Keeping PC N at bay
Keeping PC N at bay
Potato cyst nematode is on the increase, now infesting two-thirds of the UKs potato growing land. Peter Read reports on how to keep on top of this virulent pest
POTATO growers are not winning the battle against potato cyst nematode (PCN). The bald figures – average yield losses of 5% to 10% costing UK growers £40m a year – conceal at worst, crop failure and at best, significant expense on control tactics.
Why are we failing? Complacency and lack of field knowledge on PCN populations, according to Pat Haydock at Harper Adams University College: "And it needs to be on a field by field, year on year, basis."
Carefully timed, site-specific soil testing in every rotation is a must as part of a three-pronged integrated approach. "All potato growers should be routinely soil sampling," he says. Then they can confidently take one or more of the necessary control measures:
• sowing resistant potato varieties
• lengthening the rotation
• applying nematicides.
Soil sampling method and timing will depend on your objective. Is it simply to confirm the presence of PCN? "If so, maximise the chances of finding low populations by spot sampling in gateways, field perimeters, under graders and suspect unhealthy rows."
Or is the sampling to establish control strategy effectiveness, or to provide a natural decline estimate? "Then, sample after cultivations, when the cysts have been spread evenly throughout the soil profile. On a one hectare block basis, at least 50 cores should be bulked to provide a 1-2kg sample, of which 200 and 500g will be used for laboratory testing. Subsequent annual sampling for estimating PCN decline can be made less costly by only sampling a small area, perhaps one 20m x 20m block per field."
Soil sampling results should influence varietal choice. The two species Globodera rostochiensis and Globodera pallida can be identified using polymerase chain reaction (PCR) genetic finger-printing of cysts from a soil sample.
Most potato varieties have resistance – with virtually no cyst multiplication – to G rostochiensis reproduction. Unfortunately rostochiensis is giving way to pallida. For this strain only partial resistance is available, in a few varieties – notably Nadine, Sante and Rocket.
Resistance must not be confused with tolerance. Tolerance is a measure of a varietys ability to withstand PCN damage, and in no way reflects a varietys ability to reduce the multiplication of PCN. NIAB has insufficient data to produce ratings but, as a guide, varieties exhibiting vigorous growth tend to be most tolerant. "Local conditions influence tolerance, therefore use local knowledge of a variety," says Dr Haydock.
Tight potato cropping has been an important causal factor for the high level of PCN. "Half of the potato rotations are one year in five and shorter. If all rotations were one year in 10 we wouldnt have the PCN problem. By lengthening the rotation, growers can capitalise on the nematodes natural decline rate – between 10 and 40% annually."
Golden rules
Even at low PCN populations, granular nematicide application, at £350/ha (£142/acre), can be justified on the grounds of long-term control. But there are some golden rules for successful application. "Granular products – be it Temik, Vydate or Nemathorin – should be applied and incorporated on the same day as planting – because they all degrade rapidly – with an applicator that is correctly calibrated for the specific granules."
Many years of research by Harper Adams Dr Simon Woods have shown that the method of application is important. "Broadcasting in front of a bed tiller and incorporating granules with the bed tiller rotovator, is an excellent method for getting even distribution, providing it is followed by a star-type separator," says Dr Haydock.
"But, bed tiller application followed by a webbed stone and clod separator may mix the granules too deeply, reducing the effective nematicide concentration. Such separators can be successfully used, provided that the applicator is mounted at the front of the separator." And soil conditions are equally important as method of application. "A fine tilth is necessary to enable thorough mixing of granules and soils."
Soil fumigants are the cavalry to call when granular nematicides are not preventing yield loss. Telone is contractor-injected in the autumn, at a cost of £570/ha (£231/acre); it is effective at any rotation stage. "Samples should be taken in the first spring following the potato crop. Combined with previous years decline estimates, the need for Telone can be predicted and planned into the rotation. By sampling early you have the widest possible window for injecting during suitable soil conditions."
Soil conditions are the key to Telones efficacy: a light, well aerated structure, free from cultivation pans, together with the necessary temperature and moisture content. "Soil temperature at 20cm depth should be 7íC to 15íC. And soil moisture content should be 40% to 60%. To get the right temperature and moisture content, consider irrigating after harvesting while soils are still warm. September and October are the normal injection months – any later, and it is often too cold."