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Dr Marta Streminska’s latest research has focussed on how best to prevent the soil-borne pathogenic fungus Fusarium from infecting the roots of ornamental horticulture crops such as Lisianthus, Chrysanthemum, Gerbera, and Phalaenopsis.
Microbial ecologist Dr Marta Streminska at Wageningen University & Research Centre, Business Unit Greenhouse Horticulture & Flower Bulbs, highlights the importance of beneficial microorganisms in the greenhouse cultivation of different crops. Her goal is to help growers utilise them instead of only focusing on microbial plant pathogens.
They say that as one door closes, another one opens – and this is arguably the case when it comes to ornamental horticulture production. On the one hand, many chemical crop protection chemicals are being removed from the market due to their harmful effects on the environment and human health. Yet, on the other hand, scientists are gaining a greater understanding of the fascinating and complex world of microbiology, specifically, the role that microbial communities play in helping to grow more resilient crops.
Microbial communities
Indeed, microbial communities – in and around the plant – are the specialism of Dr Marta Streminska, a microbial ecologist at Wageningen University & Research Centre, Business Unit Greenhouse Horticulture & Flower Bulbs (where she has worked since 2010).
She explains: “A microbial ecologist is a scientist who tries to understand the small community of bacteria, fungi, viruses, protozoa, and so on that are present in different growing systems in the greenhouse – and what the role of these microbes are. And we know that a lot of them are very important and beneficial for plant growth and resilience.”
The ‘good’ and the ‘bad’
Streminska’s interest in microbes started in the late 1990s when she began her Master’s studies in her home city of Warsaw, Poland. Her desire to help tackle the city’s pollution inspired her to study the ways in which microbes can degrade organic pollutants in wastewater systems through denitrification. This led her to further study the process of nitrogen cycling in forest soils, gaining a PhD in Biology (Environmental Microbiology) in 2007. After obtaining her PhD, she worked as a trainee at TNO in Utrecht and, again, studying dissimilatory nitrate reduction to ammonium (DNRA) in soils as a postdoctoral associate at the University of Aberdeen, Scotland.
She recalls how, throughout most of her busy career, plant pathogens —namely, the “bad microbes”—have (unfairly) received most of the press. But now, the “good microbes” and their communities are finally gaining the attention they deserve.
“Beneficial microorganisms are very important in the greenhouse cultivation of different crops. My goal is to, therefore, help growers utilise these beneficial microorganisms instead of only focussing on microbial plant pathogens.”
Dr Marta Streminska promotes the use of a standing army of natural enemies.
Preventing Fusarium
The scientist explains that, in the greenhouse, each plant has its own microbiome. “So, microbiomes can be different from one plant species to another. Some of their microbes might be plant pathogens, but these represent a very small proportion of the microbiome. Most microbes are very beneficial. So, my job is to figure out what the microbes are doing and how they are helping the plant to grow better and protect itself against plant pathogens so that they won’t die when they attack them.
To that end, Streminska’s latest research has focussed on how best to prevent the soil-borne pathogenic fungus Fusarium from infecting the roots of ornamental horticulture crops such as Lisianthus, Chrysanthemum, Gerbera, and Phalaenopsis.
“A lot of different ornamental and fresh produce horticulture crops are having problems with Fusarium at the moment. So, we’re trying to figure out how to prevent the Fusarium infections by making the growing system more resilient. It’s about having enough active microbes to actually prevent Fusarium from developing.”
For instance, Streminska has experimented with changing the growing medium by adding some organic matter into it “to work on the microbes that are in the system so that they will, in the end, tackle Fusarium, or any other pathogens.”
She adds: “We’ve carried out a number of experiments with different crops with varying success. Not all Fusarium infections can be prevented. We’ve made some steps forward, but we can’t prevent Fusarium in every system yet.” Streminska, therefore, emphasises the value of developing a holistic approach to tackling pests and diseases. “It’s very important that more work is carried out on plant cultivars’ genetic resistance to pathogens, for example.”
Biostimulants
Interestingly, growers are increasingly choosing to apply biostimulants to the root zone. The EU’s definition of these products asserts that biostimulants are used to improve plant growth (and they are not to be confused with biopesticides). There are non-microbial and microbial biostimulants available on the market. However, the list of microorganisms allowed to be included in the EU’s new Fertilising Products Regulation only contains four genera of microbes. “The other ones cannot be certified yet, but I think that might change in the future,” opines Streminska. “I cannot say when it will happen, but from science, there are a lot more microbes – such as those that can produce plant hormones – that could become biostimulants and positively influence plant production.”
Biopesticides
Growers are also now commonly incorporating biopesticides into their integrated crop management programmes – applying them preventively to help control diseases such as root rot. However, the choice of microbe-based biopesticides on the market is still limited by the fact that it is not always easy to formulate microbes into products that can remain stable and retain efficacy for an extended period of time. Endospores, such as those seen in Bacillus bacteria, can remain dormant and stable for many years – meaning that it’s much easier to create a commercial product that can be stored longer. “So, this is something that limits possibilities. But there are some initiatives that are figuring how to produce products with other microorganisms in them, such as Pseudomonas bacteria.”
In fact, the future could hold some exciting new solutions as Streminska describes the biopesticide products currently on the market as being “the first generation of biocontrol.”
“If you look at the science of biocontrol, there is currently a shift towards synthetic communities made up of very many microbes. So, in future, I think we will see the development of more communities of microbes because we now know that around the plant’s roots, you have loads of microbes with different functions that actually work together.”
The benefits of adding a community of microbes
Streminska explains that if you manipulate a natural microbiome or add a complex community of microbes to help the plant, it probably has a higher chance of producing reproducible results than just applying one type of microbe. “Because this one type of microbe has to compete with loads of microbes that are already present in the system, and it has to find its niche. Firstly, it has to be established around the roots or in the growing medium – and then it has to produce a specific metabolite (or similar) to prevent disease. These are very important steps, and sometimes they don’t work because the microbes are not really compatible with a given crop or with a given growing medium.”
With an increasingly wide range of growing media now being used in horticulture, Streminska admits that a lot more knowledge is needed before we enter the next generation of biological control. “We need to learn more about the microbial species that are present and their functions. And these are the steps we are making at this moment.”
