Eying up eyespot in winter wheat

Common eyespot, caused by the fungal pathogens Oculimacula yallundae or O. acuformis, is a disease that can affect all cereal crops. It causes lens-shaped lesions that are most commonly located below the first node, but they can appear above it in severe cases. These lesions become more distinctive later in the season and eyespot gets its name from those lens-shaped lesions.

While the disease can be a problem in all cereals, its effects are most often felt in winter wheat. Unfortunately, it can be difficult to accurately quantify these impacts.

Impacts

Being a disease of the lower stem, eyespot adversely effects yield in a number of ways. Firstly, any disease that affects the stem affects the vital supply of both nutrients and water upwards.

This will be most evident in warm dry seasons and it often manifests itself as individual white heads scattered throughout crops late in the season. While these heads will contain grain, fill will be affected due to their premature death.

If these are few and well scattered, any impacts on yield are likely to be small and of no significance. If they are more widespread, there may be an impact on yield.

White heads caused by eyespot are unlikely to form distinct patches or be stunted in appearance, which is often the case with take-all. In our typical cool wet springs and summers, we are less likely to experience the typical white head symptoms.

While lesions would still restrict the movement of water and nutrients, the abundance of water means that the crop may not be under as much stress and premature senescence may be less of an issue. This does not imply no impact on yield, it just makes quantifying potential impacts more difficult.

Singular white heads in a crop of winter wheat that was badly infected by eyespot.

Singular white heads in a crop of winter wheat that was badly infected by eyespot.

Secondly, even where the availability of water and nutrients is not limited by stem infection, there is still the fear of stem breakage.

As we store more carbohydrates in the grains in an aim to achieve high yields, crops become top-heavy and the greater the yield the heavier the crop. And anything that reduces stem strength places such crops at increased risk of lodging.

While factors such as variety, rainfall, wind, crop density, nutritional status, etc, all affect lodging, the role of eyespot should not be overlooked. In fact, the presence of lodging should always force a check for the presence or absence of eyespot symptoms to aid decisions on how to prevent similar problems in future.

Favourable conditions

As with any fungal disease, there are specific conditions in which eyespot thrives, as well as conditions where it struggles to survive. If we understand these, and how we might manipulate them without adversely affecting the growth of the crop, we can begin to think about sustainable control measures.

Based on research from the UK, the risk of eyespot development appears to be related to sowing date, rainfall over the winter months, tillage system, crop rotation, soil type and crop variety, with each contributing a differing level of importance.

If we break these down and apply them to Irish wheat crops, we should possibly, with the exception of Dublin which receives slightly less rain, regard all Irish winter wheat crops as being at high risk of eyespot infection.

However, this does not necessarily mean that all Irish winter wheat crops will develop eyespot, which can often be seen by the different levels of the disease present in crops in mid-July. Some will show clear lesions, while the disease can be very difficult to find in others.

Infection factors

This may be a reflection of the fungicide control measures used or it could also relate to the factors described previously. Understanding the significance of these different factors in Ireland could be very useful.

As part of the EPIC-IPM project, funded by the Department of Agriculture, Food and the Marine, a survey was initiated to determine the importance of each of these factors to the development of eyespot in Irish winter wheat crops. This typically involves intensive crop sampling at stem extension in as wide a cross-section of crops as possible, followed by an evaluation of the presence and intensity of eyespot at this stage.

Over 200 crops have been monitored to date since 2015, with a follow-up sample occurring later in the season. Results to date show that the cultivation of wheat after wheat shows higher levels of eyespot (Figure 1) and this is hardly surprising.

There will understandably be reasons why wheat is grown after wheat, whether as long-term continuous wheat or as part of a wider rotation. However, the risks associated with take-all and eyespot must be understood and measures taken to counteract them. This should include the use of specific varieties that show resistance to eyespot.

It is known that the presence of a specific resistance gene will reduce the risk of infection considerably. While the suitability of these varieties to Irish conditions (such as resistance to septoria, sprouting, etc) is not always perfect, they should not be ignored outright.

While some may have slightly lower yield potential, harvesting a crop off the floor also severely affects potential yield.

As with septoria, later sowing reduces initial inoculum and the build-up of the disease over the winter months. Again, the capacity to delay drilling could well be a yield-restricting factor and be part of the reason why it is in continuous wheat. But understanding that this is a factor must be considered (as with septoria) when deciding when to sow and what to sow.

Whether these factors can be offset by reduced tillage systems, which are associated with reducing infection levels, is debatable given the generally mild wet conditions that prevail in Irish crops over the winter months.

An eyespot lesion at the base of a stem of winter wheat at the early infection stage.

An eyespot lesion at the base of a stem of winter wheat at the early infection stage.

Chemical controls

Chemical control should never be the first and only action taken and should only be considered after all of the other factors that affect eyespot development have been assessed.

The success of chemical control alone is also difficult to determine. Ideally, it should result in a reduction in symptoms or severity. However, the subsequent relationship with yield is not always clear cut.

To maximise the efficacy of fungicides that have eyespot control capability, it is essential to get the chemical to the position where it is required, ie the stem. Typically, this is best achieved at the start of stem extension, which often coincides with leaf-three fully emerged. However, it must be noted that this involves a compromise, as the need to maximise foliar disease control, including septoria, remains the biggest threat to Irish wheat crops.

As it is essential to get the fungicide to the target area in high-risk crops, the water volumes used must reflect this objective alongside the density of the target crop.

Finally, the choice of fungicide active will play a major role. Over the past decade, two specific fungicide actives – the azole prothioconazole (eg Proline) and the SDHI boscalid (eg Venture) – have been regarded as the most effective fungicides for eyespot control.

However, the arrival of the range of other SDHIs since 2012 has provided an additional range of fungicides that provide equal levels of control.

However, while we currently have effective chemistries, they are single-site and carry a medium to high risk of resistance development. Where possible, measures to reduce their application should be taken and they should be mixed with effective partners.