Biological treatment against pests is the plant protection of the future

Modern agriculture is the fundamental requisite for global food supply, and to a high degree focused on maximal productivity. An effect of this is that agriculture is almost exclusively monocultural, i.e. the same crop grown over large areas. This makes would-be harvests very vulnerable, since a single specialized pathogenic microorganism could eradicate entire fields. Disease management is thus an absolute necessity for food production throughout the world. Resistant crops and dynamic crop rotation are of great importance for the prevention of outbreaks, but the vastly prevailing method has over the last century been chemical pesticides. These days, however, other alternatives are on the march.

Biological control of pests and pathogens is usually defined as using living microorganisms (antagonists), typically bacteria and fungi, to protect and strengthen other organisms, usually agricultural crops, against various types of assailants. The method has been discussed since the 70's, and while having been a worthwhile endeavor in several cases it has so far failed to cause a lasting paradigm shift. The main reason for this is that chemical treatment has been economically superior. Such products has been cheaper to develop and purchase, simpler to apply, effective against a wider range of organisms, and resulted in larger harvests, which is why biological treatment with biocontrol agents (BCA) for many years has been viewed as a promising, but ultimately utopic, branch.

Drawbacks to chemical methods

Presently however, there are strong indications that BCAs will play a much greater part in the future. This concept is covered in a new review from the Department of Forest Mycology and Plant Pathology at the Swedish University of Agricultural Sciences, written by among others Professor Dan Funck Jensen.

"There are many reasons for this turn of events", Dan says. "Global food supply is one factor. Chemical pesticides have resulted in greater harvests,  but already at today's level of usage, there is a tangible concern regarding what potential health effects these chemicals convey to the environment, or to humans if they end up in our drinking water or remain on the crop post harvest. Thus, steps are taken to decrease the use of pesticides. New EU-legislation has already banned several types of chemicals. Instead, the ambition is to switch to other methods, such as biological control or practices to pre-empt outbreaks of disease."

There are also other risks involved in a wide spread use of pesticides. Several chemical substrates attack a wide range of organisms, some of which might have beneficial effects that should be preserved, or play an important part in ecological balance. If upset, some other, hitherto less serious pathogen might suddenly find a niche to exploit. Another risk lies in benefiting individuals resistant to the chemical. Widespread resistance forces the dosage of chemicals to ever increase, and might eventually lead to entire groups of pesticides losing their efficacy. This has already proven to be a great issue in control of some of the most important agricultural diseases, where more target-specific pesticides has been used. Resistance or tolerance may also be benefited in other microbes in the environment where the substrate is being used. These may not be seen as a risk today, but if benefited by the pesticide treatment in relation to other microbes, the ecological balance may be displaced with unforeseeable consequences. Furthermore, chemical treatment has other issues, such as not being allowed to use prior to harvest or on the final product, or when the farmer seeks to classify his business as organic.

"These causes of worry have led to ever stricter laws regarding chemical treatment in the entire EU", says Dan Funck Jensen. "At the same time, legislation is simplified and clarified regarding development of BCAs, which is now causing leading pesticide manufacturers to tap into this developing field."

Integrated plant protection

This quest for a more dynamic disease treatment is usually referred to as IPM, "integrated pest management". It is worth noting that IPM does not intrinsically strive for chemical free control methods. Sometimes, a combination of chemical and biological methods may be a viable solution, among other reasons to diminish the risk for acquired resistance in pathogens, or to prevent that other microorganisms in the crop competes with the BCA. In fact, this combination seems potent enough that developers today has resistance against chemical pesticides as a prerequisite in their putative BCAs; there are examples of otherwise promising antagonists whose development into a BCA has been abandoned since they do not possess such resistance. The IPM line of thinking also covers limiting disease through crop rotation, tilling and other forms of soil treatment, retaining naturally resistant hosts, prognosis and minimizing the risk of human-associated spread of the pathogen.

New technology provides new insights

Alongside a growing outer pressure on a more dedicated push towards IPM with BCAs, scientists now have very potent tools to their disposal to meet this demand. The past decade's dramatic development in DNA-technology has made it possible to study in detail interactions between the BCA, the pathogen, and the host. For example, this has led to the insight that some BCAs are effective in two ways; the root-colonizing Trichoderma harzianum, for example, is a mycoparasite, feeding off of the pathogen, but is also inducing immune responses against the pathogen in the host, thereby making it better prepared for the attack.

"Another insight derived from these new technologies, is how some organisms that has been deemed suitable as BCAs deal with the toxic environment created by the pathogen", Dan says. "Clonostachys rosea, another mycoparasite and promising BCA, has been shown to carry an unusually large number of so-called transporters in its cell membrane. These are present in all cells and serves to detoxify the cell by pumping out unwanted substances. The fungus' ability to survive around the pathogen thus seems to come from its ability to constantly cleanse itself in this way. This also provides Chlonostachys with resistance against pesticides, making is suitable for IPM."

Searching for new antagonists

Thus there is a societal interest in and technical potential for the development of new BCAs from promising antagonists, but how is this process conducted?

