As potato growers gear up for the new planting season, Syngenta’s technical manager, Michael Tait, highlighted growers need to be thinking about harvest skin finish with treatment options as seed goes into the ground during the recent series of meetings, Potato Science Live.

At the events, one of which was held in Dundee, he reported effective suppression of black dot and rhizoctonia solani with Amistar used in-furrow, which had become an industry standard. But difficult spring conditions, leading to extended growing seasons, combined with the challenges for store management, posed a future threat.

Mrs Tait also presented Dutch research that demonstrated the recently introduced fluxapyroxad soil treatment had very limited activity on the black dot pathogen, compared to Amistar, which is intrinsically highly active. For soil-borne rhizoctonia, the research showed Amistar was also equally effective as fluxapyroxad.

“Furthermore, in field trials looking at possible effects on crop emergence, there was no difference in emergence and successful crop establishment between Amistar and fluxapyroxad,” he added.

PCN prevention

Whilst the new SDHI fungicide, fluopyram, applied at planting may have a role to protect short season potato crops from PCN damage, Mr Tait emphasised it cannot be used in any combination with Nemathorin if the crop is grown for less than the statutory 119-day PHI.

“There has been some discussion around use of fluopyram with half-rate Nemathorin, but it makes no agronomic sense to reduce the rate of the most effective component,” he advised. “For yield, PCN control and management of multiplication, trials have consistently shown Nemathorin is superior.

“There is some research work from Scotland that suggests there may be some benefit from full rate Nemathorin and full rate fluopyram together.”

Environmental protection

The new Syngenta’s ‘green headland mix’ has proved highly popular with potato growers for environmental protection and soil improvements.

Planted on non-cropped headlands it can serve to capture nutrients to the value of £200/ha, as well as offer significant benefits in soil structure enhancement for the following crop, claimed Syngenta ecology specialist, Dr Max Newbert

Speaking at the Potato Science Live event, in Dundee, he reported more than 300 ha has been established under the ‘Operation Pollinator’ initiative. And the numbers of insects that have been found in monitoring has been truly mind-boggling.

In 2017, entomologists captured more than 41,000 insects, compared to 11,000 the previous year. A more detailed study of ground beetles had shown a significant increase in beneficial predators, he added.

“Of the 312 species identified, 60 were pollinators and at least 113 known to be predators of pest species at some point of their life cycle. There was a very high level of predator to pest ratio.” Importantly, no aphids were found in any of the insect monitoring.

“We will be further evaluating the role of ‘green headlands’ to mitigate risk of virus spread into potato crops,” he said. “In addition to the number of predators present, any aphids that spend time in the margin as they pass through would lose the capability to transmit non-persistent viruses.”

In carrot crops surrounded by the mix, the 2018 trials had shown a 70% reduction in yellowing symptoms of virus, he added.

Aphid threat

Where aphids are a threat in the potato crop, Dr Newbert advocated that for non-persistent viruses, growers need to start with a fast-acting knock-down option, before aphids start probing plants for food.

Infection can take just seconds, he pointed out. His strategy for 2019 would be to start with Hallmark Zeon, before switching to thiacloprid and then pymetrozine, or flonicamid where required for seed crops. If pyrethroid resistant aphids are present, always use an alternative mode of action, he emphasised.

Whilst spirotetramat had historically given good levels of aphid kill, its slow action will have little effect on points of non-persistent virus infection. In research trials, thiacloprid appeared to initially irritate treated aphids and stimulated greater probing, leading to increased virus transmission before the aphids were killed.

However, Dr Newbert urged growers to carefully assess risk and aphid migration patterns, to only use insecticides where truly required and justified. He also warned that the loss of seed treatments in cereal crops for BYDV aphids, growers should be aware of issues with over reliance on foliar insecticides, particularly pyrethroid sprays – which would also have implications for control in potato crops. Treatments should only be applied when thresholds are reached, he advised.

Blight changes

The ever moving target of blight races has seen further changes to challenges evolving for growers, according to Mr Tait. He highlighted that monitoring in 2018 involved a relatively low number of samples from across a limited geographical spread, but still gave some clear indications.

The genotype 36_A2 had increased as a percentage of the total population in this sample, he reported. When tested under very low doses of fungicide actives, in trials commissioned by AHDB Potatoes, 36_A2 also developed larger lesions than other genotypes.

“It’s something to watch but it’s important to remember that these tests were conducted at extremely low doses and there is no evidence of resistance to 36_A2 in any of the actives which were tested,” he added.

“It has, however, reinforced the message around the need for application to get the full dose throughout the canopy, and a tight application interval to maintain an adequate dose on the leaf,” he advised growers and agronomists at Potato Science Live.

He also urged caution where blight fungicide tank mixes could see one active decline quickly on the leaf, which would leave the other potentially exposed to resistance developing. “The experience with blight strain 37_A2 and its reduced sensitivity to fluazinam should be a salutary lesson for growers,” Mr Tait warned.

However, with the significantly reduced use of fluazinam, there could potentially be a lower incidence of 37_A2 race in the future. “It has absolutely proven the benefit of robust rates and tight spray

intervals as well as alternating modes of action, such as Revus, through the programme, rather than blocks of the same product or active group for the 2019 season.”