Late blight arrived on to our shores over 175 years ago, carving a history not just into our own island but across the world. Caused by the oomycete pathogen Phytophthora infestans, late blight under specific environmental conditions is an astonishingly destructive disease of potato and tomato crops.
A walk through the Teagasc late blight trials in late August every year is a dramatic reminder of the destruction and desolation that faced our nation at that time.
In the ensuing 175 years, P. infestans and late blight have, if anything, become more destructive to potato crops.
But we have also greatly increased our understanding of the pathogen and, hence, our ability to control and minimise its impacts.
We shouldn’t fool ourselves into thinking that we have tamed the beast, however.
On the contrary, we have come to appreciate its potential destruction and know that if we don’t take the necessary precautions, this destructive potential will quickly emerge. This is a process that needs to be continually reviewed to ensure that the strategies deployed are still relevant to changes in production and to changes in the pathogen itself.
Predicting late blight
The many epidemiological lessons that were learnt following its arrival in early autumn 1845, and ensuing destruction of the Irish potato crop over the following seasons, are as relevant in 2021 as they were then.
First and foremost, the disease is extremely dependent on local weather conditions.
Mild and damp conditions are ideal for its development.
Dry and warm conditions severely restrict its development. Specific interactions between temperature and moisture, be that relative humidity or rainfall, are critical to different components of the pathogens lifecycle.
Through understanding these factors we can start to forecast the development of the disease.
In its simplest form, a wet damp forecast during mid-summer is enough to suggest conditions are favourable for late blight development.
Understandably for commercial growers this is woefully inadequate in terms of enabling them to make decisions on control.
Through the Department of Agriculture-funded EPIC project, Teagasc, Met Éireann and Maynooth University have made revisions to the model used in Ireland to forecast late blight development, with work ongoing to ensure this information is made available to Irish growers.
Unlike the 1950s, our tolerance for late blight in crops is understandably extremely low given the costs of production and the strict requirements of end users
The current model, which was originally developed in the early 1950s, is highly conservative in its predications and, as such, often misses events that can be conducive to the disease.
In the 1950s, as the availability of fungicides was extremely limited it was essential to ensure that when they were used they protected against the most conducive events, hence its conservative nature.
Other major differences between then and now are the varieties of potatoes grown, the systems of production, including the various types of fungicides available now, and of course the specific strains of P. infestans that now dominate.
Each of these has greatly impacted, not only our ability to forecast the disease, but on our wider ability to control it.
Understanding these factors and their implications for late blight control is essential if measures are to be adapted to limit initial infections and to quickly identify and restrict its development.
Unlike the 1950s, our tolerance for late blight in crops is understandably extremely low given the costs of production and the strict requirements of end users. Therefore, most growers are very risk adverse towards the management of this disease.
From a disease control perspective, one of the key tools at our disposal is our ability to prevent the disease through the use of varietal resistance.
Neither Rooster nor Queens, which dominate the Irish market, can be regarded as resistant to late blight, highlighting one of the first hurdles we have to accept in late blight control.
As consumer habits change, the potential for more resistant varieties to be included in production will of course increase.
Detailed work is ongoing on the means through which these varieties can be developed and tailored to suit the Irish market.
Until then, however, we have to ensure that all other measures which are essential to limiting initial infections are maintained.
As P. infestans is not native to Ireland, Europe, or indeed the majority of regions globally. It has an extremely narrow host range. In the majority of regions it is also only present as clonal populations. This means that it is likely that the same strain, or only a handful of strains, are infecting most crops.
This has implications for how it survives between seasons. In the absence of sexual recombination and the long-lived oospores, P. infestans can only survive between seasons on living potato material, mostly infected tubers in the form of cull piles, seed, ground-keepers or volunteers left behind from the previous harvest.
From these sources, P. infestans can directly sporulate and spread spores, or grow into the emerging crop or volunteer and subsequently sporulate. For these reasons, it is critical to limit this risk as much as is feasibly possible.
Planting seed grade material is a must, of course, not just for late blight management but for a multitude of diseases, including viruses, bacteria and fungi.
While the impacts of planting poor grade material will be immediately visible and is rarely practised, the management of the previous seasons cull piles or ground-keepers may not always be as strict.
If weather conditions are favourable for the germination of infected tubers, they can act as a continual source of inoculum from early in the growing season.
Simple management practices that ensure their destruction may well be the most successful control strategy employed in any given season.
Attention to detail
Even with the best cultural control management strategies, it is highly unlikely that suppression of potential inoculum will be 100% successful.
Inoculum may not come from your own cull piles or volunteers in neighbouring fields. It could well come from within the field or even a garden plot.
While a garden can be a source of inoculum that you have limited control over, creating such a source in your own crop is only inviting problems. To put it into context, in highly conducive conditions late blight will cycle in under four days.
