How to control tuber blight in potatoes in 2022

Planning a blight programme from mid-season can be akin to picking the winning lottery numbers the week before the draw: the temptation is to go with the same combination of numbers or products you always use and hope you get lucky.

Unfortunately for potato growers there is often more at stake than the £1.50 fluttered on a lottery ticket. The arrival and subsequent rapid spread of fluazinam-resistant 37_A2 in 2016 effectively meant the end for one of the industry’s most widely used active substances. The pressure this placed on strategies was compounded with the arrival of 36_A2 (also in 2016) which has been found to be more aggressive than the strains that dominated for the past decade. 36_A2 is now dominant across England while the discovery of 41_A2 in 2021, another strain which has shown resistance to fluazinam and also phenylamines such as metalaxyl-M, means successful blight control is set to become more complex and, potentially, more expensive.

Managing this threat led to a change in application practices. To reduce the risk of selection pressure, a product based on a single mode of action should be mixed with a suitable partner. In addition, it is good practice to alternate modes of action rather than practice ‘blocking’: where the same product is applied in consecutive applications.

“It’s not sensible, given what we know about the aggressiveness of 36_A2, to be applying Revus (mandipropamid) without a mixing partner such as mancozeb during the rapid canopy phase,” said Greg Dawson of farmer-owned advisers Scottish Agronomy in 2019.

1. Plan use of QiI products carefully to protect against tuber blight

With 13 modes of action available for late blight control ensuring crops are suitably protected might not seem too challenging. However, only three are considered to have good activity against zoospores and therefore against tuber blight.

Fluazinam is one, but for those who don’t want to run the risk of insensitivity that leaves just two: Quinone inside Inhibitors (QiI), which belong to FRAC code 21, and the pyridinylmethyl-benzamide group containing fluopicolide, belonging to FRAC code 43.

Both Shinkon/Gachinko (amisulbrom) and Ranman Top (cyazofamid) are QiI fungicides meaning that to comply with an effective resistance management strategy, they should not be used consecutively or in mixtures with each other.

Furthermore, FRAG-UK resistance management guidelines for the QiI group are that they should not form more than 50% of the intended programme or that CAA group fungicides should not exceed six applications when used in a mixture or four when used alone and no more than 33% of the programme.

This is reasonably straightforward to overcome so long as growers make use of products other than QiIs in the rapid canopy phase and alternate use of the remaining QiI applications with Infinito (fluopicolide + propamocarb), which can be applied up to four times per crop.

For example, if Zorvec Enicade (oxathiapiprolin) is mixed with amisulbrom during rapid canopy growth this limits the number of QiI applications that can be made later in the programme when the focus turns to tuber blight protection.

2. The practical realities

The arrival of yet another more aggressive strain challenges two points: the suggestion that variety resistance has a greater role to play in structuring programmes and the intention of those planning to extend intervals to more than seven days. It’s high risk in both instances.

All the clones active in the UK are acknowledged as aggressive, as genotype information is as yet unable to predict the risk that a clone presents to crops in a particular region. This means from a control perspective all clones are equally threatening.

Advisers often talk about new strains of blight as having a ‘fitness advantage’ that marks them out as being ‘more aggressive’ than those that preceded them, but what does it mean, and does it apply equally to foliar blight and tuber blight?

The term ‘fitness advantage’ is often used to cover several characteristics that separate new genotypes from those that preceded them. It can be used to describe its biotrophic nature (the term used to consider its ability to sporulate while keeping the leaf alive for longer), the length of life cycle (the interval in days between spore releases), or the ability to overcome plant resistance genes.

From the practical perspective of crop protection, these differences between genotypes are largely irrelevant. Under controlled conditions it is possible to observe small differences in the way the symptoms manifest, but these are not so large as to support a change in crop management strategies in the field.

The implications of this reality are two-fold. First, when it comes to blight, all genotypes should be regarded as aggressive. This means crops should be suitably protected against tuber blight from the start of tuber initiation through to lifting if crop breakdowns in store are to be avoided.

Second, since there is likely to be more than one strain of blight present in a crop, relying on products containing an active ingredient known to be insensitive to one or more strains of blight, as is the case with fluazinam, effectively leaves the crop unprotected. Doing so will support the spread of insensitive strains while increasing the risk of tuber blight infection occurring.