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How the potato industry should respond to reduced fluazinam sensitivity in late blight

Publication Date: 
15 May 2018

Knowledge Exchange Manager, Claire Hodge discusses new guidance on how the potato industry should respond to reduced fluazinam sensitivity in late Blight populations

By Claire Hodge

Funded by AHDB, Scotland’s Rural College (SRUC) worked alongside ADAS to produce guidance on the management of blight given the prevalence of a strain of the potato blight pathogen, named genotype EU_37, which has proven to be less sensitive to the common fungicide fluazinam than other established strains in Great Britain.

Increasing substantially between 2016 and 2017, the prevalence of this genotype rose as Blight Scouts, who form part of AHDB’s “Fight Against Blight” network, saw an increase from 3% to 24% of GB P. infestans population samples collected. As a result, the efficacy of fluazinam is likely to be further affected if EU_37 continues to increase at the same rate.

Currently there are blight fungicide active ingredients (a.i.s) with 13 different modes of action available in the UK. Consequently, resistance to fluazinam should not be a major issue for potato production, provided there is sufficient diversity in their use.

However, in the absence of fluazinam only two modes of action remain available for highly effective tuber blight control, both of which should be considered as also being at risk of fungicide resistance.

Clearly, robust anti-resistance management is a pressing requirement. There is concern that fungicide a.i.s representing those two modes of action (i.e. fluopicolide, cyazofamid and amisulbrom) will not provide a sufficiently long period of tuber protection against blight for some crops, because of the label restrictions on the maximum number of applications per crop.

Changes in the P. infestans population

Available evidence suggests that EU_37 is associated with decreased sensitivity to fluazinam and has comparable aggressiveness to another dominant genotype in GB, 6_A1.

Consequently, the proportion of EU_37 in the population is likely to increase if fluazinam is used, but may remain high even if no fluazinam-based products are being applied. Part of the P. infestans population in Scotland has potential to reproducine sexually, which allows new strains to arise which allows new strains to arise which may combine some degree of fluazinam insensitivity

However, miscellaneous strains are not uncommon in Scotland and we have not seen any unique strains spread any further than the north of Scotland and are not considered as virulent as European strains.

 However, these ‘miscellaneous’ novel genotypes are not common in Scotland and have to-date appeared locally and only in a single season which suggests they are not as fit or aggressive as the dominant aggressive clonal lineages.

Reducing the risk of poor control

In areas where EU_37 has already been confirmed, there is a risk of less effective foliar blight control and increased risk of tuber infection. These risks should be mitigated by using products other than fluazinam.  In areas where EU_37 has not been confirmed,

The following risk factors should be fully considered before the use of fluazinam:

  • Geographical location of crops, i.e. EU_37 confirmed in region or not. Consider if sampling intensity in the area is sufficient to avoid false negatives (i.e. that EU_37 has        not been detected, but is in fact present).
  • Seed source used for your crops and others in the locality. Did it originate from a region in which EU_37 has been confirmed? This comment applies to imported seed           plus UK seed
  • Is fluazinam needed for Sclerotinia control?
  • Is the crop to be desiccated and stored (and hence at higher risk from soft rots originating from blighted tubers), or harvested green top and marketed immediately?        
  • Check who carries the risk if fluazinam is used and there is loss of control linked to resistance.

Implications for control of Sclerotinia stem rot and powdery scab in potato using fluazinam

Fluazinam can provide incidental control of Sclerotinia stem rot and is sometimes applied to the soil for the control of powdery scab. The efficacy of fluazinam against these pathogens is not affected by the sensitivity of blight strains. 

However, if application of fluazinam targeted at other pathogens also exposes P. infestans to fluazinam, then this could increase selection for insensitive strains of P. infestans. Such exposure could, in principle, occur from either foliar application or exposure of P. infestans from blighted seed tubers to fluazinam applied to the soil.  If, in future, insensitivity in P. infestans affects the availability or economics of using fluazinam in potatoes, then other control measures for Sclerotinia are likely to become more important.  

Examples of such measures are : extending rotations, limiting the number of crops in the rotation that are susceptible to Sclerotinia and providing good control of Sclerotinia by use of fungicides in other host crops (e.g. oilseed rape, peas and carrots)

Recommendations

  • Implement cultural controls
  • Prevent oospores contributing to late blight epidemiology by having long rotations (at least 1 in 5 and free from volunteers) and also by maintaining a high level of late blight control
  • In areas in which genotype 37_A2 has been confirmed, there is increased risk of crop damage due to late blight if fluazinam is used and proves ineffective.
  • In regions thought to be free from 37_A2 (but be aware of the potential for a rapid change in status) fully consider the risk of using fluazinam
  • Plan an appropriate strategy across the entire fungicide programme which will protect foliage and tubers from late blight, and provide good resistance management
  • Place much greater emphasis on modes of action when choosing products.  Make full use of the different modes of action available; especially during the tuber protection phase of growth
  • Using mixtures of fungicides with different modes of action (tank mixes or co-formulations) and alternating products throughout the fungicide programme are both effective resistance management strategies.
  • Ensure mixing partners are used at doses  that provide similar efficacy and persistence
  • Recent research in The Netherlands, with the less aggressive fluazinam insensitive genotype 33_A2, found that cymoxanil was ineffective at slowing the increase in fluazinam insensitivity when used as the only mixture partner with fluazinam.
  • Avoid repeated application of the same product or mode of action and never exceed the label recommendation
  • Multisite fungicides, e.g. mancozeb, should be used where possible throughout the fungicide programme
  • The reliance on zoospore-active fungicides to control tuber blight can be reduced by: boosting foliar blight control during stable canopy; the inclusion of fungicides with good anti-sporulant activity; ensuring the haulm is protected until dead; preventing regrowth of haulm and by taking account of varietal resistance to tuber blight when deciding how soon to harvest after desiccatio
  • Consider possible selection pressure on P. infestans from the use of fluazinam soil treatments

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