Biotic stress tolerance and resistance

Published on July 1, 2014   38 min

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Hello. My name is Professor Robert Park. I work at the University of Sydney Plant Breeding Institute where I hold the Judith and David Coffey Chair in Sustainable Agriculture. I'm also the director of the Australian Cereal Rust Control Program. Today I'm going to talk to you about biotic stress tolerance and resistance plants.
Today's lecture will cover, firstly, the importance of plant diseases, secondly, options for disease control, and thirdly, I will discuss the genetic control of plant pathogens. In this section, I will cover the terminology that we use in host pathogen interactions. For example, what is resistance and what its pathogenicity? I will also discuss the gene-for-gene hypothesis which is the foundation of almost all the work we do in resistance breeding. And then finally, I want to spend a little bit of time and talk to you about the wheat rust diseases and resistance breeding. This is an area that I've worked in for the past 30 years.
Plants are the basis of all life on earth. We as humans need clean air, water, food, shelter, and fuel. All of these things have been provided to us at one point in time or another by plants. Plant diseases are caused by viruses, bacteria, fungi, oomycetes and nematodes. And globally, these diseases have been estimated to reduce plant production by about 30%. In thinking about plant disease, it is important to remember that the organisms that cause these diseases have co-evolved with their hosts in resilient natural ecosystems. We as humans have domesticated many plants and this, in turn, has placed huge selection pressure on pathogen populations. As a consequence of that, agriculture has had to deal with resilient mutable pathogens that can change and undo the hard work of breeders.
When it comes to disease control options, broadly speaking, we have three approaches that can be used. Agronomic practices, chemical control and genetics. The first of these, agronomic practices, can involve things such as fallow management, crop rotation and the eradication of alternate hosts. An example of fallow management is the destruction of the "green bridge" to control biotrophic pathogens. Biotrophic pathogens require a living host on which to survive. And in order to get from one cropping cycle to the next, they need living plants, self sown plants, also known as volunteers. In many parts of the world, destroying these self sown plants between cropping cycles can contribute significantly to the control of these biotrophic pathogens. Crop rotation can include the use of break crops to control soil-borne diseases. And in the case of eradication of alternate hosts, a spectacular example was the eradication of barberry which contributed significantly to the control of stem rust of wheat in North America.
The second disease control option is the use of chemicals or pesticides. There has been a huge reduction in the cost of many of these chemicals in recent years due to patent expiry. One example, the triazole fungicides are used quite commonly to control fungal diseases in many countries. However, two major concerns have arisen from the use of these chemicals. The first is that some pathogens have developed insensitivity to them. For example, wheat and barley mildew in Europe. Both of these pathogens have become insensitive to triazole fungicides. A second concern is that of Maximum Residue Limits, the amount of chemical that is permissible in the end product of a food item. And this is now a concern for some fungicides. One example is tebuconazole which is known to break down slowly in plants. The Maximum Residue Limit set for tebuconazole in weight in Australia are such that, depending on the formulation and the rate used, only one or two applications of this chemical can be applied before Maximum Residue Limits are exceeded.
The third and final disease control option is that of genetics or resistance breeding. A plant variety, or cultivar, is in essence a package of genes. These genes control yield, maturity, quality and so on. Some of the genes in the package that is a variety confer resistance to diseases. These resistance genes have been used to great affect in controlling plant diseases. For example, in Australia, genetic resistance to rust diseases in wheat alone was estimated to save more than $1 billion per year. Resistance breeding is the most environmentally friendly and economical approach to disease control. However, we must remember that pathogens are variable. They have the ability to overcome resistance chains by generating races, strains or pathotypes. The terms races, strains or pathotypes are, in essence, synonymous. The important thing to remember is that not all resistance genes give complete protection against yield loss.

Biotic stress tolerance and resistance

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