Nematode crackdown threatens most table potato growers

Press, 11 July 1986, Page 12

Nematode crackdown threatens most table potato growers

By

DAVID LUCAS

Much of the traditional potato-growing land on the outskirts of Christchurch could be banned from potatoes under potato cyst nematode measures set by the Ministry of Agriculture and Fisheries. New restrictions on growing potatoes on P.C.N.-infected land near Christchurch will take effect from the 1987-88 planting as part of a tightening in P.C.N. control.

The Plant Health Division of the M.A.F. maintains the measures are necessary to safeguard New Zealand’s high reputation for plant health. P.C.N. is regarded as a major international plant health pest and is attracting increasing attention in trade access matters, say plant health officers.

The M.A.F. has decided that potatoes will be al-

lowed to be grown only on paddocks which during sampling contain no live P.C.N. eggs per litre of soil.

Previously Christchurch growers had been granted a concession allowing potatoes to grow on a four-year rotation on land which contained no more than 2000 live eggs per litre.

The removal of the concession will bring the Christchurch district into line with other parts of the country, but in so doing could make potatoes a banned crop in up to IOOOha of land. Land at Marshland, Harewood, Broadfield, Yaldhurst, West Melton and Clarkville, already scheduled under P.C.N. policy, will be affected.

Just over 1161 ha in 129 titles is at present classified as schedule one

where the concession allows potatoes to be grown every four years providing the live egg count does not exceed 2000. Of schedule one properties, growers on 56 titles last season were allowed to grow potatoes, but under the new zero live egg requirement only three growers are likely to be given permission.

The new restriction will virtually eliminate the availability of black soil, Chippewa potatoes which command a price premium in Christchurch markets.

But more importantly, it will have severe economic consequences for growers, many of whom have invested thousands of dollars into specialised potato machinery. News of the removal of the concession has come as a blow to growers who maintain that P.C.N. had been contained and been kept under control by the existing measures. They are disappointed the M.A.F. has decided to change the rules, but are reluctant to talk about the issue as they have asked the Director-General of Agriculture, Mr Malcolm Cameron, to reconsider the removal of the concession. Existing growers have

almost exhausted the supply of P.C.N.-free ground available for leasing close to their properties and because of the removal of the concession some growers intend moving their whole potato-grow-ing operations further afield. This could include moving to traditional seedgrowing districts round Courtenay, increasing the risk of a rapid spread of P.C.N. if the pest is transferred during the shift. Plant protection officers believe firm control measures (including the zero live egg requirement) offer the best chance of containing P.C.N. and possibly, in the long term, even eradicating the pest. Mr Gavin Harper, of the M.A.F. plant protection centre at Lincoln, feels that if the Christchurch 2000 egg concession remained in force for another five years, any chance of eliminating the pest would probably be lost.

The nematode population was still in a localised state and very few paddocks contained a uniform spread of infection, indicating it had not yet become well established. No serious crop reduction had yet occurred.

At Pukekohe, under a zero live egg requirement, an increasing percentage of scheduled paddocks were being allowed back to potatoes, indicating that egg levels were dropping significantly, he said.

Mr Harper pointed to Israel and South Africa where hard-line policies had apparently been successful in reducing or even eliminating P.C.N. The success was difficult to substantiate because countries were naturally hesitant to discuss their P.C.N. status.

Just how P.C.N. arrived in New Zealand is not known, but scientists suspect it could have been introduced with potatoes which were allowed to be imported without many restrictions until 1969. P.C.N. was first noticed in New Zealand at Pukekohe in 1972, at Marshland three years later, and Dunedin the following year.

Along with the control measures, the stable size of New Zealand’s potatogrowing area (about 10,000 ha) had helped prevent a rapid spread of P.C.N. If potatoes had become a highly profitable or “boom” crop many newcomers might have

been attracted to the industry, accelerating the spread of P.C.N. Mr John Hercus, director of the M.A.F. Advisory Services Division, has acknowledged the difficulty in drawing the fine line between minimising the effect on growers’ incomes and preserving the effectiveness of the P.C.N. programme. The decision to withdraw the concessions was made in view of the international plant health implications of P.C.N. and concern at the scientific soundness and effectiveness of the concessions, said Mr Hercus. The matter had not been treated lightly, he said. The P.C.N. programme was the best way to contain the pest to limit its effect on production in New Zealand and to allow New Zealand to maintain its high plant health standing with its trading partners.

Under the new conditions affecting Christchurch growers, potatoes will be permitted to be grown in a previously scheduled paddock only if the soil sampling shows no sign of viable eggs and provided the paddock is fumigated with a nematicide or a nematode-resist-ant cultivar is used.

Growers have to agree also to pay the cost of fumigation if viable eggs are later found in a preharvest fork survey.

Growers will be not able to use seed from schedule one areas, bringing a halt to the normal practice of saving half the seed from the previous year’s crop and buying fresh seed for the balance.

The M.A.F. endeavours to survey every potato crop each year and last season covered 620 ha in Canterbury between Waimate and Oxford. Since the first nematodes were found in the Christchurch area in 1975, the number of new finds has dropped dramatically. In 1975, 60 discoveries were made, dropping to 12 new finds the following year, six in 1980, and one in each of the last two years.