Immunising Plants To Control Disease

Published on 8 January 2010 in Sustainability and Communities , Ecosystems and biodiversity

Introduction

It is a stark fact that more than 800 million people worldwide do not have sufficient food, and some 1.3 billion people survive on less than $1 a day. To make matters worse, it is estimated that there will be an additional 1.5 billion people to feed by 2020, requiring farmers to produce 39% more grain. Since it is estimated that some 12% of global crop production is lost to plant disease annually, there is clearly a need for efficient, reliable and affordable disease control measures.

To date, ensuring adequate levels of disease control has relied heavily on the use of fungicides and resistant varieties of crop plants. However, the pathogens that cause plant disease can become resistant to fungicides, causing fungicides to become less effective, while the pathogens can also overcome the resistance bred into crop plants, so resistant crop varieties lose their ability to fend off pathogens. In addition, recent EU legislation (Directive 91/414/EEC, now known as the Sustainable Use Directive) aims to remove older pesticides which might harm people and the environment and to promote the safe and sustainable use of remaining pesticides. Although pesticides will continue to be useful, there is an urgent need for new approaches to control crop diseases.

When a farmer grows a crop variety that is resistant to certain disease-causing pathogens, the invader is quickly recognised by the plant and specific defences are activated. These defences stop the pathogen in its tracks and no disease develops. If the crop variety is not resistant, but is susceptible to those pathogens, the invading pathogen manages to avoid recognition by the plant. The pathogen can then infect the plant, colonise it, and cause disease. The susceptible plant has all of the defences it needs to deal with invading pathogens, but because it fails to recognise the invader, the defences are not activated. However, it is possible to by-pass the recognition process and to directly activate the defences. This is known as induced resistance and it can be triggered in plants using compounds known as elicitors. These elicitors can work in two ways, they can activate defences directly, or they can activate defences only following pathogen attack. The latter is called priming and is potentially very useful, since it leads to the deployment of defences only when there is real danger.

Key Points

Figure 1: Application of an elicitor to prime a plant against disease.

Disease is responsible for considerable crop losses every year
Fungicides and resistant crop varieties lose effectiveness over time as pathogens adapt to them
New EU legislation will curtail fungicide use
There is a need for alternative approaches to control crop diseases

Research Undertaken

Research at SAC has shown that elicitors can provide disease control under field conditions. The control achieved depends on the elicitor, the particular crop (e.g. barley, oilseed rape) and variety used. Elicitor combinations have been shown to provide better disease control than elicitors used individually, and in barley, best disease control is achieved using elicitors together with fungicide applied at reduced rates. Here, application of elicitor early in the season (at GS32) reduces the amount of pathogen in the crop, enabling less fungicide to be applied later in the season. This approach not only provides excellent disease control and yield increases, it also reduces fungicide use on the crop.

On oilseed rape, disease control provided by elicitors can be significantly better than that provided by fungicides (see Figure 2 below). Here, the most effective fungicide reduced light leaf spot on oilseed rape by 82%, while an elicitor combination provided complete disease control.

Figure 2: Effect of three different fungicides and an elicitor treatment on light leaf spot on winter oilseed rape.

Current SAC research is aimed at determining how best to use elicitors in crop protection programmes and will provide information on the most appropriate elicitors to use on different crops, which crop varieties are most responsive to elicitors, the best time to apply elicitors, and how elicitors can best be used to reduce fungicide application to crops.

Policy Implications

The new EU legislation will have a profound impact on crop protection and will drive the search for alternatives to fungicides. However, given the adaptability of the pathogens which cause disease, it is likely that careful use of fungicides, in conjunction with other control measures, will be required in the future. This should help to protect the environment and reduce the crop and food waste that results from plant disease.

Author

Professor Dale Walters dale.walters@sac.ac.uk

Topics

Sustainability and Communities , Ecosystems and biodiversity