Mystery bacteria linked to US potato disease

Last updated 00:56 25/07/2008

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A plant disease scare in tomatoes, which triggered biosecurity bans on New Zealand potatoes, tomatoes and capsicums, has now been linked to a disease in American potato crops.

And the potato disease, known as "zebra chip", has now been discovered in an Auckland spud crop, NZPA has been told.

The disease-causing Candidatus liberibacter bacteria species disrupted New Zealand exports last month when agriculture authorities revealed its discovery in three Auckland commercial hothouses in January.

The announcement by NZ scientists that it was a previously unrecorded form of C. liberibacter spooked some key export customers, as it is related to huanglongbing disease – also known as citrus greening.

The warnings also unfortunately coincided with overseas headlines about an unrelated bacteria – salmonella – on tomatoes in America making people sick.

Australia blocked imports of tomatoes, capsicum and potatoes, and tamarillos, Fiji blocked tomatoes, capsicum and potatoes – and is now running short of spuds – and Japan also stopped key imports.

Overall tomato exports are worth $7.3 million annually and capsicum exports are worth $34 million. Growers are keen for constraints on exports to be lifted by October, when they are due to ramp up harvests from new crops they are now planting.

But now the Ministry of Agriculture and Forestry (MAF) has told growers that DNA tests developed in New Zealand show that the bacterium is in Texan potato crops, and is probably the cause of "zebra chip" disease which has been hitting American crops for years.

"The liberibacter detected in the USA is the same species as that detected in NZ," a MAF spokeswoman said.

Zebra chip converts part of the starch in a potato to soluble sugar, and when the potato is cooked, causes zebra-like stripes and breaks up the chip.

Spread by potato psillid insects – scientifically known as Bactericera cockerelli – the disease is thought to have been introduced to the US 15 years ago from Central America.

This insect also mysteriously turned up in New Zealand – found in an Auckland greenhouse in 2006 – and has since dispersed as far south as Nelson.

Zebra chip is a serious problem in regions such as Texas, particularly on species used for manufacture of french fries, and New Zealand technology companies have supplied American growers with NIR (or near-infrared) scanners to screen out affected potatoes.

MAF said it had provided American researchers with genetic markers for the bacteria and two laboratories detected it in Texas, in russet norkota potatoes showing zebra chip symptoms. The US has not reported the bacteria causing problems in tomatoes.

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The identification means that trade officials re-negotiating access for NZ fruit and vegetables to overseas markets are no longer dealing with an unknown disease – they have been able to tell customer countries that the bacteria is probably the cause of zebra chip.

MAF said that after the announcement that the mystery bacteria was likely being spread by potato psyllids, it was first told of zebra chip symptoms in domestic potato crops.

Symptoms resembling zebra chip showed up in potatoes harvested from a breeding trial in South Auckland in May.

"These potato tubers tested positive for Liberibacter and was our first report of such in potatoes in NZ," the MAF spokeswoman said.

Potato psyllids feeding on spuds weakly infected with zebra chip spread the bacteria to other potato plants.

American experts said that controlling the potato psyllids and planting seed potatoes certified free of zebra chip can reduce the spread.


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