Global spotlight on fungicide resistance and crop disease

Dr Megan McDonald and Associate Professor Peter Solomon from the Australian National University report on information gathered at the 10th International Congress of Plant Pathology in Beijing.

‘Bio-security, Food Safety and Plant Pathology: The Role of Plant Pathology in a Globalized Economy’ was the theme of the 10th International Congress of Plant Pathology (ICPP) hosted by the Chinese Society of Plant Pathology.

The congress attracted plant pathologists from around the world, many of whom raised issues of relevance to the Australian grains industry.

For example, fungicide use in Australia relies heavily on triazoles (demethylation-inhibitors – DMIs) and strobilurins (quinone outside inhibitors – QoIs).

Dr Cassiano Forcelini, from the University of Passo Fundo in Brazil, reported that heavy application of a mixed DMI-QoI fungicide has led to rapid emergence of fungicide resistance in soybean rust (Phakopsora pachyrhizi) in his country.

There, a decline in effectiveness of the DMI portion of the fungicide mixture has resulted in losses exceeding US$2 billion (A$2.1 billion) for soybean growers. This is the first report of fungicide resistance to this devastating rust pathogen in Brazil. Currently, 99 per cent of Brazilian growers apply three to seven fungicide sprays per season to control the disease.

However, promising results were seen in the control of soybean rust after substitution of the DMI component of the fungicide mixtures with a new mode-of-action fungicide, the succinate dehydrogenase inhibitors (SDHIs). However, this success will be limited to the continued efficacy of the QoI component.

In another fungicide resistance presentation by Andrew Leadbeater from Syngenta, QoI fungicides were categorised under the ‘high-risk’ category for development of resistance.

Heavy use of QoI fungicides in Europe has resulted in the emergence of populations of fungi that carry a single point mutation conferring full resistance. This mutation was acquired in less than five years after widespread use of QoIs in the field.

In another approach to disease management, a new gene technology known as clustered regularly interspaced short palindromic repeat (CRISPR) was presented by Dr Sophien Kamoun from the UK’s Sainsbury Laboratory. This technology has the potential to target and modify specific genes in any crop or pathogen species.

The technology involves selectively removing a small piece of a desired plant gene. This is potentially the first technology to allow targeted gene deletion in crop plants, while avoiding the consumer-wary label ‘genetically modified’.

New ideas on more traditional methods of disease management were also presented. Professor Bruce McDonald, from ETH Zurich in Switzerland, provided evidence from several field studies that limited genetic diversity in our major crops is increasing their vulnerability to diseases.

He proposed that increasing crop diversity is key to the future sustainability of agricultural ecosystems. Low-tech solutions, such as crop rotation, cultivar mixtures (either by row or randomly through seed mixtures) and intercropping (planting of two different crop species in alternating rows) were suggested as quick and easy options.

Professor McDonald also proposed that temporal rotations of dominant resistance genes would prolong the life of these genes used in the field.

Several high-tech solutions to increasing crop diversity were also presented. The most promising hypothetical solution involved the creation of ‘resistance-gene cassettes’.

These cassettes would comprise several resistance genes of choice that could be inserted into a finished cultivar after the completion of conventional breeding. These cassettes would be interchangeable between years and places, allowing breeders to have finite control over which resistance genes would be deployed in a given area.

'This new technique works by trapping the spores of the pathogen and automatically germinating these in growth medium within the trap.'

While this technology does not yet exist for conventional breeding, new technologies such as CRISPR may eventually make resistance-gene cassettes used in breeding a reality.

Emerging crop diseases were also a strong theme of the congress.

Professor Paulo Ceresini, from Brazil’s Universidade Estadual Paulista, reported on the recent emergence of wheat blast disease (Magnaporthe oryzae) in Brazil. This is a major pathogen of wheat throughout that country and is now commonly detected in neighbouring countries.

Populations collected from distant regions in Brazil demonstrated that this pathogen has potential for long-distance dispersal, highlighting the need for Australia to maintain strict biosecurity controls when exchanging material between continents.

New technologies were also presented on the development of automated early disease warning systems for growers.

Dr John West, from Rothamsted Research Station in the UK, has developed an air sampling technique to enable the rapid detection of sclerotinia stem rot (Sclerotinia sclerotiorum).

This new technique works by trapping the spores of the pathogen and automatically germinating these in growth medium within the trap.

If the spores of the pathogen are present, it emits a known chemical signal (oxalic acid) that is detected by sensors within the trap. The sensor automatically contacts growers in the region by text message, informing them that the disease is in the area.

While initial investment costs are high for these automated sensors, there are clear benefits to growers in terms of informed disease management based on rapid and automated disease detection.

More information:

Associate Professor Peter Solomon,
02 6125 3952, 
peter.solomon@anu.edu.au

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