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Routine survey of tomato growers underway

9 February 2001

Routine survey of tomato growers underway

The New Zealand Vegetable and Potato Growers' Federation (VegFed) and the Ministry of Agriculture and Forestry (MAF) are working together to determine if the potato spindle tuber viroid, which affects potatoes and tomatoes, is present in commercial production sites in New Zealand.

MAF contract staff started working in the field yesterday (Thursday 8 February) inspecting commercial tomato crops in the Auckland area. The survey is expected to take a maximum of two weeks.

MAF received confirmation of the presence of the viroid on one Auckland tomato growing property on 26 January 2001. No other possible sightings have been received. The survey is a routine progression of an investigation that started at the Auckland property last year.

The potato spindle tuber viroid can affect tomatoes and potatoes. In tomatoes it can cause yellowing of leaves and shortening of stems, making them appear malnourished, except that the yellowing occurs in the tops of affected plants and the leaves may curl downwards.

In potatoes, when symptoms show, the plant may be darker green than normal, the leaves are smaller with fluted margins, and the leaves near the ground are noticeably shorter and held upright.

"In some plants, the viroid shows no symptoms at all, as severe and mild strains exist and response also varies with crop variety," says Barney Stephenson, MAF's Plant Pest Surveillance and Response National Advisor. It is known only to occur in plants and poses no risk to human health.

Dr Stephenson said the viroid has only been found in one tomato crop in Auckland, and the field survey will focus on checking major commercial tomato growers. Potato fields around the infected tomato crop will also be checked. Laboratory tests are the only way to confirm the presence of the viroid.

The viroid can be eliminated by management practises, particularly through planting clean seed and crop rotation. It is spread principally from seed, and in tomatoes, between plants by the transfer of sap during routine management such as pruning.

Aphids can also spread it, but only if the potato leafroll luteovirus, which is found in New Zealand, is present and acting as an encapsulating agent. MAF considers the occurrence of aphid transmission is unlikely at this point in time.

The most likely source of entry of the viroid into New Zealand is by imported hybrid tomato seed, which is required by industry for their commercial crops.

This has traditionally been seen as a low-risk form of entry and MAF is now reviewing its position with respect to modifying import requirements for tomato seed. The field survey may assist with tracing the source of any problem seed.

MAF and VegFed are investigating the potential market impacts, and at this stage the detection is not expected to have a major influence.

For further information contact:

Barney Stephenson, MAF's national adviser, Plant Pest Surveillance and Response. Telephone 04-474-4102

or Gita Parsot, MAF Communications. Telephone: 04-498-9806

or Peter Silcock, Chief Executive, Telephone 025-487-036

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