While a garden can be a source of inoculum that you have limited control over, creating such a source in your own crop is only inviting problems
If a small part of a field is left untreated, such as a handful of plants around a pole which may miss a single spray, it provides sufficient time for a single P. infestans spore to land, infect and already be producing further spores before the next application is due.
If it is a benign season and each spray timing runs like clockwork every seven days, the impact will only be on those few plants.
However, these seven-day treatments can easily stretch to eight or nine days so at the very least, a fungicide with curative chemistry will be required to ensure a scenario such as this doesn’t become a much greater issue.
Careful planning at crop planting to ensure all crops can be sprayed and are not missed, is important.
The same attention to detail is required when fungicides are actually being applied.
Even the most curative fungicides work best as preventative treatments. If they are not applied in a manner that provides complete coverage, you risk developing the same scenario with those handful of plants around the pole throughout the entire crop.
In most seasons, you may not notice any differences. However, in the season where weather conditions are conducive to late blight development, these differences won’t lead to a lesion here or there, but will instead lead to multiple lesions. Fungicide chemistries can take us so far but we need to help them help us.
The various control measures above may seem a little over cautious, especially in the context of past systems which were much more relaxed.
Costs of production have gone up and with increasing consolidation of the market, the potential implications of blight from an economic standpoint become inevitably greater.
At the same time, tolerances for blemishes in the marketplace are long gone.
However, the pathogen we are tackling has also changed, with any weakness in control now exploited even further than previously. As mentioned earlier, most global populations are dominated by single or a limited number of linages or strains, with these being displaced every few years as more aggressive or fit strains emerge and quickly dominate.
The most recognisable of these displacements has been the emergence of the strain often referred to as Blue-13 or EU_13_A2 and its rapid displacement of older European populations throughout the 2000s.
As an island nation, Ireland was among the last of the European countries (outside of the Nordic countries which are unique with sexually active P. infestans resulting in highly diverse populations) to be overcome by EU_13_A2.
By the end of 2008 it had become the dominant strain of P. infestans infecting Irish potato crops, in some instances with devastating consequences.
The combination of phenylamide resistance without the previously associated fitness penalties and an ability to cause infections at slightly lower temperatures quickly focused our minds on blight control, resulting in increased intensities of fungicide applications.
Since then, the Irish population has been dominated by three strains, the older EU_8_A1, Pink-6 or EU_6_A1 and EU_13_A2, with all three potentially present in the same crop. More recently, further displacements have been occurring throughout European populations, with these strains eventually making their way to Ireland.
This includes two strains that are of equal concern. Firstly, EU_36_A2 has rapidly dominated populations throughout the continent and UK, and, as it has displaced EU_13_A2 and EU_6_A1, can be assumed to be very aggressive and fit and will further exploit weaknesses in control programmes.
The other EU_37_A2, in addition to being fit and aggressive, exhibits reduced sensitivity to the fungicide fluazinam, with serious consequences for programmes relying on this active. Both strains have been detected through the Irish population in recent seasons and it must be assumed that they will contribute to Irish late blight outbreaks in 2021.
The presence of both strains in the Irish population has implications for disease control programmes.
The immediate change is of course the impact that EU_37_A2 has on fungicide choice.
Relying on fluazinam for control is now seriously questionable. As a strong protectant product it was often used either very early in the programme before rapid canopy development or late in the programme for tuber protection.
Clearly, late-season applications are now high-risk as fluazinam will select for EU_37_A2 and prolonged desiccations will require strong tuber blight control.
The imminent loss of mancozeb potentially removes an early season protective alternative.
While the loss of both these fungicide actives for differing reasons is unwelcomed, compared to other diseases, an abundance of fungicide chemistries is still available and remain effective for late blight control. Although these fungicides may still remain available, as each have different strengths and weaknesses, using them appropriately throughout the different stages of crop development is key.
The specific requirements of a fungicide during rapid canopy development, where the leaf sprayed this week is no longer visible in the following week, is vastly different to late season tuber protection.
Here, the canopy is naturally senescing and preventing zoospores from reaching the tubers underneath.
The specific fungicide required at both of these stages will also be different.
Know your fungicides
As the conditions that allow late blight to thrive are the same as those that limit our capacity to apply protection, being aware of the rainfastness of the different fungicides can be as important as knowing their curative or protective qualities.
The various key attributes of the major fungicides available for late blight control are evaluated by independent experts and their ranking agreed upon as part of the Euroblight network.
The result is the Euroblight Fungicide Table (https://agro.au.dk/forskning/internationale-platforme/euroblight/control-strategies/late-blight-fungicide-table/) which should guide the development of late blight fungicide programmes, both at the start of the season and also during it given the likely requirement of amendments and adjustments to reflect how the season progresses.