"The most important step is really the first", says Dan Jensen, "that is, finding a species that impedes disease development in the field. It is an old truth that the best place to start looking is where the disease seemingly has every reason to exist, but yet does not. In those cases, chances are good that it is facing competition from some other species, whose assumed antagonistic properties thus would be of interest to explore. In order to discover the identity of this species, it is important to screen wide, for as many candidates as possible, and above all to search in environments where the would-be BCA is supposed to act. Species previously labeled as promising antagonists solely based on interaction studies with the disease in laboratory conditions, has oftentimes proved inefficient in nature."

Dan Funck Jensen also mentions that modern molecular methods may be adopted to develop markers for certain genes or traits previously known as beneficial to a BCA, such as transporters to deal with toxicity or chitinases to degrade fungal cell walls. Such markers can be helpful to identify the candidate species that do possess these traits. This approach is risky however, as it introduces the chance of unknowingly excluding organisms that lack the wanted genes but carry others, whose antagonistic effect may yet remain to be discovered. Such screening criteria should be employed with great caution.

Each pathosystem is unique

"It is also advised not to focus solely on antagonism in a strict sense when considering what organisms can be potential BCAs," Dan continues. "If by antagonists one only regards those directly attacking, parasitizing or physically blocking an assailant, one also disregards those benefiting host growth, which is both an economical advantage and an indirect protection, since large plants are hardier than small. Such stimulation can occur by microbes initiating the host's growth hormone production or root system development. In such ways, mycorrhizal fungi or other microbes conveying benefits to the host may also be included in an integrated biological plant protection, together with other, more directly antagonistic species. In fact, sometimes one begins by selecting fungi with growth promoting effects and subsequently screens these for antagonistic traits, in order to develop potent BCA's."

Each BCA-pathogen-host pathosystem is unique and requires a unique regime in order to be efficient. These treatments vary widely. The BCA ContansWG, including a parasite on pathogenic Sclerotinia-fungi, is mixed into the soil following harvest, long before the new crop is sown. The BCA attacks the dormant parts of the pathogen, the sclerotia, which otherwise would infect the growing seeds. Another BCA, used against powdery mildew, is based on the mycoparasite Ampelomyces quisqualis. This agent is slow-acting and inefficient at high infection pressure, but may be used to great effect  in combination with chemical agents to stop the growing resistance against the pesticide the mildew has been developing. Yet another example is gray mold, attacking strawberries around the time of fruiting, when chemical treatment is prohibited. The fungus Ulocladium otrum then serves as a viable option to alleviate the attacks. However, the most common form of application of BCA's is on seeds, which are treated both in order to prevent disease and to stimulate faster germination. All in all, the usage of BCA's is potentially very versatile but also demanding in terms of the developer's knowledge of pathogen, antagonist and host life cycles, not to mention his imagination.

Antagonists in consortia

Today, most BCA's are being applied one by one, with or without accompanying pesticide treatment, with a certain crop and a certain pathogen in mind. In line with the IPM philosophy, it has long been desired to instead apply them as a consortium, with several BCA's acting simultaneously together, in hope that additive or even synergistic effects will convey a stronger plant protection. Such effects may for example include one microbe attacking the pathogen, another inducing host defense and a third enhancing root uptake of water. Such attempts has often been a failure, possibly because the antagonists’ compatibility is a sensitive relationship that may be disturbed in the high concentrations necessary in the finalized BCA. There are hopes, however, that new genetic methods will assist the search for species tolerant to each other even when applied as a consortium. The principal viability of the concept has already been demonstrated by nature. So-called compost tea, i.e. compost dirt fermented in water, has on several occasions proved to convey antagonistic properties against pathogens in both soil and foliage. Exactly which mechanisms are causing this effect is unclear, but since sterilized compost tea loses this trait, it is plausible that it has to do with its complex microflora.

Amelioration of antagonists

One single individual rarely carries all traits one could wish for in the optimal BCA. For example, those able to harm the pathogen with toxic substances and enzymes might not cope well with long-time storage or lose its efficacy in the specific environment where the pathogen thrives. Hence, it is oftentimes desirable to cross individuals, or if possible even species, to gather as many of the beneficial traits as possible.

“Such changes can of course also be accomplished by gene modification”, Dan Jensen says. “This is particularly convenient in the case of microbes; it is much easier to manipulate those than to, for example, develop resistant plants. It is not difficult to remove or add single genes in one fungal or bacterial strain, and there are several cases where such enhanced organisms are used in commercially available BCAs. However, the EU enforces a strict regulation on GMOs, and I don't view this approach as viable to us in the foreseeable future."

Risks in biological plant protection

Biological control methods confer the advantage of not having to release large amounts of chemicals in limited areas, which means that may contribute to a better environment and result in agricultural products without pesticide residues. However, even though the common stance is that this is an environmentally friendly and sustainable method to treat plant diseases, the EU puts strict demands on potential organisms before these can be used as BCAs in the union. This legislation covers BCAs based on natural, non-modified microbes, and requires thorough studies on every antagonist in order to make certain they will not constitute any risk to the environment or cause unwanted effects on other microbes, plants, animals or humans. Gene-modified organisms are covered by a different, much stricter legislation, which makes it unlikely that GMOs will be used as BCAs in the near future.

In conclusion, biological control of plant diseases is of growing importance to agriculture. New BCAs continue to reach the market and the scientific field attracts a lot of attention, both regarding fundamental research and more applied research that analyzes the effects of biological control in the field. This development will naturally also benefit Swedish agriculture, and the SLU is strongly represented in this regard.