Potatoes Update – August 2020

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 29 July there are two outbreaks in Scotland confirmed both in the IV12 postcode. In addition, blight has recently been confirmed on groundkeepers in South Ayrshire (KA6).

Also, check BlightWatch or BlightCAST for the general blight-weather risk where your crops are located. In the last 7 days many areas in Scotland have had between 1 and 3 Hutton Periods.

Dealing with blighted crops

• Act quickly to minimise the development of blight in the crop

This not only has the obvious benefits of minimising the impact of the disease on the crop and limiting the inoculum available to infect other crops, but it should also reduce the risk of oospores being formed. For crops entered in the seed classification scheme discuss with SASA inspectors before desiccating any plants.

• The fungicides applied need to provide curative activity but also protection of healthy foliage (and tubers if formed).

The active ingredients with good curative activity are benthiavalicarb, cymoxanil, dimethomorph, mandipropamid + cymoxanil and propamocarb.  Numerous trials have demonstrated that a tank mix of  propamocarb + cymoxanil + cyazofamid is particularly effective in limiting blight development in infected crops.  Alternatively, tank mix dimethomorph + cyazofamid or as another alternative amisulbrom + mancozeb + cymoxanil. The crop buyer may place restrictions on which curative active ingredients can be used.

Fungicide products vary in how curative they are. However, in general the kick-back period will be less for very susceptible varieties, where the temperature profile is high (i.e. between 20° and 23°C rather than 10° to 15°C) and/or for the more aggressive genotypes of blight that are now present in the UK (13_A2, 6_A1, 37_A2 and 36_A2).

• Shorter than normal fungicide spray intervals will be required until control is regained.

This is likely to require alternating products so that the spray intervals specified on the product label are adhered to.

• Ensure that haulm coverage is optimised, e.g. through the addition of an anti-drift agent to the fungicide(s). Also, a higher volume of water than normal may improve coverage, but timing is key.
• Don’t use a fungicide product where the label states that it is not to be used where blight is present.
• To minimise the selection for insensitive strains alternate products with different modes of action and/or use tank mixtures of active ingredients, or co-formulations, from different FRAC resistance groups.
• The option to quickly desiccate distinct patches that are blighted has gone with the loss of approval of diquat.  However, it remains important to apply the desiccant used some metres beyond the edge of the obvious patch because although plants at the margin may appear healthy, they may have been infected but symptoms are not yet visible.
• If irrigation is necessary, then carefully consider the impact on tuber blight

Try to avoid irrigating after periods of high-risk weather, i.e. warm and humid conditions. The combination of weather conducive to the production of sporangia followed shortly after by irrigation substantially increases the risk of tuber infection.

The decision over whether to irrigate or not can be easier to make if there is good information on the extent of foliar blight in the crop. The implications for tuber infection are clearly different if haulm blight is restricted to one or two small localised patches compared with it being present throughout the field.

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.

1. 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 to just three. Some ways of boosting tuber blight control when applications of these three fungicides can’t be made are listed below. In principle, these 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 the 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 has quantified the relative risks for some cultivars. The 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.

2. 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