Blight control challenge
Keeping late potato blight (phytophthora infestans) out of potato crops, is a constant challenge. As part of an integrated pest management approach, timely fungicide programme and product choice are two key components in protecting potato crops against the disease.
Changing factors such as the loss of certain active ingredients and the development of fungicide resistant blight strains in continental Europe have refocused attention to not just the fungicides applied but how they are sequenced throughout the growing season to protect crops and minimize the risk of fungicide resistant blight strains developing.
Cultural Control
Fungicides aren’t the only way to get on top of the blight challenge. An integrated pest management (IPM) approach needs to consider all factors which contribute to blight spread. Individual varieties vary in their susceptibility to blight. Unlike in cereals where a grower may choose to grow a variety with good disease resistance, the potato market a farmer is growing for largely determines the variety. Although growers may have little variety choice, by at least knowing their varieties blight susceptibility they can manage accordingly and prioritise blight susceptible varieties for spraying in high pressure situations.
Identifying the sources of infection is also important. As the disease can be spread from seed, sourcing good quality stock is important. Keeping other sources of inoculum to a minimum will require careful management of out grade piles, keeping machinery, boxes and yards clean and managing groundkeepers in following crops. This can be particularly difficult when taking potato land for a single year. Some growers are choosing to treat growing ware crops with sprout suppressants to reduce groundkeepers or supplying herbicide to control groundkeepers in the following cereal crop particularly if they will have potatoes in the adjacent fields the next year.
Mancozeb withdrawal
For decades mancozeb has been a cornerstone of late blight control programmes at an affordable cost. As a multisite fungicide, arguably its greatest strength was offering robust protection to partner actives against fungicide resistance developing. The withdrawal of Mancozeb has meant several heavily relied on products are no longer available for blight control in Northern Ireland. High mancozeb usage for late blight control programmes consequently protected against Alternaria or Early blight, a different disease that mancozeb had good activity against. For Alternaria susceptible varieties growers also need to actively consider including fungicides to protect against it as well as late blight.
Fungicide resistance blight strains
Late potato blight consists of a population of different individual strains. Due to the way blight reproduces new strains are constantly developing and evolving. While many will pass unnoticed some strains develop traits which could be a threat to potato production. Those traits could be the ability to reproduce or spread more quickly, to be more aggressive in their severity or they may have the ability to overcome the protection fungicides provide, i.e. fungicide resistant strains.
The first and for a considerable period only known resistance issue to blight fungicides in the UK was the EU13_A2 strain affecting Metalaxyl-M. This has latterly been joined by Fluazinam resistant strains EU33_A2 and EU37_A2. On the continent EU43_A1 strain with resistance to Mandipropamid and the entire CAA group of fungicides has become widespread. Most recently resistance to Oxathiapiprolin has been reports in EU43_A1, EU46_A1 and EU36_A1 strains.
A change in Blight control programmes
Faced with a reduction in the number of effective fungicides, the industry is looking for the best way to protect crops against blight considering the challenges highlighted previously. Resistance develops when blight strains are exposed to the same active ingredient repeatedly, some will evolve and overcome the fungicide defence. Once that occurs further applications with the same active ingredient eliminate susceptible strains and very quickly, we are left with a resistant population. To combat this, we must provide a different challenge to the blight population each time a fungicide is applied, ensuring all strains are controlled and resistance is significantly less likely to develop. There are two main strategies to achieve this, firstly partnering active ingredients and secondly alternating modes of action.
Partnered active ingredients are either available as co-formulated products or the same outcome can be achieved by mixing two single active ingredient products. If picking single active products, it is important to select partners that come from fungicide group with a different mode of action.
The second strategy involves alternating active ingredients with different modes of action to those applied at the previous spray. In most circumstances this means in any two consecutive blight sprays fungicides with 4 different modes of action will be applied.
A key piece of information needed to compile an effective spray programme is what fungicide group does an individual active ingredient belong to. These are summarised in the table below.
