but the most spectacular results can be achieved when growers tackle the problem in a different way and are open to advice,” he says.
The number of virus-tolerant varieties has been on the rise for some years now, giving growers more options for keeping the virus under control. The Dutch vegetable breeder Rijk Zwaan, which was the first company to launch highly resistant varieties, offers a number of varieties that fit the bill in its BonDefense range.
Most growers don’t plant resistant varieties in the first cycle, crop coordinator Marcel van Koppen says. If the virus pressure increases, they often opt for resistant varieties in a second or third cycle. “In the past, cucumber growers would often switch to growing tomatoes in the autumn. Luckily, they now have an alternative with these new varieties. Virus resistance is important but it’s not the only solution to an infected nursery. Varieties are resistant, not immune. So they can become sick,” he explains. He believes that good greenhouse hygiene is at least as important as resistance because it reduces the pressure from disease.
Young plants need to be given time to develop resistance, Peters says, and that can take time. “So you first have to give the plants a chance to grow towards the wire. And that goes better if the infection pressure is lower.”
Advisor Piet van Adrichem is also keen to emphasise the relationship between a resilient crop and a viral infection. “High-wire growing and viruses don’t go together,” he says. “Mature plants have to work extremely hard to get the water to the top of the plant. Every plant essentially has to make its own climate. And that makes them vulnerable.”
Other diseases, too, such as Fusarium, should not be underestimated, the advisor says. Growers must continue to prioritise starting clean and adhering to clean working methods. Resistant varieties are an alternative but they are not as high yielding as conventional varieties, he says. Moreover, they are more susceptible to stress.
Everyone must join in
In effect, cucumber growers now have several ways of keeping the virus under control. By following a good hygiene protocol, they are not only helping themselves but the whole community. Peters: “It’s really important that everyone joins in because that way you’ll get the whole area clean.”
Cucumber grower de Gier believes that the discussion around paying for advice is irrelevant. “Just think what it would cost you if you stopped producing for several weeks and had to clear the crop and maybe buy a whole new set of slabs halfway through the cycle. What we’re talking about is no more than half a cucumber per square metre. So what’s the big deal?”
The hygiene protocol for cucumber growers is based on many years of observations and logical thinking. In two years, growers in parts of the Netherlands and abroad have managed to rid themselves of cucumber green model mosaic virus. Breeders are working on producing resistant varieties, but it’s still important to adhere to clean working methods. The cost of this combined approach easily outweighs the financial impact of losing production halfway through the season.
Text: Pieternel van Velden.
Images: Pieternel van Velden, Rijk Zwaan and LD Photography.
Reducing energy consumption in greenhouses is associated with higher humidity levels. Many growers are concerned that this could make the crop less active, impacting on transpiration from the top in particular. Last year, a Dutch research project was launched with the specific aim of measuring that. The conclusion: rather than a reason for concern, intensive screening is actually a way of improving the crop.
The “Transpiration from the Top” study took place at the Wageningen University & Research greenhouses in Bleiswijk, the Netherlands. Over two months, greenhouse climate and energy researcher Feije de Zwart tested a measurement method in a mature tomato crop and evaluated the quality of the technique. “We looked at the extent to which we could specifically measure transpiration at the top of a crop using a thermal imaging camera. And it worked. But this method requires an extremely accurate camera and a lot of attention. So for the time being, although this camera is excellent in the lab, it is not really suitable for use in the commercial setting.”
Low-energy cultivation not only requires better greenhouse insulation, but it also means the crop has to be grown in higher humidity levels. Growers tend to use their screens more often and don’t often open gaps in them. This increases the humidity in the greenhouse. “One of the reasons why growers are reluctant to accept this situation is that they are concerned the crop may not transpire enough,” de Zwart says. “After all, the more humid the air in the greenhouse, the lower the difference in vapour pressure between the crop and the greenhouse air, and the less the crop will transpire. They are most concerned about the top of the plant.”
These concerns stem from the fact that inadequate transpiration can affect nutrient transport to growing tips. He adds: “To measure is to know, so we developed a measurement system that can accurately determine transpiration at the top. A sensor of this kind could put growers’ minds at ease and could result in wider acceptance of higher humidity levels, particularly in vegetable cultivation.”
Camera plus artificial leaf
To test the system, the researchers installed a thermal camera above the crop. Besides the leaves on the crop itself, there were always two artificial leaves in the camera’s line of sight. One of these artificial leaves was fitted with a PT-100 temperature sensor to check the temperature registered by the camera. With this setup they were able to compare the temperature of a leaf at the top of the plant with that of an artificial leaf that was not transpiring, in the same conditions. The lower the temperature of the real leaf compared with the non-transpiring one, the greater the transpiration.
