Nematicide application and persistence are vital for the best levels of reduction of spraing, according Dr Pat Haydock, leading nematologist at Harper Adams University College.
Speaking to growers from Herefordshire, Shropshire and the West Midlands last week (12 January) he said the key is to provide sufficient concentration to stop them getting to the roots and feeding, where there is the risk of Tobacco Rattle Virus transmission and the physical feeding effects resulting in reduced yield of misshapen tubers.
Dr Haydock reports there are clear varietal differences in the susceptibility to spraing tuber damage. French fry processing crops face the biggest problems, with poor resistance to spraing in the major cultivars – particularly Russet Burbank and Pentland Dell. Crisp crop growers are better served, with the exception of the new variety Courlan which has low resistance rating for spraing and no PCN resistance.
With the introduction of Nemathorin (fosthiazate) for spraing this season, Dr Haydock advocates growers take a proactive approach to tackling the free living 'stubby root' nematodes responsible for the spread of TRV. He highlights a number of key risk assessment measures growers can look at:
• Previous crop history; if you have suffered spraing in the past it is almost impossible to rid it from the soil.
• Soil type; light textured sandy loam soils allow nematodes to move freely – clay soils rarely suffer spraing.
• Soil moisture; nematodes need moisture to lubricate movement through the soil. Dry soils are generally problem free, but irrigation provides the necessary soil moisture.
• Weeds; free living nematodes will survive and feed on a range of arable weeds, including Chickweed, Fat Hen, Mayweed and Groundsel – all of which can perpetuate TRV. Control of weeds through the rotation can help minimise TRV carry over, but virtually eliminates the chance of ridding soil of disease.
• Docking-disorder in Sugar Beet; if you have seen sugar beet docking-disorder in the field then the spraing nematodes are present (although not necessarily carrying TRV).
• Seed sources; aim to protect clean land from the introduction of TRV with clean seed sources. Key problem is that some seed may be carrying the virus, but show none of the symptoms. Crops grown from infected seed may also show no visible symptoms, but will leave a legacy of infection in the soil.
• Soil sampling; PCR tests will show if you have nematodes, and if they are carrying TRV. Nematodes move freely through the soil, so sample in warm, moist conditions, to a depth of at least 20cm. Handle soil samples with care to avoid killing nematodes.
Risk assessment of virus risk is a necessary part of product justification within an Integrated Pest Management strategy, says Dr Haydock, but growers may well need to apply nematicide where they have a risk of rejection. The effects of TRV, mainly associated with internal brown arcs in the flesh, can also be to reduce ware yields by 20 to 30%, he warns, with increased tuber numbers by decreased tuber weight and an increase in the number of misshapen tubers. In severe cases, emergence can be delayed.
Growers' options have increased this season with the approval of Nemathorin for spraing, including an in-furrow application recommendation along with overall incorporation. Syngenta Technical Manager, Jon Ogborn, believes the in-furrow application will prove an attractive option for growers targeting spraing and wireworm, with trials showing similar results to overall application, but at a lower rate and the convenience of application on the planter.
Dr Haydock adds the relatively high water solubility of oxamyl nematicide, compared to fosthiazate, increases the risk of active leaching away in wet conditions that are most conducive to spraing nematode movement; when conditions do warm up and nematodes get active potatoes could be left unprotected. "With contracts allowing only five to 10% for total defects, if there's any other damage it could be zero tolerance for spraing," he added.