FRAC code | Mode of Action group | Active Ingredient | Example product name |
P07 | Phosphonates | potassium phosphonates | Privest (2) |
21 | Qil fungicides | amisulbrom | Gachinko/Shinkon (1) |
cyazofamid | Ranman Top (1) | ||
22 | Benzamides (toluamides) | zoxamide | Presidium (2) |
27 | Cyanoacetamideoxime | cymoxanil | Lieto (2) |
28 | carbamates | propamacarb | Raport (1) |
29 | Uncouplers of oxidative phosphorylation* | fluazinam | Shirlan (1) |
40 | Carboxilic acid amides (CAA)* | benthiavalicarb | Versilus (1) |
dimethomorph | Diprospero (2) | ||
mandipropamid | Revus (1) | ||
valifenalate | Pallene/Voyager (2) | ||
43 | Benzamides (pyridinylmethylbenzamides) | fluopicolide | Infinito (2) |
45 | QoSI | ametoctradin | Enervin SC (1) |
49 | OSBPI* | oxathiapiprolin | Zorvec Enicade (2) |
* Fungicide groups with known or suspected resistance issues
Blight monitoring
Blight forecasting tools or apps can be useful in identifying high blight risk Hutton Periods using local weather data. If blight outbreaks do occur it is useful for growers to submit blight samples to the fight against blight. This may seem like closing the door once the horse has bolted but those samples can still provide useful information. Fight against blight analyses samples that blight scouts submit and genotype the strains present. This information is gathered from all potato growing areas producing a map of where outbreaks occur and what the predominate strains are. Advice can then be tailored accordingly the following season. A good example of this was when fluazinam resistant strain EU37_A2 started to become more widespread, growers were advised reduce or eliminate fluazinam from programmes. As fluazinam usage fell over several seasons so did the proportion of EU37_A2 in the overall blight population. This means we are now in a position where fluazinam can be reintroduced to blight programmes to play a useful role as a partner product to other effective active ingredients.
Compiling a blight fungicide programme
The need to protect both the crop and active ingredients from developing resistance using fewer active ingredients and products already compromised by resistance add further complication to compiling a blight programme.
As always growers need to target each growth stage with the most appropriate product. First application at emergence requires a product with some antisporulant activity to cover the soil. During the rapid canopy stage products with good systemic or translaminar activity give protection to the entire plant including new growth after the fungicide was applied. Once the biggest flush of growth is over during stable canopy stage systemic activity is less important, however a strong protectant mix is still required as this usually coincides with some of the highest blight pressure conditions. In the final senescence and burndown phase, products with good tuber blight activity are important and the programme should continue right up until all traces of living green haulm have gone.
When outbreaks do occur act immediately with products with good curative activity e.g cymoxanil, propamacarb mix. Remember cymoxanil has only 3-4 day persistence so aim to come back in after day 3 with the next programmed spray. It may be difficult to achieve but aim for a little flexibility in the programme should there be the need for more applications either due to persistent high blight pressure conditions resulting in shorter spray intervals or a slower than expected burndown. Flexibility is most easily achieved by utilising a wide range of fungicides from across all the modes of action available. This allows substitution should product availability be an issue, or conditions dictate an alternative which has better activity or have improved rain fastness.
FRAG and fungicide manufacturer guidance
Fungicide Resistance Action Group (FRAG) guidelines give up to date detail on best practice to be followed.
Resistance to Zorvec, (an OSPBI fungicide) has been reported in continental Europe. Whilst still one of the strongest blight products available, Corteva, the manufacturer is keen to protect this useful active for as long as possible. Update application advice for the use of Zorvec in 2024 is available here.
FRAG have also produced guidance for use of OSPBI fungicides for a range of crops including potatoes.
Syngetna the manufacturers of Revus (mandipropamid) have also produced guidance for the responsible use of their product, which can be found here. FRAG guidelines for the CAA group of fungicides can also be found here.
Summary
The many factors at play make blight a difficult disease to manage. To summarise the key points:
- Identify and manage sources of infection e.g. out grade piles, groundkeepers where possible.
- Use quality seed.
- Choose varieties with good blight resistance or if market dictates variety – be aware of its blight susceptibility.
- Blight monitoring tools can be useful as can local/farm knowledge – you know your crops best.
- At each spray timing partner fungicides with different modes of action.
- Alternate modes of action between successive sprays.
- Consult FRAG and manufacturer guidance for advice on product efficacy and anti-resistance protocols.
- Using a wide range of fungicides from across all the modes of action available gives more spray mix options and lowers the chance of resistance developing.
- Good spray application will boost performance, angled nozzles, higher water volumes and lower forward speeds all improve target coverage.
- Continue spray programme until all traces of green haulm have been desiccated.
- If blight outbreaks do occur submit samples to fight against blight for genotyping.