De Zwart again: “In fact, we noticed that the temperature of the real leaf fell quite a bit below that of the artificial leaf at night and that the temperature difference increased as the humidity level dropped. The best thing was that the behaviour of the real leaves was very much in line with our expectations based on our calculations. When we took another close look at the calculations, we noticed that the reduction in radiated heat loss brought about by screening really does increase the temperature at the top of the crop quite significantly. And this in turn leads to higher levels of transpiration at the top.”
De Zwart’s conclusion is therefore that transpiration at the top of the plant simply continues when screens are used, even if humidity is higher. “Intensive screening can limit transpiration from the crop as a whole but, conversely, stimulates it from the top of the crop.”
Vertical differences in the crop
These results could perhaps explain why good yields were achieved in all those practical trials with Next Generation Growing, despite the expectation that the high humidity would cause problems. De Zwart again: “If you look at water uptake, for example by comparing the amount irrigated and the drain, or by using a weighing gutter, you can barely see the effect of screening at all. But if you look at the increase in temperature in the crop, then you can see that closing the screen increases the temperature at the top, while often lowering it slightly further down in the canopy. This is because the use of the screen means less heating is needed further down.”
In any case, the temperature gradient across the crop drops, making transpiration more even throughout the crop. “That last factor, the evenness, had never really occurred to me,” he adds. “So rather than being a reason for concern, intensive screening is actually a way of improving the crop. This realisation is essentially the most important outcome of our research. It is such interesting information that it has been added to the Radiation Monitor.”
Growers and other interested parties who attended the Next Generation Growing course in the Netherlands will already be familiar with the Radiation Monitor. This online simulation model calculates the effects of screening and greenhouse covering materials on energy consumption and vertical temperature distribution.
De Zwart used the same model to establish the expected difference in temperature between a transpiring and a non-transpiring leaf. However, the data obtained from the project mentioned above demonstrated that the original calculation method was too inaccurate. Following improvements, the program now calculates transpiration at each layer of the crop.
The basis for this is that the difference in vapour pressure between the greenhouse air and the leaves plays a bigger role in driving transpiration than the local leaf temperature. The program can be used via the greenhouse horticulture models website.
De Zwart is slightly less enthusiastic about the results of the original project setup. “The measuring equipment was quite tricky to set up. We didn’t have a problem collecting images with a thermal camera, but focusing the lens was difficult. The plant was growing, so we had to constantly refocus the lens. Also, you have to use artificial leaves. We now know that a tomato leaf transpiring at the normal rate at the top of the plant is around 0.4°C cooler at night than a non-transpiring leaf in the same place.”
If this measurement method is used to distinguish normally transpiring leaves from leaves transpiring at a lower rate, the temperature differences measured should be in the magnitude of 0.2°C. These are such small differences that you would need to know exactly how warm a non-transpiring leaf would be in that position. That is why you need artificial leaves and a very accurate camera. He adds: “Actually, you do wonder whether the information you get is really worthwhile. After all, we now know that transpiration at the top of the plant simply continues when screens are used intensively, even if the air humidity is higher.”
Low-energy cultivation means a lot of screening hours and higher humidity in the greenhouse. Research shows that although higher humidity causes transpiration to decrease, the use of screens does not affect transpiration from the top of the plant. The project used a thermal camera and artificial leaves. The setup worked and, besides providing figures for the top of the crop, it also highlighted the vertical temperature distribution in the crop. The data was integrated into the online Radiation Monitor.
Text: Jojanneke Rodenburg.
Images: Wageningen University & Research and Jan van Staalduinen.
The predatory bug Orius has been used to control thrips in sweet pepper for many years with great success, but the results have so far been disappointing in ornamentals. Researchers Marjolein Kruidhof and Gerben Messelink now think they have found a solution. With a new method of using the bugs that involves supplementary feeding, thrips can now be successfully controlled in chrysanthemums.
Thrips are the biggest threat to ornamental growers’ crops. Research into biological predators for this pest has been going on for many years. Good results have been achieved with predatory mites, but this has often failed to eliminate the problem because the predatory mites only attack the young larvae. The predatory bug Orius is a very effective weapon against thrips in both the larval and adult stages but it has trouble establishing in ornamental crops. Numerous ways of overcoming this problem have been investigated, ranging from banker plants to feeding stations, but there has been no real breakthrough. Until now, that is.
In the spring of 2017 the Wageningen University & Research Greenhouse Horticulture business unit in the Netherlands started experimenting with a new approach to thrips control in chrysanthemum cultivation. Instead of starting off with chemical crop protection products, the researchers are now introducing biological agents in the cuttings phase. The predators are given high-quality supplementary food so that they can form a strong population or a “standing army” to nip the outbreak in the bud.
“The results that have been achieved this time are due to good coordination between two projects: the PPS Thrips project, in which we are looking for a good alternative supplementary food source, and the Green Challenges project, in which we are optimising the role of biodiversity in crop protection and achieving paradigm shifts,” says researcher Marjolein Kruidhof.
