Chemistry will continue to play a key role in fighting weeds, pests and diseases for years to come, but legislation coupled with new diseases, weeds and pest threats means finding new approaches will become crucial for UK arable farming to remain viable.

UK researchers are leading the way, looking at novel solutions such as coatings for leaf surfaces, herbicides that work on resistant grassweeds, genetic resistance and plants that emit alarm pheromones to ward off insect attack.

These are just some of the research areas showing promise, as the science community looks for ways to help growers in their battle against pests and pathogens – many of which are genetically adaptable and proving difficult to combat with existing control methods.

At the Scottish Rural College (SRUC), Dale Walters has been investigating the use of protective coatings for plants, with some of his recent results coming as a surprise. The concept is based on the fact that the leaf surface provides the first barrier that fungi and insects have to overcome, in order to gain access to the plant.

“The leaf surface also offers a variety of physical and chemical cues that are needed, either by a fungal pathogen for the development of its infection structures, or by an insect to locate and accept the plant as suitable host,” he explains.

Disguising the leaf surface prevents some diseases, as they can’t develop as they need to, he adds. “Yellow rust and mildew are two good examples. If a thin polymer film has been sprayed on to the leaf surface, these pathogens can’t recognise it and don’t develop.”

Changing the visual and physical characteristics of the leaf can also deter insects, by affecting the taste and smell of the plant, so repelling the insect and preventing it from feeding and egg-laying, he comments.

But spraying a polymer coating on to a leaf surface isn’t as straightforward as it sounds, he continues. “The material used is important. If it stopped gas exchange from occurring, or interfered with photosynthesis, it wouldn’t be a starter.”

Anti-transference materials, such as those used as stickers or extenders in spray adjuvants, have proved to be suitable and have worked well in the field, Prof Walters confirms. “They don’t give the same level of control as a fungicide, but they are effective.”

However, once they’ve been sprayed on, the crop continues to grow, causing the coating to split, he points out. “So there’s a need to make more than one application in most circumstances.”

His latest work has involved the use of plant-derived polymers as coatings, which have the advantage of being a renewable resource. Again, they have been successful at controlling disease, but not by acting as a barrier, as originally believed.

“Instead, they worked by inducing the plant’s own resistance. They triggered or stimulated the plant defences.”

Field trials

Field trials showed they reduced disease by between 50% and 70%, he reveals. “That’s in line with other induced resistance work. They are best used as the first part of a planned programme, with fungicides only used later on, if required.”

Induced resistance can last for several weeks or for the whole of the crop’s life, notes Prof Walters. “So there’s potential for these materials to come to the market in a variety of forms – as seed treatments to protect the crop at a very vulnerable stage or as foliar sprays, applied at key timings.”

Root exudates are a subject of interest at the John Innes Centre, where their potential for blackgrass control is under investigation.

“It’s come from the observation that when oats are grown in the rotation, you get a suppression of blackgrass in the following crop,” reveals Jonathan Clarke of the John Innes Centre. “A compound which is released from oat roots is responsible for the effect.”

This effect is known as allelopathy, which is not a new idea, he acknowledges. “But we now have the tools to make better use of it and we can combine it with genetics. So exudates with herbicide activity could be employed in a number of ways.”

Whether plants are used to make the exudates or they are produced synthetically remains to be seen, he says. “There’s also the possibility of breeding this ability into wheat, for example, so that grassweeds are suppressed by the crop they are growing in.”

Blackgrass control is also on the agenda at the Universities of York and Durham, where the discovery of a gene that plays a key role in controlling herbicide resistance has given researchers the potential to produce herbicides that can control resistant weeds.

“Resistant grassweeds have a gene that gives them a general detoxification mechanism,” explains Rob Edwards of the Centre for Novel Agricultural Products at the University of York. “This gene is called AmGSTF1 and it makes an enzyme called glutathione transferase.”

This enzyme, known as GST, causes the plant to make lots of protective antioxidants, so detoxifying the herbicides that are applied. However, when resistant grassweeds were sprayed with a GST-inhibiting chemical by Prof Edward’s team, they became susceptible to herbicides.

