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Keeping one step ahead of tuber blight

Publication Date: 
15 March 2018
Author/Contact :
Author/Contact: 
Claire Hodge

The  aggressive and fit new blight strains 37_A2 and 36_A2 have been detected as tuber blight in GB-stored potatoes. The latest results and analysis will help the sector keep one step ahead on how to best manage this latest threat.

Of the genotyped batches of tuber blight samples typed in the storage season to date,  the new genotype 37_A2  has been confirmed  in six samples, and 36_A2 from two,  reveals David Cooke of the James Hutton  Institute. Other samples have identified  nine outbreaks of  6_A1, one of 13_A2 and one ‘Other’ genotype.

Dr Cooke says: “Most of the 30 batches received have been confirmed as infected with tuber blight but secondary bacterial rot has been an inevitable problem and, in some cases, it has not been possible to obtain even a candidate blight sample. In other cases, the tubers have been infected with only Fusarium dry rot.”

Of the 30 batches, 22 have been from crops grown in England, four from Scotland and four from the Netherlands (which were intended for planting as seed).

Samples have ranged from a single tuber to  more than 20  and have been in a range of conditions.  He and his team have attempted to genotype 55 samples from the 30 batches provided so far.

“In some cases, sample processing is still underway and in others no samples could be recovered that were worth genotyping.  Moreover, some diseased tuber tissue samples remain to be tested. We currently have data from 17 of the 20 tuber batches tested and these comprise 40 genotyped samples.”

He adds that due to the variable number of tubers provided and their condition he has been unable to process a fixed number of sub-samples per batch.  A total of 98 samples have been processed from 18 different potato varieties. 

Dr Cooke explains: “As the crops moved to store concerns were raised about elevated levels of tuber blight in some circumstances.  In response to this The James Hutton Institute responded to a request within the industry and conducted some screening which confirmed the presence of 37_A2 and other genotypes in stored tubers. We have also identified 37_A2 in a sample grown in Suffolk which is a concern for crops in this area in the 2018 season.”

He reminds growers and their advisers that a report from Wageningen University in June 2017 indicated the reduced sensitivity of the 37_A2 lineage to fluazinam, an  active which has formed part of the management of tuber blight late in the potato growing season. This highlighted the risk of selection pressure on the seemingly aggressive 37_A2 population that was reported to comprise 24 per cent  of the population in the 2017 FAB monitoring of GB crops. 

“Because of the importance of the evolving blight  some additional financial support from AHDB Potatoes has been made available  to conduct further tuber blight screening in the 2017 stored crop.

 “If you suspect an outbreak, it is not too late to send in samples with dry, firm tuber blight symptoms so we can determine the genotype; this will help us assess overwinter survival of this fit and aggressive genotype of Phytophthora infestans in tubers.”

Genotyping

Genotyping samples from in-store blight outbreaks is giving growers and agronomists a clearer idea of the  inoculum which was in their locality at the end of the season, says Graeme Skinner of Provenance Potatoes in Thanet, Kent.

Blight infections are usually from local sources and he believes this year it will be particularly important to know the strain affecting the crop because of the increase in outbreaks of 37_A2, which is less sensitive to fluazinam.

Mr Skinner says:” The increased findings of 37_A2 requires greater focus on tuber blight control for the coming season, as fluazinam was heavily relied upon in the past. If one has not found 37-A2 in your locality in 2017, it is by no means a guarantee that it will not be present for 2018. Genotypes within a local population seem to be able to switch very quickly.”

 “At the moment tuber sampling is not scientifically scheduled, it happens as and when we suspect soft rot; one of the challenges is to get the sample in for testing before it becomes a ball of water.”

In addition, sensitivity may not be the only issue with this strain, which is fit and aggressive, he adds.

“It is important to build the ever-changing picture of blight so you can take better-informed decisions.”

He found AHDB’s change in policy useful as receiving an immediate result about which strain is in the crop can drive better decision making and understanding of when the strain present at the beginning of the season is overwhelmed by a fitter, more aggressive strain.