In chrysanthemum cultivation, there is usually only a short time window in which you can start using biological control, according to Kruidhof. “Also, the presence of chemical residues delays the growth of populations of natural predators,” she says.
The researchers experimented with a biological start using the predatory bug Orius. They ordered cuttings that were almost pesticide-free, rooted the cuttings themselves and added the bugs a few days before the plants went into the greenhouse. “A biological start is a real change in thinking,” says Kruidhof’s colleague Gerben Messelink. An important part of this strategy is the supplementary feeding, he stresses. “After a series of trials in which we compared different types of food, we ultimately went with Artemia, the cysts of the brine shrimp. This is a potentially good food source and has a long shelf life.”
Trials using Artemia as a feed supplement for predatory bugs had been carried out before but with only moderate results, he says. “The quality of the Artemia that is available on the market at present is good enough for feeding predators like Macrolophus in tomato but not for Orius.”
The researchers therefore got together with the University of Ghent to come up with a good quality food source. Meanwhile, the Israeli company Biobee had also started producing high-quality Artemia which the researchers were able to use in subsequent experiments.
The results exceeded expectations. The number of Orius rose substantially as a result of the supplementary feeding. Having started with fewer than one bug per cutting, by the end of the production phase the researchers were counting 40 bugs per plant. What’s more, the natural predator seemed to respond very well to the availability of food. “It turns out that they are highly mobile,” says Kruidhof. “This has potential because it allows you to manage your biological control better. Plus it means you will very likely be able to reuse the bugs. If you end up with 40 bugs per plant, it would be a shame to spray them dead. That’s destruction of capital. You might be able to lure the adult specimens to new cuttings with targeted supplementary feeding.”
More effective than predatory mites
The impact on thrips damage was significant. “In the control section, in which no Orius or Artemia were used, half the younger leaves were damaged by thrips,” says Kruidhof. “The figure for the plants with the bugs was less than two percent.” The predatory mites did less well than the predatory bugs in terms of thrips control, despite the fact that they had built up a good population with the chosen food source. Researchers still found about 20 to 25% thrips damage on plants following the use of these biological predators. “So Orius really are more effective than predatory mites because they also attack adult thrips,” says Messelink.
“We have proved that the system works,” says Kruidhof. “We can build up the population of bugs by using biological controls and good quality nutrition right from the start, and this population provides good thrips control even in the presence of another food source.” However. that doesn’t mean that this method can simply be replicated in the commercial greenhouse setting. “We still need to optimise certain aspects,” she says. “For example: when is the best time to introduce the bugs? Should they be used in the rooting phase or can they be brought in later? How many bugs should you use? What will your feeding strategy be? How much food should you provide?”
This method of control is based on one generalist. What do you do as a grower if you also have to deal with leaf miner or aphids? “Growers will have to control leaf miner with additional biological measures or selective chemicals. Aphid control can become a problem, but the expectation is that high densities of this predatory bug will also keep aphids under control. Other possibilities for controlling aphids are parasitic wasps, gall midges or perhaps other predatory bugs. We therefore want to investigate whether other types of bugs can be combined with Orius to deal with aphids.”
Crop protection specialist Helma Verberkt of the Dutch growers’ organisation LTO Glaskracht sees this as an excellent development. “It is a good addition to developments in the commercial greenhouse setting, where good results have been obtained in recent years using predatory mites,” she says. “For use in practice, there will need to be enough affordable, good quality Artemia available and it is important to ensure that Orius is compatible with other biological agents and pesticides used.”
The question is also whether cutting suppliers and producers will be willing to come on board. Cuttings with few or no crop protection product residues are currently hard to find. “It’s a bit of a chicken-and-egg situation, but I think we will manage,” says Messelink. “There’s also a real change in thinking going on among cutting suppliers. More and more growers want to start biological control earlier and are asking for cuttings with fewer or no chemical residues. Cutting suppliers are also looking for alternative options. I think biological control is the solution.”
“We have shown that it works now, and that is quite a breakthrough,” Kruidhof adds. “We plan to carry out another greenhouse trial this year and we expect growers themselves to start developing the strategy further as well. As a result, the market for pesticide-free cuttings will only get bigger and more demand-driven. So producers and suppliers will have to meet that demand.”
Both projects are funded through the Top Sector Horticulture & Propagating Materials and are being implemented within this sector with funding from the government, various crop cooperatives and Koppert. The projects are coordinated by LTO Glaskracht Nederland.
Researchers in the Netherlands have made a breakthrough in controlling thrips in chrysanthemums. By starting biological control early on and providing good quality nutrition, it is possible to build up a good population of the predatory bug Orius. This population controls infestations well, even in the presence of food.
Text and images: Marjolein van Woerkom.