More and more research is revealing that endophytes – microorganisms that live in the plant without harming it – can significantly boost a plant’s resistance to pests. These findings prompted researchers to investigate the potential of endophytes in pest control in greenhouse horticulture. Experiments in tomato, sweet pepper, bedding plants and chrysanthemum have shown that while the fungal isolates tested thus far don’t deliver complete pest control, they do significantly inhibit population growth.
Endophytic fungi are therefore of particular interest alongside other control measures, but there are still many unanswered questions about how they work and how to use them. A recently launched fundamental project on endophytes is expected to provide more basic knowledge to support applied research into endophytes.
The study of microbial ecosystems in humans, animals and plants has been the focus of much research in recent years. Just as gut flora is essential for good health in humans, microorganisms in plants also play an important role in resilience to pests and diseases. Precisely how this works is not always clear. Fungi can have a direct effect on pests by secreting certain metabolites, but it has also been shown that fungi and bacteria in plants can accelerate resilience in the plant (known as priming).
Some of these fungi are also known as entomopathogenic fungi, such as those of the Beauveria, Metarhizium, Lecanicillium and Isaria genera. Many of these fungi are used as a product in crops to control insects such as whitefly and thrips. We are increasingly discovering that most of these types of fungi can also grow in plants without harming the plant. Over the past two years, Wageningen University & Research in the Netherlands has been running a number of studies looking at the effects of endophytes on pests that are relevant to greenhouse horticulture.
Inhibiting aphid growth
Aphid control in greenhouse horticulture is still often based on the use of chemical crop protection products, but new regulations and customer requirements are making chemical control increasingly difficult. The alternative – biological control with natural predators – is not always as effective. Aphids are known for their ability to grow explosively on plants, making it difficult for natural predators to get the infestation under control fast enough. Measures that could inhibit the explosive growth of aphids would therefore be a welcome addition.
A study funded by the Dutch Horticulture Product Board was set up to investigate whether endophytic fungi could inhibit the growth of aphids in sweet pepper and the bedding plant Calibrachoa. Many isolates produced no effect at all, but the fungus Lecanicillium muscarium was able to inhibit the increase in aphids by 25-30% in both Calibrachoa and sweet pepper. This may not seem very much, but on a rapidly growing aphid population and in combination with other pest control methods, it can make a big difference. In further research, it is being investigated whether these effects actually result in better control when combined with natural predators.
Another major problem in greenhouse horticulture is the western flower thrips, or Californian thrips, Frankliniella occidentalis. This pest is particularly difficult to control in ornamental crops, partly due to the ever diminishing package of chemicals available and growing resistance to pesticides. In a large public-private project (Masterplan Thrips), various endophytic fungal isolates are now being screened for their effectiveness against these thrips in ornamental crops.
In the fight against thrips, increased plant resilience would be a welcome addition alongside other measures. More than 40 fungal isolates have already been screened, some of which show significant inhibition of 40-50% in the population growth of thrips. Here too, therefore, what we are seeing is not complete control but inhibition of growth. However, the results are promising and the expectation is that they will provide a very useful addition to the various measures for keeping thrips in check. In research carried out in other countries, good results have also been achieved on the tobacco thrips in onion, with endophytes reducing thrips damage by 80%.
The next step is to take a look at potential applications in greenhouse horticulture. As with aphids, the question in terms of thrips is how to combine endophytes with biological pest control. A delayed development time could be beneficial for predatory mites, which would then have longer to work on the susceptible stages (predominantly L1), but there may also be effects that impact negatively on predatory mites. These and other predators could be affected by certain substances that enter their prey (the pest) via plants containing endophytes. So it is important to test the effects in the overall system of natural predators.
The initial results with endophytes in greenhouse horticulture indicate that there is definitely potential for using these fungi to increase resilience to pests. However, there are still a lot of questions to be answered in the research: for example, what spectrum of pests they are effective against and whether they have any adverse effects on some pests and plant pathogens. Other very practical questions concern the methods of inoculation and longevity during cultivation. It has been found that there are several types of microorganisms in plants (bacteria and fungi) that interact with each other but also with the host plant. These interactions are complex and as yet not fully understood, but they will certainly affect the behaviour of introduced endophytes.
The basic questions surrounding the plant microbiome are being investigated in a new fundamental endophyte project led by Leo van Overbeek of Wageningen Plant Research. In this project, he initially wants to determine how endophytes can best be introduced into the plant. He is looking at methods of application in practice, such as via treatment of seed or young plant material, possibly in combination with irrigation water treatment.
The researchers are also taking a close look at how endophytes behave in a plant, for example which parts of the plant they spread to which also contain the various pests and plant pathogens.
Using modern DNA sequencing and other so-called ‘omics’ technologies, including metabolomics, the research is also determining how and to what extent the microbiome of the plant is influenced, based on a) the presence of the causative agent, b) the presence of the endophyte applied, and c) the combination of both. This should shed light on how pests and endophytes respond to each other and what factors play an important role in that.
The hypothesis is that endophytes will affect the composition of the microbiome, which would be expected to have an effect on the metabolism of the plant and, in turn, on pests and diseases. The researchers hope to be able to correlate measurable effects with parameters that provide an indication of a plant’s resilience. This fundamental project is expected to culminate in an integrative model that indicates how to boost resilience in plants by applying endophytes. The model will then be used to develop new technologies to make plants more resilient to pests and diseases.
Fungi and bacteria that live in plants can help make the plants more resilient to pests and diseases. It has been shown in a range of crops that population growth in aphids and thrips can be inhibited by certain endophytic fungi. Further research is looking at developing practical inoculation methods for different crops, integration with natural predators, endophytic behaviour in plants and the underlying mechanisms that increase plant resilience with endophytes.
Text and images: Gerben Messelink and Leo Overbeek, Wageningen University & Research.