As featured in Arable Farming Magazine
Preparing crops to resist attack
by Arable Farming
While so-called elicitors are no silver bullet in disease control, the loss of multisite fungicides and the growing need to target pathogens from all angles means they could have an important role to play. Alice Dyer reports.
Plant elicitors, which work by priming the plant’s natural resistance mechanisms to react faster to the presence of disease, offered great potential when they were first identified many years ago, but widespread adoption never came to fruition.
This was largely because when used on their own, resistance elicitor products do not offer the reliability or disease control efficacy that conventional fungicides alone can.
This is according to Prof Adrian Newton, cereal pathologist at the James Hutton Institute, who says: “Research [on resistance elicitors] started years ago when we were looking for ‘just another fungicide’, but now they are very much a part of integrated pest management and if you’re using lots of different components to achieve healthy plants, this is one which can be used effectively.
“They don’t induce much resistance as such, because if they did, they’d cost a lot in energy and yields would suffer.
“Instead, they have a small cost but a big potential benefit if the pathogen that causes the disease then arrives [on the plant].
“Something like mildew on wheat or barley, will land on the leaf surface and the plant recognises it and there’s a cascade of molecular responses which happen
within minutes and hours.”
A plant’s response to the presence of disease spores causes cell walls to build a defence that has anti-fungal properties and if it reacts fast enough, stop the mildew penetrating the leaf.
Often the difference between a resistant and a susceptible plant, whether by breeding or otherwise, is how fast this response happens.
Prof Newton adds: “Elicitors make that response quicker – they can be applied to a susceptible plant and make it behave like a resistant plant. It’s mimicking it being a resistant plant on a temporary basis.”
This means most elicitors contain no biocidal properties because the plant itself combats the disease, rather than the product. Despite this, resistance elicitor products must be registered as pesticides because they reduce disease levels, which has led to problems bringing products to market.
- Elicitors work by priming the plant’s natural resistance mechanisms to react faster to disease
- Elicitors are normally applied at T0 or T1
- Many can be tank mixed with fungicides
- Current products claim good control of septoria and some control of rusts
- Most elicitors contain no biocidal properties
- Resistance elicitor products must currently be registered as pesticides as they reduce disease levels, which has led to problems bringing products to market
- Used as part of a fungicide programme, it is claimed elicitors can reduce disease risk or work alongside a lower dose of fungicide
Prof Newton says: “Regulation is a nightmare area. Because elicitors cause control of the disease, they have to go through pesticide regulations, but it’s difficult to demonstrate the same level of efficacy. The way around this is manufacturers don’t make that claim, but instead make a general plant health claim. Some of these products are also sold as biostimulants. Ideally, they should be claiming more than a biostimulant, but it gives them a market.”
Part of the problem with demonstrating good efficacy is that because elicitors work in an indirect way across multiple defence pathways in the plant, all those pathways can be influenced by environmental factors, unlike a fungicide which simply works directly on targeting a fungus.
“They can be very effective against disease, but they are often not. In trials, we have seen mildew [infection levels] fall by 87% which was almost as good as the fungicide, but the following year they were nowhere near as good as the fungicide, despite being applied in exactly the same way.”
However, used as part of a fungicide programme, elicitors can reduce disease risk or work alongside a lower dose of fungicide.
Prof Newton says: “Do not use them by themselves because they’re not an alternative to fungicides. However, at the moment, adequate information on the best way to [combine them with fungicides] isn’t out there and there needs to be a lot more trials done.
“Using one in a programme with a fungicide, you are unlikely to lose out, but you could well benefit, particularly if you are then able to lower subsequent
fungicide doses. Even between crop varieties there are large differences between the efficacy of the product, which we still don’t really understand.
“Potentially, I think it’s a really valuable area because it works through the plant and we need to get much better at bringing together all the factors that contribute to the agronomy of the plant.”
Taking the pressure off chemistry
Hutchinsons has been exploring a range of biological solutions to disease, including the biostimulant Scyon which contains a blend of four naturally-occurring signalling metabolites and potassium, zinc, sulphur and manganese.
Scyon is claimed to help the plant optimise uptake and transportation of nutrition, causing it to increase its metabolism and put more energy into defence against diseases.
