14 March 1996

Pest build-up risk when nematicide used with separator

In the second of our series of articles taking an "In Depth" look at arable issues we consider nematicide use. Careless application of nematicides may go unnoticed in this seasons crop, but will create future problems, reports

Robert Harris

TWO-THIRDS of UK potato growers apply nematicides while stone or clod separating. Poor technique could leave them at considerable risk, says Rhône-Poulencs David James.

"Unfortunately, such machines are not good at mixing soil. They act as a sieve, applying nematicide in a layer, rather than incorporating it thoroughly."

According to RP trials carried out with a range of independent institutions, that has little affect on yields of potatoes planted in the same year. "Its easy to protect yields using a nematicide in a routine way."

However, long-term control suffers. "The hidden danger is a population build up." Independent work at Rothamsted and ADAS Arthur Rickwood has shown numbers, measured as eggs/g of soil, can double when products are applied through a stone or clod separator, compared with other methods, says Mr James.

Other work with ADAS Leeds and the SAC showed why. "On light land, soil tends to fall through the web quickly. That means chemical tends to be applied in a thin band high in the ridge. In heavier, wetter ground soil remains on the web for longer, so chemical is applied deeper – but still in a band."

The technique will remain popular with growers, because it saves an extra pass, reduces compaction, and ensures minimum delay between application and planting. But separators are difficult to keep at a level working depth. Soil cover, and consequently where it falls through the web, varies.

That makes it difficult to correctly position fishtail applicators to ensure chemical ends up near the middle of the ridge. The best compromise is to position them as far forward as possible, just behind the front roller, he advises.

Another disadvantage is that separators leave a narrow strip on each side of the bed unlifted, and consequently untreated. Any nematodes present will hatch when potato roots reach those areas, increasing numbers. Once the field is cultivated after harvest, eggs sacs will be spread throughout the soil ready to infect future crops.

Growers with high potato cyst nematode counts (such as 20 eggs/g plus), and/or those with tight rotations, should consider alternative application methods, Mr James suggests. "Temik, and other granular nematicides, work best when they are incorporated in to the top 15cm, or 6in of soil."

Improved distribution can give up to 1t/ha more yield, but more importantly it typically halves population increases.

An L-shaped or spiked rotovator is the best tool. Trials in Yorks in 1992 showed this method of incorporation maintained an initial nematode population of 30eggs/g at that level. Using a separator to apply chemical allowed populations to double during the season.

Even distribution

The reason is that rotovators distribute the chemical evenly through the top 15cm of soil. Power harrows which act in a horizontal plane, tend to keep most of the chemical in the top 5cm band, says Mr James. On peat soils at Arthur Rickwood that allowed numbers to rise 50% above rotovated plots.

Bed tillers are gaining favour to knock down clods in over-wintered beds before separation or planting. Those may be the ideal incorporation tools, says Mr James. The spiked rotovator action incorporates chemical throughout the profile and also works soil inbetween beds, ensuring the whole field is treated.

Although expensive, Mr James believes growers with high PCN populations will recoup cost quickly. "With only limited land available, unless growers keep on top of nematodes by thorough incorporation, they could soon have a big problem. Once you have more than 80 eggs/g, you may have to use Telone as well as Temik. That will add another £460/ha to growing costs."

Mounting applicator fishtails at the front of the separator is the best compromise when applying granules, says Rhône-Poulencs David James. Growers with a big PCN problem should consider other methods.

Population shift

Pallida nematode populations only decline by 12-15% a year, compared with 30-40% for rostochiensis. Pallida is becoming more prevalent, mainly due to the introduction of rostochiensis-resistant varieties which select for them. Control is declining as a result. But small changes in application technique can make a big difference.


&#8226 Mix 15cm deep for best control.

&#8226 Spiked or L-blade Rotavator best.

&#8226 Separators apply chemical in shallow band.

&#8226 Mount fishtails near front of machine.

&#8226 Plant as soon as possible after application.