What is stem rust of wheat, and why does it matter?

What is stem rust of wheat, and why does it matter?

Hundreds of millions of people rely on wheat as the main ingredient for at least one meal in a day or all meals in their day. Globally, a staggering 670 million metric tons of wheat are produced annually. In the past 10 years, however, a devastating disease of wheat, called stem rust, has become a regular topic of discussion in the media. Stem rust may reduce wheat yields dramatically; in severe circumstances up to 100%. If humanity loses the ability to control stem rust, it will result in mass starvation. There is no reason for immediate alarm though since successful control measures are in place. If stem rust went rogue, it will be evident. Shortages of products that contain wheat flour will be experienced and that is far from becoming a reality at present.

Stem rust is a plant disease, but the topic of plant diseases is not general knowledge. We know that humans and animals fall ill, but it is not so widely known that plants alike are attacked by unfavourable organisms and that it also falls ill.  When we have flu, our energy levels are low, and our bodies are weakened. This holds the same for plants, which are similarly infected by microorganisms in such a way that they become diseased and when severely infected they die.

In the plant kingdom, the organisms mainly responsible for diseases are fungi. An example of fungi that we encounter in our everyday life is the green mould that appears on old bread and cheese. These green fungi, known as Penicillium species, survive and proliferate on numerous kinds of substrates, which also include oranges -and a citrus fruit turned green is a familiar sight.

The stem rust fungus (see photo), as its name also describes, survives on the stems of plants. Its host range is restricted to mainly the stems of small grain plants which include barley, triticale, oats, rye and most importantly wheat. The fungus lives inside the tissue of the plant and reproduces via a mass of spores (which are to fungi what seeds are to plants – reproductive structures) that break through the surface of the stem. These groups of thousands of spores appear in small patches on the stem are known as pustules. The colour of the fungal growth is a rust-brown, which describes the latter part of the name of this disease – stem rust.

The stem rust fungus survives in the plant as a parasite where it extracts nutrients for its survival. This weakens the plant and harms productivity, which results in yield loss. The loss in yield is proportional to the severity of the disease. Where severe stem rust epidemics occur and no control measures are in place, 100% yield loss is possible.  The disease is fully under control in most countries, although very low and sporadic incidences of stem rust outbreaks and yield loss of thousands of hectares of wheat are reported annually. This serves as a reminder that stem rust always needs to be held in check.

There are two main methods of controlling stem rust. The first is by breeding wheat lines that will have a natural resistance to the disease. The second is by the timely application of fungicide.The full article is for subscribed members only. To view the full article please subscribe. It’s FREE!Log In Register
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When a plant is resistant to stem rust, it means that it will naturally be able to withstand stem rust infection. This is a highly effective and environmentally friendly method of stem rust control which has kept stem rust at bay around the world for decades, particularly in developing countries. Resistant plants as a measure of control, however, is not fail-safe since wheat cultivars can lose their ability to resist the disease.

This loss of stem rust resistance in wheat has, in recent years, become of major importance. Notably, one resistance gene, known as Sr31, was widely incorporated in the wheat cultivars of the world as a reliable source of resistance. In 1999, the resistance conferred by this gene was overcome by a stem rust type (or race) found in Uganda. Since its first discovery, this type of stem rust has spread to numerous countries where it has caused havoc as the natural protection of the wheat plants against stem rust was lost. Currently, such loss of resistance presents the greatest challenge to wheat production in the world. However, successful breeding efforts by wheat breeders, who collaborate internationally, strive for the continual and timely development of lines that will withstand stem rust.

Traditionally, the breeding of resistant lines is a time-consuming process and the development new cultivars can take up to ten, sometimes 20 years. In 2017, a new speed breeding technique, inspired by the way the National Aeronautics and Space Administration (NASA) were growing plants in space was developed. This technique puts plants under continuous light, for up to 22 hours a day, and tricks plants into growing much faster. Using this technique development of plants from seed to seed can take as little as six weeks and four to six generations of the plants can be achieved in one year. Traditional techniques allow the growth of only two generations of plants per year, and generation turnover is a major restricting factor in wheat breeding and the release of resistant cultivars. Speed breeding may, along with the integration of another state-of-the-art technologies, speed up the availability of new cultivars from 10 years to two or three years.

Globally, the control of stem rust does not only rely on stem rust resistance, but the well-timed and accurate application of fungicides is also a highly effective method of control. In developed countries particularly, and in South Africa, fungicides are routinely applied for the control of stem rust. In South Africa, no serious epidemics have been experienced in recent years. Nevertheless, although yield loss because of this disease, when not controlled effectively, is still a reality.

In South Africa, stem rust appears predominantly in the Western Cape wheat-growing region, usually around September, but it may be seen as early as the beginning of August when the climate is highly favourable for the survival and proliferation of the fungus.  Stem rust thrives under warm and moist conditions and with hot days of 25-30 ̊C and mild nights of 15-20 ̊C under which circumstances the growth and reproduction is swift. The fungus can infect one plant and nearby plants repeatedly. Thus, every time a pustule bursts open the same plant and neighbouring plants will be re-infected. This means that the disease if left unchecked, can spread extremely fast which renders it difficult to control if there was no early intervention.

Since the fungus favours warmer climates, stem rust was never a major disease in European countries as it was in Africa and Asia.  Moreover, with the introduction of resistant wheat varieties bred in the 1950s (by Norman Borlaug and co-workers) to European wheat production, the disease virtually disappeared as neither the host nor the climate was suitable for the fungus to flourish.

In recent years, however, two factors are likely to have led to the re-emergence of stem rust. The first is the occurrence of a new, highly virulent, rust type or strain which has overcome the resistance of numerous wheat varieties grown in Europe. Thus, inherent resistance within the wheat lines is not a reliable way of protecting the plants anymore. The second is global warming, which could have warmed the European climates to such an extent that it is more conducive to the occurrence and spread of stem rust than ever before.

The combined threat of the loss of resistance in the wheat lines along with the increased likelihood of climate supporting the survival of the stem rust fungus has rightfully raised the alarm in Europe. In 2013 severe yield loss was experienced in Germany for the first time in decades, and more recently, in 2016, 30 000 hectares of wheat were severely infected with stem rust in the Italian island of Sicily. This was one of the largest outbreaks of the disease seen in Europe since the 1950s. This means that for the first time in decades, European countries need to breed new resistant lines and be on high alert to prevent spread and loss of yield because of the disease.

Stem rust is, therefore, presenting a threat to wheat production in the world, but scientific innovation has in the past, and will in the future keep the problem at bay. An underlying message is that of the wheat farmers must manage the risk of stem rust, but also other diseases. The production of food in the world is therefore not to be taken for granted and farmers should rightly receive recognition for overcoming threats to ensure food security (and seeing to it that the birthday cake finds its way to your birthday party).

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