Over the last three years trials have shown Scyon to be effective in reducing septoria and rust levels in crops, says David Howard, Hutchinsons’ head of integrated crop management (ICM).
“Plant defence is an energy expensive process and this is the reason why plant defences are not turned on at all times, because this could exhaust the plants resources.”
Instead, defences are turned on when the plant senses the arrival of a pest or pathogen.
Scyon works with this reaction to strengthen the plant’s defences, upregulating the plant’s metabolism by optimising nutrient availability and balance in the plant which increases energy available for defence.
Mr Howard adds: “It stimulates the plant’s natural SAR [systemic acquired resistance] pathway, releasing defensive proteins and metabolites. At the same time, it stimulates the abscisic acid pathway which reduces plant stress.”
While these products will not replace the need for chemical fungicides yet, they can act as an important building block in ICM and by applying Scyon at GS30/T0 in wheat, pressure on following sprays can be reduced, says Mr Howard.
“When this product’s effect on disease response wears off, defences will drop, so it is important to still use a fungicide. It looks like once you generate that [resistance] effect it remains at effective levels for around 30 days, so that gives us the opportunity to lower our disease pressure for the following sprays.
“It creates consistency of control, which might be because the plant defences are better at dealing with certain isolates that some fungicides might struggle with. It’s hard to know as a farmer or agronomist which isolates you’re challenged with at T0 or T1, so this product helps by letting the plant’s evolved defences deal with some of those isolates.
“It’s all about strategic management of the crop. There are a lot of agronomic decisions made before this point, from variety choice to drilling, and we have to look at all aspects of managing the crop, particularly when it comes to nutrition, to improve the plant’s health and making it ready to fight off disease.”
Hutchinsons is now exploring how Scyon can be used in other crops, including for control of ramularia in barley and light leaf spot in oilseed rape.
Triggering systemic resistance
One elicitor-based fungicide that has achieved full registration as a pesticide is UPL’s Iodus, which is currently registered for use in winter wheat.
The product contains the active laminarin which is extracted from seaweed but behaves differently to seaweed extracts typically found in biostimulants, says Greg Hanna, technical specialist and trials manager for UPL. This molecule tricks plants into thinking they are already under attack from disease.
Mr Hanna says: “Normally when a plant comes under attack from a pathogen it will take around 48 hours to mount its natural defence processes.
“Using laminarin ahead of disease transmission, you’re telling the plant to arm its defences before a pathogen attack so you don’t have that 48-hour window where the pathogen can get in without too much resistance from the crop.”
Laminarin is said to offer multiple lines of defence – it signals for increased lignification to create a thicker cell wall that is harder for the pathogen to penetrate. Secondly, it promotes the release of phytoalexins and proteins produced by the plant that attack a fungal pathogen. If present in good numbers, these will fight back and cut down on the pathogen’s ability to create a reservoir of infection in the crop, says UPL.
“Phytoalexins can almost be thought of as a multisite response because they will bind at different sites on a pathogen and interact in a number of ways. We’ve seen good data on reducing the rates of other chemistry and getting a good return on investment and strengthening some of the chemistry under pressure.”
Iodus is best used in a preventative scenario and is positioned to be sprayed at T0. Although only leaf 4 is present as an immediate target, the flag leaf and leaf 2 are bound within the stem but not yet visible, making it a good point to prime the plant, says Mr Hanna.
“You’ve got all the structures and organs that the plant’s going to need going forward.
“Because laminarin triggers systemic resistance, you’re not just priming the cells you hit with the spray, the reaction is cascaded through all cells and plants and the whole plant is switched on in terms of defence. You’ve effectively protected leaf 1, 2 and 3 before they were ever vulnerable to pathogen attack.”
Whether the product needs to be applied alongside other fungicides will depend on the target disease and how high pressure the scenario.
Unlike some biofungicides, elicitors can generally be tank mixed with other products because they are less susceptible to damage from chemistry, says Mr Hanna.
“We’ve seen Iodus perform well with other fungicides, but it depends on the pathogen you’re going after. For septoria, it performs very well on its own. However, for yellow rust we see a bit of efficacy, but not at a level that could be relied upon, so that’s where we’d suggest mixing with a triazole or a strobilurin.”
Laminarin’s mode of action has potential against a number of diseases and can trigger elicitation in a wide range of crops. Registration trials for further arable crops are ongoing.