Following a cold, wet spring, potato crops have had a protracted planting period. Some growers will want to keep crops going as long as possible to make up yield; it’s likely there will be a knock-on effect and extended harvesting.

Weather conditions are now conducive to blight and pressure is very high. From the beginning of haulm removal to lifting is a high-risk phase for tuber blight infection. In a late-lifting year this can be exacerbated by:

1. Potential delayed haulm removal, which is likely to push the crop back into a condition with shorter, duller, wetter days, frequent Hutton Periods and later harvesting

2. Delaying desiccation slows the process itself and extends the time the crop is vulnerable to blight spores being washed down to tubers

3. Longer, slower haulm removal leaves more time for regrowth, giving blight a further opportunity to get into the crop

Seasonal advice is therefore:

• Kill crops as quickly and efficiently as possible. Use Gozai desiccant to begin haulm removal and get a good start on foliage, stolon detachment and skin set, and hit any weeds present

• Watch out for regrowth and manage accordingly. This can be worse in tramlines and other areas where spray coverage is compromised; wet fields make for difficult travelling and mud splash onto foliage

• Continue blight programme until the end: if any foliage or stem remains, it needs protection

• Choose products rated for tuber blight control such as Ranman Top, the UK’s leading potato fungicide for foliar and tuber blight

• Use a tank mix supported by manufacturer Certis Belchim: Gozai 0.8 L/ha + mso 1.5-2.0 L/ha with Ranman Top 0.5 L/ha in 300 L water

Across Europe, it’s becoming more challenging to control late blight, with more aggressive strains holding dominance.

Monitoring of the Phytopthora infestans genotypes causing late blight outbreaks in Great Britain’s potato crops is conducted by Dundee-based James Hutton Institute (JHI).

Its researchers isolated some 453 genotypes in 2022 from samples sent in by a network of blight scouts across the country. Fig. 1 shows the results.

Fig. 1 – 2022 GB P. infestans population change

Source: James Hutton Institute

It reveals that 13_A2 and the fluazinam-insensitive 37_A2 have all but disappeared, with the population now dominated by 36_A2 at 53% and 6_A1 at 35%.

England had the highest proportion of 36_A2 at 67% and 37_A2 disappeared in Wales from 41% in 2021, while 6_A1 continued to dominate in Scotland.

The 41_A2 genotype, which was found for the first time in Fife the previous year, was not found again.

Certis Belchim has undertaken work to learn more about the fitness of recently established and emerging strains at its Londerzeel trials station in Belgium.

Results have shown the latent period of 36_A2, which now dominates in GB, is almost one day shorter than 6_A1 at 18C and 24C.

Growth (cm2/day) and sporulation capacity (spores/cm2) of 36_A2 lesions at the lower temperature of 10˚C are more than five times that of 6_A1.

With faster-growing lesions and huge sporulation capacity, even at lower, less “blighty” temperatures, growers cannot afford to take any risks.

Maintaining protection with appropriate fungicide choice and spray intervals is crucial to avoid outbreaks that can spread rapidly.

A genotype known as 43_A1 is insensitive to carboxylic acid amide (CAA) fungicides and has spread from Denmark, where it was first discovered, to the Netherlands and Belgium in recent seasons.

Although the clone has not yet been discovered in GB, experts believe it is only a matter of time before it jumps across the Channel.

There is also the threat of other aggressive or fungicide-insensitive genotypes from the “Other” category in sampling.

This category comprises genetically diverse late blight isolates from oospores created by sexual reproduction between two mating genotypes and is what drives population change over time.

Growers and agronomists need to be aware of the potential implications of known and unknown genotypes for fungicide use and take a pragmatic approach to late blight control in 2023 and beyond.

For both efficacy and resistance management, growers should always avoid applying blocks of the same mode of action and always mix modes of action in each spray.

Growers should leave little to chance, particularly in relation to inoculum sources, and use preventative fungicide programmes to limit epidemics.

This means halting the proliferation of volunteers, which can be poorly controlled in other crops, stubbles and at potato field edges beyond the reach of the sprayer boom.

Members of organisations such as the Association of Independent Potato Consultants (AIPC) are increasingly finding late blight on hairy nightshade (Solanum sarrachoides), which should be controlled.

Volunteer potatoes in cover crops are a worrying problem that threatens the uptake of “regenerative farming” where potatoes are grown.

Cull piles also need to be managed, either by covering them with sheeting or spraying off any new growth with glyphosate.

With aggressive late blight strains and the threat of insensitive isolates, resistance management should be front of mind.

Growers should adhere to tight intervals between sprays to maintain a protective covering within the crop canopy, particularly on new growth.

In 2006-07, growers stopped using phenylamide fungicides like metalaxyl-M when 13_A2 became prevalent and, more recently, fluazinam use dropped after the discovery of 37_A2.

Both genotypes have almost disappeared from GB populations and because these two active substances are effective against 36_A2 and 6_A1, they can now again play a valuable role in late blight control.

Using these alternative modes of action will help manage resistance to other fungicide groups and help reduce the risk of insensitive genotypes becoming established in the GB population.

Shirlan (fluazinam), which is effective at controlling zoospores, brings back another useful tool against tuber blight.

Growers previously had access to just two fungicide groups – QiIs like cyazofamid and amisulbrom, and the benzamide active fluopicolide.

Fluazinam products like Shirlan will help reintroduce a third mode of action, Group C5 (uncouplers of oxidative phosphorylation), into tuber blight sprays. Mixed with a suitable partner, Shirlan works with QiIs like Ranman Top (cyazofamid), which is an important bedrock of tuber blight control.

Furthermore, Shirlan is a useful tool at flowering as it provides activity against Sclerotinia, Botrytis and Alternaria.

Earlier than many growers may think. As soon as tubers initiate, tuber blight infection can occur, so it is essential to use products that protect against sporangia and zoospore, the late blight spores that cause tuber blight, activity early, rather than waiting until the end of the season.

Key tuber blight products like Ranman Top can be used early on, or later in programmes.

Actives like cymoxanil, or cymoxanil + propamocarb in higher pressure situations, are effective partners, combining the curative kick-back of Cymbal (cymoxanil) with the protectant nature of Ranman Top (cyazofamid).

Shirlan is a cost-effective option with zoospore activity for use early in the programme and again, cymoxanil, mancozeb, or stronger chemistry, are suitable partners.

Shirlan is also suitable in green-top crops later in the season, but is not supported in the last three sprays of stored crops. Ranman Top is the main alternative option here.

Burn patches of infected crop out immediately and treat surrounding crops quickly with the best chemistry available, as once blight gets established it is extremely difficult to stop, especially with aggressive genotypes like 36_A2.

This could involve an application of fluopicolide + propamocarb hydrochloride followed within three days using another robust product such as benthiavalicarb + oxathiapiprolin.