Growers fear pest's impact on potatoes

Tomato and potato-growers are grappling with a new plant pest found on plants in Auckland and Nelson.

The pest, identified as potato spindle tuber viroid, has been discovered stunting tomato crops but is thought to be more of a threat to potatoes.

A tomato-grower's checks on symptoms of a suspected exotic disease led to a crown research company's alerting the Ministry of Agriculture and Forestry to the problem last May, but samples checked by MAF did not show the presence of the viroid, which is much smaller than a virus. It consists of a small nucleic acid molecule lacking the protein coat of viruses.

The grower then had DNA from the affected plants sequenced in the Netherlands, and the problem was identified as the potato spindle tuber viroid.

A MAF survey confirmed its presence on tomato crops at three sites around Auckland and one at Nelson.

The viroid has never before been found in New Zealand in either tomatoes or potatoes. It has not yet been found in potatoes.

MAF's national adviser on plant pest surveillance, Barney Stephenson, said it was most likely the disease came into the country through imported seed contaminated with the viroid.

But it had proved difficult to find common factors in the seed used by the four infected properties, and it was likely the viroid had entered the country at some other property and been spread between crops through infected sap.

Dr Stephenson said the original symptoms had been similar to nutrient deficiencies in tomatoes, and the stunted growth was estimated to be likely to reduce production of an infected plant by between 10 per cent and 20 per cent.

"We feel that the overall production loss in tomatoes is not that serious," he said.

But in potatoes, the disease could be potentially more harmful, with a drop in production of up to 60 per cent in susceptible cultivars, though a non-susceptible cultivar infected with a mild strain might lose only 10 per cent production, according to overseas figures.

MAF did not believe a Government-driven eradication programme was feasible, but had involved the Vegetable and Potato Growers' Federation so it could formulate an industry plan. This would probably mean taking tomato crops through a season and then having a break, which would clean out any viroid that could be carried over.

Concerns about transmission by insects were low-key because the aphids needed to also be carrying another virus, such as tomato yellow leaf-curl or potato leaf-roll to transmit the viroid as well.

Vegfed executive Ken Robertson said the organisation was helping tomato-growers with information that would make it easier to identify any infection, and advising them on improving their greenhouse hygiene.

A meeting of MAF and industry growers this week would discuss with seed companies factors they needed to take into account when importing seed for greenhouse tomatoes.

Potato-growers are expected to combat any spread of the disease by using seed certified to be free of pathogens.

- NZPA