Potatoes – Late August 2020

The first season without diquat is throwing up a couple of issues.

Many crops have come up to size quickly and were flailed early. We have seen copious regrowth after flailing for some seed crops of indeterminate varieties. Our advice remains to monitor flailed crops for regrowth, and if necessary, apply a desiccant.

Potato blight

Blight risk

Check the AHDB Potatoes Fight against Blight website regularly for the most up-to-date information on the appearance of confirmed blight outbreaks. Up to 26 August there have been relatively few outbreaks, with only seven, confirmed in Scotland (postcode areas AB41, IV12, PH13 and ZE2). In addition, there is blight in KA6.  To provide perspective, the number of confirmed outbreaks per season in Scotland over the last 10 years has ranged between 10 and 145.

Also, check BlightWatch or BlightCAST for the general blight-weather risk where your crops are located. In the last 7 days some areas of Scotland have generally had zero or one high-risk period (Hutton).

Tuber blight control

Fluazinam-based products should not be used for tuber blight control.

It’s crucial, especially at the tuber protection stage of the fungicide programme, that resistance management guidelines are followed.

Control in the growing crop

One new issue for 2020 is the prolonged period of crop desiccation, now that diquat is no longer approved, for crops that can’t be flailed. For the control of tuber blight crops should be fully protected by fungicide until all haulm is dead, therefore prolonged protection by fungicide will be required. Efficacy ratings for tuber blight control are provided by EuroBlight for some fungicide products and mixtures.

EuroBlight tuber blight ratings

¹ Zero to five scale (0 = no effect, 5 = perfect control)

Another issue is tuber blight control in long-season ware crops given that the number of permitted applications of QiI fungicides is limited. Some ways of boosting tuber blight control when applications of these fungicides can’t be made are listed below. In principle, the three most-effective fungicides for tuber protection should be used to cover weeks when the risk of tuber infection is greatest. This is fine in principle, but it’s recognized that it can be very difficult to forecast accurately the amount of rainfall (one of the key risk factors for tuber infection) over the coming 7 days.

The main risk factors for tuber infection in the growing crop are the presence of foliar blight, together with high-risk weather to encourage sporulation, then lower temperatures to favour the production of a large number of zoospores, combined with wet soil and irrigation or substantial rainfall. Note that for tuber infection, foliar blight does not need to be severe if the other factors are favourable.

The risk of tuber infection can be reduced by:

• Ensuring highly effective control of foliar blight, to minimise the inoculum available to infect tubers.  Foliar blight control during stable canopy and desiccation can be very significantly improved through, for example, using fungicides that are more effective, adding an additional fungicide to the spray tank, or tank mixing anti-drift agents with certain fungicides.
• Maintaining effective fungicide protection until the foliage and stems are dead
• Using fungicides with good EuroBlight ratings for anti-sporulant activity and products that are oxathiapiprolin + benthiavalicarb + partner fungicide (either co-formulation or tank mix).
• Avoiding the cracking of ridges
• Avoiding, or restricting, the irrigation of crops with foliar blight

We all know that the risk of tuber infection depends on cultivar resistance. Scottish Government-funded work at SRUC has quantified the relative risks for some cultivars. The SRUC field experiments have shown that several cultivars, each with moderate ratings for both foliar and tuber blight, reduced the risk of tuber blight to between one-twelfth and one-fortieth compared with a susceptible reference cultivar (rated 2 and 3 for foliar and tuber resistance respectively). Genotypes 13_A2 and 6_A1 were present at the trial site in all 3 years. In 2018 and 2019 37_A2 was detected. 36_A2 was also present in 2019.

Control during harvest

• If test digs reveal blighted tubers pre-desiccation then the number of blighted tubers harvested into store can be reduced by delaying harvest and thereby allowing blighted tubers to disappear pre-harvest through secondary bacterial soft rotting. However, it should be noted that the success of this approach is greater for soils that are warmer and wetter. Trials experience suggests that on occasions tuber decay can be limited in free-draining soils, especially once soil temperatures fall significantly.
• Ensure thorough haulm desiccation to prevent blighted haulm re-growth smearing blight spores onto tubers on the harvester.
• For crops that have, or have had, foliar blight, if the variety is particularly susceptible to tuber blight then lengthen the period between desiccation and harvest for an extra week or two to allow longer for a greater number of viable spores in the soil to die off. Fewer spores are required to infect tubers of tuber-susceptible cultivars.
• Ventilate and dry tubers immediately after harvest

Haulm Destruction

This is the first season where diquat cannot be used, so it is timely to review the principles behind haulm destruction, as well as options that are available.  The goals of haulm destruction aren’t complicated: halt plant growth to achieve as many tubers as possible within the desired size fraction. Good haulm destruction should also help when it comes to harvest; ensuring high levels of stolon detachment and less residual haulm when the crop is lifted.  Fast skin set is also desirable, as the longer tubers are in the soil, the greater the risk of diseases affecting quality.

There’s a growing body of evidence that rate of canopy death doesn’t correlate well with what’s happening under the soil.  Trials (at SRUC/SAC and elsewhere) have provided evidence that skin set and stolon detachment are broadly similar when flailing and chemical-only desiccation are compared.

