Wetenschap - 1 september 2011

A grip on Phytophthora

The wet, relatively cool summer weather has led to potato disease running riot throughout the Netherlands - with the exception of some Wageningen UR experimental fields. The Wageningen GM spuds are resistant to Phytophthora thanks to an accumulation of resistance genes.

Researcher Geert Kessel of Plant Research International has three experimental fields with potatoes in the Netherlands. The fields consist of around a hundred plots, each with six plants, ranging from wild relatives of existing potato varieties to genetically modified (GM) potatoes. They all have a different combination of genes that are thought to protect potatoes from Phytophthora, which causes the dreaded potato disease.
'We use an assortment of resistance genes in our research', says Kessel. 'The good news is that certain combinations of these R-genes keep potatoes free of Phytophthora. We have seen that one resistance gene is not enough, and also that weak resistance genes are not up to the job. Phytophthora is an extremely stubborn pathogen but you can keep potatoes free of disease by adding several strong R-genes.' The Wageningen potato researchers use genetic modification to add a specific set of R-genes to the potato genomes of existing varieties.
The existing potato varieties were bred without using GM. They are generally highly susceptible to Phytophthora and arable farmers currently have to spray a lot of fungicide to keep the mould under control. Alternatively, they have to kill the potatoes by spraying and then harvest quickly, which gives much lower yields.
Whether or not the potatoes were GM was not a decisive factor in the experiment, says Kessel. 'What matters is the quality of the resistance genes.' But Kessel still needs some fungicides even when using a combination of strong R-genes. 'If we don't spray when disease pressure is high, there is a risk that Phytophthora will even get around combinations of R-genes through natural mutation and selection pressure. Using strong R-genes means we can achieve sustained resistance while using 80 percent less fungicide.'
Another experiment is being carried out on potatoes' resistance to Phytophthora in Wetteren, Belgium. Belgian websites contain shocked reports that Phytophthora has been found on genetically modified potatoes. That is only to be expected, says Kessel. 'The pathogen is everywhere and disease pressure is high. But we will only know after lab analyses whether resistance genes have been broken.'
 

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