As commercial herbicide performance continues to slide, this is a development that will be of interest to all growers with autumn-sown cereal-based rotations.

Prof Edwards recognises the importance of the development, but cautions growers about being too optimistic at this early stage. “We’ve been able to identify a group of compounds that could be applied alongside existing herbicides, to restore their activity. But the compound we tested originally proved to be toxic to the crop.”

Resistance problem

However, other versions could be developed, he believes. “We’re also developing a test for AmGSTF1, so that growers know if they are likely to have a resistance problem.”

Identifying a certain type of resistance to fusarium is occupying Paul Nicholson and his team at the John Innes Centre, who are keen to bring Type 1 resistance into commercial wheat varieties and reduce the need for ear sprays.

“Type 1 resistance is important because it prevents the initial infection of all types of ear blight pathogens,” he explains. “In contrast, Type II prevents the spread of the fungus around the ear.”

Genetic lines from CIMMYT in Mexico, which are thought to contain the Type I gene, have been introduced into the spring wheat Paragon using conventional breeding techniques, he explains.

The resulting progeny will be used to identify the Type I gene, using genetic markers, so that infection can be prevented and toxin levels reduced. “Combining Type I and Type II resistance in the same plant would give a lasting solution to the problem.”

What About GM?

At Rothamsted Research, the GM aphid-repelling wheat trial is being extended this year so that an autumn-sown crop can be evaluated.

The trial combines the use of second-generation GM technologies with the institution’s knowledge of natural plant defences, to develop crops with their own in-built ability to repel aphids.

The wheat plants in the trial have been modified so they produce an alarm pheromone called (E)-β-farnasene, which is the same as the one that aphids produce to alert one another to danger and leave the vicinity. While this approach has worked well in the laboratory, it now has to be tested in the field.

Another important part of the study is to examine how any changes in aphid behaviour influence the behaviour of other insect populations or have an effect on biodiversity.

“Autumn infestations of aphids are a real problem for growers, so it makes sense to adjust our trial slightly to look at this crop development stage,” explains John Pickett. “The loss of certain active ingredients also makes it extremely relevant, as growers are going to have to find other ways to control barley yellow dwarf virus.”

Projects at a glance

• Disguising the leaf surface with plant-derived polymers –Scottish Rural College and University of Edinburgh, funded by Grain Research and Development Corporation (GRDC) of Australia
  
• Using GM technology to repel aphids in wheat – Rothamsted Research, funded by BBSRC
  
• The key role of an enzyme in multiple-herbicide resistance in grassweeds – Universities of York and Durham, funded by BBSRC
  
• Underground signals warn neighbouring plants of aphid attack – University of Aberdeen, James Hutton Institute and Rothamsted Research, funded by the Scottish government
  
•  Identifying multiple genetic resistances to ear diseases – John Innes Centre, funded by BBSRC

Underground communication

Researchers have discovered that plants use an underground network of fungi to warn each other of pest attack, which could help them develop a new weapon against insect pests.

Published in Ecology Letters, it is the first time plants have been found to communicate underground in this way.

The study funded by the BBSRC and NERC, saw researchers from the University of Aberdeen, the James Hutton Institute and Rothamsted Research grow beans in groups connected via underground networks of mycelia – a thread-like fungus that grows from one set of roots to another.

Aphids were introduced to one plant in each group, which triggered the release of a suite of chemicals designed to repel attack. Remarkably, plants in the group that were not under attack, but were connected via fungal networks, also began to produce the defensive chemical response. Plants without the fungal networks didn’t mount a chemical defence, so remained vulnerable to aphid attack.

David Johnson of the University of Aberdeen who led the study says: “We knew that plants produce volatile chemicals when attacked, and we knew they communicate danger to each other above ground. Now we know that they communicate danger through these underground fungal networks as well.”

The roots of virtually all groups of plants, including important food crops such as wheat, rice, maize and barley, are colonised by symbiotic fungi.

“We could use a plant that is susceptible to aphid attack to ‘switch on’ the defence mechanism in crops through the natural underground connection,” says Rothamsted’s John Pickett.

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