The Challenges of Blight Evolution - Potato Review - David Harris

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Agronomist Craig Green stressed that the increased aggressiveness of the 36_A2 and 37_A2 strains makes them the "ones to watch" and said growers must not be complacent.

Craig, who annually advises on 1,000 ha of potatoes between Kings Lynn and Norwich also predicts that seed quality, lower water levels in farm reservoirs and rising aphid populations may be other key factors for growers to consider during 2019.

"Although much of the seed I have seen so far has been generally of good quality, there will be some concerns on availability of good seed given last year's drought that effected yields in some of areas of Scotland where much of our seed is sourced, he said. "Another knock-on effect of the lengthy 2018 summer was reduced water levels in farm reservoirs. I believe some growers will be downsizing their potato acreage as a result of their current reservoir levels. The warmer weather we have experienced so far this year could also lead to a rise in populations of peach potato aphid (Myzus persicae) which would be another early season concern.”

On blight strategies, early vigilance and planning is as crucial as it's ever been, said Craig. Whilst both the two most common blight strains (Blue 13_A2 and Pink 6_A1) seem to have been largely well controlled last year and, with the fluazinam resistance of the 37_A2 strain now fully confirmed and widespread, growers should be more wary of the less well known and, potentially, less predictable 36_A2 genotype.

36_A2 produces high numbers of sporangia per lesion so it's vital growers and advisers implement good cultural control strategies right from the early rosette sprays. Close dialogue with sprayer operators is critical, and do not plant anything in a position where it cannot be sprayed effectively such as close proximity to telegraph poles or pylons. Keep seven days as the maximum spray interval but be prepared to drop that to three to five days as temperatures and humidity rise," he said.

On tank mix, Craig advocates alternating products with different modes of action to maintain control and fight resistance.

"My plan is to start with Ranman Top (cyazofamid) partnered by mancozeb or with some cymoxanil for kickback activity. Ranman Top performs exceptionally well in this early slot. It is consistent and controls both foliar blight and early Zoospore activity. If temperatures drop and there is an increased risk of Oospore activity, Ranman Top is also one of the best options to deal with that alternative scenario.”

During the rapid canopy to stable canopy phase, he will be adding some foliar nitrate and other trace elements to the sprays to boost the plant's canopy health and immune system. "From there on in, I will be mixing and matching other actives such as mandipropamid + cymoxanil and fluopicolide + propamocarb, which offers good systemic activity, whilst factoring in the weather conditions, the varieties being grown and the economics. For later season control of tuber blight, I will return to Ranman Top with cymoxanil and mancozeb," said Craig. "For pre-harvest desiccation of the crop, many growers will be using diquat for the final time so it is also worth noting that Ranman Top is an effective tank mix partner that performs well alongside diquat for that late season control as you approach desiccation.”


Heightened chance of disease

A heightened risk of late blight in main crop potatoes may be one of the lingering consequences of the hot, dry weather many growers experienced in 2018. Few rain events from May onwards last year lowered blight pressure for many during most of last season.

But size and quality was affected - something that Craig Chisholm, field technical manager with Corteva Agriscience, says could heighten the chance of disease this season.

"Because tubers were generally smaller owing to the weather, particularly in unirrigated fields, more will have passed through harvesters back into the soil" he said. "The impact will be an increase in the untreated reservoir source for blight. Some volunteers may come through carrying infection on the stems."

Craig added that tight seed supplies may require some growers to consider using home-saved seed which may also contribute to an increased blight risk.

"Add these factors to the mild winter weather not having any significant impact on volunteers and discard piles, the potential is there for both early and sustained blight pressure." he said.

He advises growers implement robust programmes from the rapid growth phase and says Zorvec Enicade (oxathiapiprolin) plus its designated partner product would be highly effective. Launched last year, Zorvec has preventative, persistent and curative properties, moving upwards into new growth during stem extension. It allows for spray intervals of up to 10 days, adding three days of flexibility for growers who were limited to five or seven-day intervals before the product's launch last year.

Corteva advises two sprays during the rapid growth phase of the crop to ensure the cleanest possible start to the season, and recommends holding treatments in reserve for the stable canopy phase.


Responsible use of fungicides

Hutchinsons'agronomist David Harris, who is involved in the management of substantial acreage of potatoes grown in Cornwall from Plymouth to Land's End, also advocated Zorvec as a robust first spray when the plastic was lifted on early crops established under plastic planted between Christmas and New Year. After putting it to the test last year, he plans to use the new chemistry again this season as part of a planned programme.

"I'm a great believer in investing in blight programmes early and putting the best products on so growers know that they have robust protection. In some cases, this may mean a higher investment in crop protection early but cost should be secondary to ensuring that you have the best insurance against blight. If you don't let blight get established, in the long term it gives better results," said David.

He said any new blight chemistry that works is vital to growers. "We have lost some older chemistry to resistance and others have been withdrawn or become less effective," said David. "We have not seen the recent dark green fluazinam resistance problems found elsewhere in the country yet but I'm sure we will get it in Cornwall at some point, so we have to think about using fungicides responsibly."