Flail and Spray

Most Scottish growers seem to have settled on a “flail-and-spray” approach as the default for haulm destruction.  Flailing takes down a crop quickly, but it also encourages regrowth in most varieties meaning that follow up sprays are almost always required.  SAC trials to date indicate similar performance between different desiccants in controlling regrowth.  Key points to bear in mind when adopting flail-and-spray are:

• Pay attention to condition of the crop and weather
  • Don’t flail in wet conditions due to risk of Pectobacterium (blackleg bacteria) spread.
• The follow-up desiccant needs a target…
  • Ensure 15-20 cm (6-8 in) of stem remains after pulverisation.
• …and that the target needs to be clean
  • Untidy pulverisation, with lots of leaf material covering stems will shield stems and reduce desiccant efficacy. If haulm and/or leaf material remains attached and covers stems, leave at least 2 – 3 days before spraying for it to dry and fall away.
• Avoid untidy flailing by maintaining equipment
  • Replace worn flails. Ensure hydraulics are in good working-order and properly calibrated.
• Monitor regrowth
  • Flailing may induce secondary growth, although this is variety dependent.
  • In most cases a single application of Pyraflufen-ethyl + methylated vegetable oil or Carfentrazone-ethyl after pulverising will be sufficient to limit regrowth but on vigorous indeterminate varieties additional sprays may be needed.
• Blight sprays should continue until all plant tissue has died. With a product that offers good tuber blight protection.  If aphid pressure is high, the same is true of aphicides in seed crops.

Whilst flail-and-spray is set to be the industry standard, a non-mechanical fall-back will still be needed.  The 2019 season was a sharp lesson that wet conditions can prevent the use of a pulveriser.  We also advise to never use flailing when blight is present in a crop due to the risk of spreading inoculum.

Desiccant Only

Chemical desiccation is still possible, but it will need a change of mind-set from the days of diquat.  The two products currently approved from haulm desiccation are Pyraflufen-ethyl and Carfentrazone-ethyl, summarized in the table below:

Both products are PPO (proroporphyrinogen oxidase) inhibitors which are slower than diquat because they have a different biochemical mode of action.  How much slower than diquat will depend on the crop and conditions, but a rough rule of thumb would be 2-4 days longer in a crop where senescence has begun, and possibly up to 7 days in still growing seed or salad crops.

The key detail of real importance here is that it is the combination of a PPO and sunlight which will kill the plants.  High humidity and air temperatures will also help as more chemical will be absorbed into plant tissues, and their metabolism will be running at a faster rate.  The best results with PPOs are seen when they are applied in the morning or early afternoon.

In our experience, there is not much to separate the pyraflufen-ethyl and carfentrazone-ethyl products currently on the market.  If a crop is of a determinate variety and has begun senescence it may be possible to achieve haulm destruction with a single treatment. Note that the labels for both products specify timing treatments at the onset of senescence.

More vigorous growing crops will likely need a sequence of sprays.  Our trials have shown very little benefit of applying “big hits” of either product as a tank mix, and with only two applications of each available you may run out of options before a crop has been killed. We recommend single product sprays. Monitor canopy death to determine if more passes are needed. When planning forward, take into account that carfentrazone-ethyl has a harvest interval of 7 days, as opposed to pyraflufen-ethyl’s 14 days.

PPOs are contact herbicides, so a judgment about water volume needs to be made to ensure good canopy penetration.  This will depend on the variety and crop condition.  Use higher water volumes for denser canopies.

• Plan in the extra time for slower kill. Start earlier to achieve similar harvest dates.
• Chemical-only desiccation with PPOs will slow passive bulking. As well as triggering skin set and stolon detachment.
• Apply PPOs at onset of senescence, if that is an option. Likely not for most seed crops.
• Apply PPOs in the morning on warm sunny days, if you can. Easy to write, harder to achieve!
• Up to three PPO treatments may be needed to fully desiccate a vigorous crop. Determine intervals by following speed of canopy death and in compliance with the label.

 Haulm regrowth

Haulm regrowth has been reported in some seed crops that were flailed earlier. This should be controlled for several reasons, not least the risk of blighted regrowth spreading blight inoculum onto tubers at harvest. The use of a desiccant (see above) may be required.

Potato groundkeepers and blight risk (in 2021)

The last 10 years of data for Scotland from the Fight against Blight outbreak mapping service (run by JHI; funded by AHDB Potatoes) has helped quantify the importance of blighted groundkeepers as a source of viable inoculum for crop infection. As expected, the vast majority of confirmed cases of blight in Scotland were in crops (88.6%). Less predictable was that over the 10 years there were twice as many outbreaks on groundkeepers (6.0%) compared with the more obvious source, outgrade piles (3.1%).

The AHDB Potatoes’ data demonstrated that on average confirmed cases in crops generally peaked in July (43.3%) and August (43.4%). The numbers of definite outbreaks on groundkeepers were commonly greatest during July (7 cases), August (15) and September (12). In other words, many were active sources of blight when new crop cases were being reported. Of course, these data are not robust enough by themselves to prove that all blighted groundkeepers necessarily lead to crop infection. However, SRUC has monitored groundkeepers in Ayrshire weekly from the time of emergence. In the two years in which blight was detected on them, it was found relatively early: 27 June in 2019 and 9 July in 2020.

Additional inoculum from any primary source makes blight control in crops considerably more difficult.

Pests

The weather has been favouring slugs this season and it is worth doing some test digs to see if there are any signs of slug or wireworm damage to tubers. Wireworm damage tends to be small holes penetrating the tuber whereas slug damage tends to be more irregular shaped holes. Unfortunately, nothing can be done for wireworm apart from lifting the crop as soon as it is practical to do so, as damage will only get worse the longer the crop is in the ground. For slugs both ferric phosphate and metaldehyde are effective molluscicides and can be used to try to reduce slug damage but the optimum timing (just before crops meet across the rows) has now passed. If planning to use a metaldehyde product note the guidelines issued by the Metaldehyde Stewardship Group and in particular the requirement for a 10 metre buffer zone to the edge of the field where no metaldehyde pellets should be applied. The best timings now available are after these has been some rain which will draw slugs up onto the surface and after burn down, again when there has been some rain.