Free-living nematode target cuts spraing tuber damage

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New independent research has provided an insight for potato growers to tackle the increasing incidence of free-living nematodes (FLN) - and the subsequent spraing damage to tubers when they transmit tobacco rattle virus (TRV).

Trials by agronomy and root crop research specialists, Produce Solutions, have shown over 150% increase in the proportion of tubers that conform to stringent processing protocol standards with Nemathorin nematicide treatment, reported Syngenta Technical Manager, Andy Cunningham.

In the field trial, in a commercial crop of Pentland Dell on a high-risk site in Shropshire, less than 24% of harvested tubers from untreated areas hit the grading acceptability for a processor, compared to more than 60% meeting the specification with Nemathorin treatment.

Tuber assessments ranked 56% of untreated area tubers in the more severely damaged categories with over 12.5% of surface area affected by spraing symptoms. That compared to just 8% of Nemathorin treated tubers exceeding 12.5% of damage and zero in the highest damage category of 50% surface area affected.   

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Andy pointed out that changing climatic conditions have been conducive to FLN activity in recent seasons. FLN move more easily in a film of soil moisture between soil particles, with lighter sandy soils - favoured for potato cropping - the most susceptible in a wet early spring season after planting.

Furthermore, stubby root nematodes can proliferate on some plants in green cover crops. New generations of nematodes can pick up TRV from specific host weed species and perpetuate the potential to infect through any rotational break.

Investigations have also shown that some potato varieties can carry symptomless TRV, whereby undetected infected seed could result in the introduction of the virus into previously clean fields.

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“As FLN move freely through the soil they can infect multiple plants from initial foci points,” warned Andy. “Tubers are most susceptible to more damaging effects of spraing from early TRV infection. FLN activity can also result in reduced root formation and make plants more susceptible to other stresses through the season.

“Nemathorin can effectively protect the crop by paralysing nematodes for long enough to reduce initial TRV infection and for crops to develop past the danger period,” he added.     

Produce Solutions’ Team Lead, James Lee (below), highlighted FLN and the associated issue of spraing can result in significant crop losses. “The major challenge with FLN and spraing is that, unlike most other visual defects, suppliers can’t reduce the level of waste through grading/sorting.  The level of infection of spraing that will result in total crop failure is relatively low.”

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He added that spraing is a major issue for both packers and processors, which means affected crops have no secondary market of any value. While skin blemished tubers could still be used for secondary packing or processing, crops with significant spraing have little outlet beyond stockfeed. 

“The FLN and spraing pest/disease complex is a key factor in our agronomy advice and planning,” he advised. “It’s often very difficult to forecast when FLN and spraing are going to cause an issue. Soil sampling is not as straightforward as with PCN, while the presence of FLN doesn’t always result in TRV (spraing).  This adds a level of complication to decision making.”

Potato processors impose a strict grading system for the acceptability of tubers with visible effects of spraing, including the diameter and the length of the internal damage. For the FLN spraing trial, Produce Solutions assessed tubers for the percentage of visible damage to the cut surface, with five categories from zero, to over 50% of area damaged. 

The use of Nemathorin at the label rate of 30 kg/ha produced a 160% increase in percentage of marketable tubers with an acceptably low level of spraing, compared to untreated. The use of fluopyram in the trial, either applied at label rates in-furrow or overall and incorporated, provided no significant increase in acceptable yield over untreated.

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The trial also included application of Nemathorin at half-rate of 15 kg/ha, as used by growers for wireworm suppression, that gave a lower, but still a significant, 114% increase in the percentage of tubers acceptable for processing. Where fluopyram was used in combination with half-rate Nemathorin there was no additional significant benefit.

Andy pointed out the Shropshire site was specifically selected for its history of FLN, with pre-trial testing revealing a population of 433 stubby root nematodes per litre of soil, including both Trichodorus and Paratrichodorus nematode species most widely associated with spraing. The testing also showed the FLN to be carrying TRV.

“The trial results showed a clear correlation between the use of nematicides to target FLN and the reduction in the incidence of symptoms of spraing,” he highlighted.

“The most effective nematicide, Nemathorin, at the highest rate, gave the best result.”

Further trials had shown where there was a lower initial population of FLN, the differences were correspondingly less, but with the same result of reduced damage and greater proportion of acceptable yield with the Nemathorin treatment.

“That has confirmed the importance of accurate soil testing to identify the potential threat from FLN and the incidence of TRV in the population,” advocated Andy. “The presence of FLN can initially be very patchy in a field, before the population spreads. Where growers or agronomists identify weakened plants or crops with shorter, stubby roots, then GPS pinpointing or mapping the patches can facilitate more accurate soil testing when potatoes come around in the rotation again.”  

For potato growers facing multiple soil pests active at the same time, Nemathorin can also target potato cyst nematode and wireworm with the one application, he added.