It also encourages blight scouts to go out searching for outbreaks.

“We find where we are that blight usually comes into the crop in spring, and then dries up, reappearing later in the summer. In the past we would have the initial outbreak sent for genotyping, but not later outbreaks.

 “In 2017 we did not have much blight, nevertheless, all the samples we sent in were found to be infected with 37_A2; a nearby grower had 36_A2 detected.

“We still need know more a lot more  about 37_A2, and as yet 36_A2 is a great unknown, so this work is critical to our sector.”

Managing in-store tuber blight – is your ventilation good enough?

Tuber blight has been identified in areas where 37_A2 was found in foliage last season, reports John Sarup of SPUD Agronomy.

Storage has been a challenge this year – not helped by big yields, temporary stores and insufficient ventilation.

John says: “Last year we had a wet harvest, so there was a lot of soil going into store and on tubers, tubers were subjected to higher than normal agitation on harvester webs, causing more damage, potentially allowing blight to develop further.

“This has resulted in more tuber breakdown in-store; analysis has shown it to be secondary soft rots causing this but it is likely that blight was the precursor.”

Adrian Cunnington, Head of Sutton Bridge Crop Storage Research, says:  “Tuber blight spreads from active, uncontrolled foliar blight as spores are washed down the stems by rainfall and can then infect tubers. Blight can enter via the stolon or damaged surface areas of the tuber, or even the lenticels.”

Tuber blight is not usually evident at store-loading, as it develops over time, but close monitoring of the crop can help by picking up early signs, such as the characteristic rusty lesions under the skin.

Once tuber blight is detected in store, decision-making on how to manage breakdown depends on infection levels, says Adrian.

 “If the tubers are still fairly dry, ventilation and cooling can help keep the problem at bay, but if the infection is more serious, good sampling and monitoring is crucial and positive ventilation – where air can be forced through the crop -  is really needed if control is likely to be at all effective,” he warns.

Adrian says: “If the problem escalates and secondary rotting becomes an issue, decisions need to be taken quickly about when to unload, or you run the risk of having nothing to sell.”

Tuber blight control starts at planting

Tuber blight protection is optimised by building it up from tuber initiation and goes right through the season to haulm destruction, says Eric Anderson of Scottish Agronomy.

Tuber infection is mainly caused by motile zoospores, which are produced in the cooler temperatures of early spring, late summer and early autumn. Zoospores produced on infected foliage are washed into the ridges by rainfall and move between the soil particles to infect the daughter tubers, he explains.

He urges avoiding fluazinam as a blight spray, and making greater use of products featuring different modes of action, including if fluazinam is used for sclerotinia control.

“Just because 37_A2 has not been found in a particular area does not mean it was not there, it may have been under-reported,” says Mr Anderson.

Looking at alternative products, he notes  Ranman Top (cyazofamid), Infinito (propamocarb + fluopicolide) and Shinkon (amisulbrom), all have activity against tuber blight.

Become a Blight Scout in the Fight Against Blight

Register now to become a Blight Scout, Claire Hodge, knowledge exchange manager at AHDB Potatoes, urges growers and agronomists. Packs for are expected to be sent out by the end of March, with  an earlier start in the south of England, she reports.

“The website problems have been solved, and so it will be live for prospective scouts to log-on and request a pack.”

The process has been streamlined to make it easier, she adds.

  1. Register as a Blight Scout
  2. Download the Fight Against Blight App
  3. Receive a blight pack in the post
  4. When blight is identified in the field, use the App to record the sample; this will give a registration number.
  5. Fill in the paper form with this number
  6. Collect  four sample points, bag them and post.

“Results on a positive blight result should be received on your App within three days, depending on the post, ” says Ms Hodge. “Genotyping information will be available later, as there will be in-season updates on genotype threats.

“If 37_A2 is detected, you will receive an email from AHDB Potatoes.”

Further content

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