To date, the 2017 growing season has been unusually dry and warm. This is undoubtedly a good thing from a cereal disease management perspective.

The major diseases of Irish wheat and barley crops have evolved over time to exploit the wet, humid and mild weather conditions that normally prevail and dry, warm weather does not suit them.

For wheat, the exceptionally dry April hampered the upward spread of septoria, and also ensured that the initial fungicides were applied protectively.

Similarly in barley, excellent planting and growing conditions during the same period allowed crops to emerge and tiller with little pressure from rhyncho.

Unfortunately, this is only half of the battle complete.

From the middle of May onwards, low atmospheric pressure has returned, bringing with it those showers which should have fallen in April. Although these showers will promote both septoria and rhyncho, the earlier fungicide interventions, assuming correct application, product choice and timings, will have greatly reduced the potential damage that these diseases can inflict.

While the main diseases should still be considered a threat, at this stage in the season different diseases take dominance. With the ability to control these through variety choice or agronomy now past, decisions on whether to apply a fungicide, or what fungicide to apply and when to apply it, have to be made, the risks to final yields in terms of both quantity and quality must be considered.

Winter wheat

With broken weather now with us since mid-May, concern has turned to controlling Fusarium head blight or FHB in winter wheat. Although the disease causes problems every year, it is 2012 since it last caused serious yield losses throughout the country.

The disease is caused by a complex of different pathogens, each occupying their own niche. These include true Fusarium species such as F. culmorum and F. graminearum, which can produce a range of toxins and the Microdochium species M. nivale and M. majus. However, predicting which of the pathogens will dominate in any given season can be difficult.

Generally, the Microdochium species will be dominant when cool and prolonged periods of rainfall occur, such as in 2012. In fact, the disease pressure was so high in that year that the grey lens-shaped lesions, which the pathogens can cause on the leaves, also occurred in many crops.

Even though these subtle differences occur between the species that combine to make up the FHB complex, their control is now the same. As resistance to the strobilurins, caused by the G143A mutation is now endemic in the Irish Microdochium population, fungicide control of FHB, caused by Fusarium or Microdochium species, is dependent on the azole fungicides.

For the true Fusarium species this comprises metconazole, prothiconazole and tebuconazole. For the Microdochium species, prothioconazole is the most effective. As it is difficult to predict which species will be dominant in any season, mixtures of azoles ensuring activity against all are recommended.

Fusarium head blight in wheat is a wet weather disease and, unlike control of foliar diseases, it is difficult to control, even with the most active fungicides. The reason for this isn’t that the stated fungicides aren’t effective, they are, but a combination of other factors intervenes.

Firstly, the plant is at its most vulnerable to infection during flowering and to achieve optimum efficacy the fungicides must be applied at this vulnerable stage. Secondly, Fusarium and Microdochium fungi will cause most infections in periods of rainfall, during which it makes no sense to apply a fungicide as it will immediately be washed away.

Finally, flowering is not uniform across a crop, meaning it is impossible to achieve 100% coverage, or even close to that when it is really needed. If this is the case, is there any point of even applying a fungicide?

Case for the T3

The quick answer is “yes”. Even if only 50% control is achieved this could be as much as 0.75t/ha, as was observed in trials in 2012.

Timing the application to achieve maximum coverage is the most important factor and is achieved early to mid-flowering across the crop. This is when the anthers have started to emerge and are still yellow in appearance.

Spray too early and this vulnerable stage is not protected, too late and the disease has already taken hold.

Splitting the application equally runs the risk of not achieving sufficient protection at the most important timing. Unfortunately, this is a disease that rears its ugly head every so often. As difficult as it is to say, we are at the mercy of the weather when it comes to FHB control. The most effective control measure will be if high pressure establishes itself over Ireland during the coming week.

If warm, dry weather returns, diseases such as yellow rust and powdery mildew, which have established themselves in some crops, may continue to be a concern. In most instances, strong preventative and curative applications of azoles at the flag leaf stage will have achieved good control of yellow rust. However, if pockets of yellow rust continue to be active in susceptible varieties, the azole mixtures used for FHB control should take good care of them.

Where powdery mildew continues to be an issue at the base of the crop, any application of a curative mildewicide is unlikely to be able to achieve sufficient penetration into the crop to kill the pathogen. In these instances, the FHB control should include prothioconazole, as it will also protect the upper canopy and ear from mildew infection.

Finally, the application of an azole mixture will provide some level of a top up on septoria control.

How much of this is actually required or how effective the treatment will be is open to debate. If control has been achieved by the flag leaf application, any additional top-up will have a minimal benefit.

As we enter an era where curativity of the SDHIs and azoles has been compromised through resistance development, if infections are present we can expect even the best of the currently available azole mixtures to be short-lived.

Furthermore, if this is the case, control achieved from the addition of a multisite will be limited as the disease will already be present. So far, septoria appears to have been kept at bay. Any further sensitivity shifts will have consequences for 2018.

Firstly, irrespective of the fungicide that may be used, Teagasc spring barley research has consistently shown that the optimum performance comes from application during awns emerging and not later. The choice of fungicides should then reflect local disease pressure and the strengths or weaknesses of your variety.

As stated previously, rhyncho levels remain low but recent broken weather may now enable this disease to develop quickly, especially in crops that may be harbouring infections down in the crop. The warm muggy weather is also ideal for net blotch, and with cases of net blotch reported throughout the country in winter barley crops earlier in the year, there is undoubtedly inoculum present.

As with winter wheat, the dry April and early May allowed powdery mildew to develop in spring barley, especially in those varieties with low resistance.

These epidemics continue to be present in places on the lower leaves and stems but control of these down in the canopy is very difficult. So control should focus on the upper canopy.

Ramularia is a threat that only shows itself post-flowering and at a stage in the crop’s life when control is no longer achievable.

Finally, FHB is a threat to spring barley also. However, as flowering in spring barley can be very rapid and occur as the ear is emerging, or even in some case before it has emerged, it is extremely difficult to achieve FHB control in through the specific use of fungicides.

With this number of threats, is disease control achievable?

Previously, this was a relatively easy prescription. A range of actives were available, and while each had specific advantages over the other, most were equally effective in controlling a broad range of diseases. Although this has not necessarily changed, concerns over the sensitivity of the major diseases to these actives are increasing.

Firstly, low levels of strobilurin resistance have been detected in rhyncho in Ireland over the last number of years. Mutations associated with moderate resistance to the strobilurins and SDHIs have also been detected at low levels in Irish net blotch populations. Furthermore, resistance to the strobilurins, SDHIs and azoles (including prothiconazole) have been reported in Germany.

Isolates of net blotch resistant to SDHIs were detected in the Irish population in 2016 and reported by FRAC. In light of these issues, fungicide programmes must ensure that individual actives are not unduly exposed.

As for Ramularia, longer periods of leaf wetness have occurred in recent weeks, which will have increased the risk of infection. In all cases, chlorothalonil should be applied as part of the final fungicide programme to control this disease. As chlorothalonil has limited efficacy against the other pathogens a mixture of fungicides from the other groups should be used.

Even though some level of resistance may be present in the Irish barley pathogen populations, the main fungicide groups still remain effective but over reliance on either one active or even fungicide group is neither necessary nor advisable.

  • Recent wet weather means that our traditional diseases cannot be ignored in the final fungicides.
  • Ear blight and ramularia are major target diseases of the final sprays.
  • Potential resistance development remains a key concern in all major diseases.
  • There is no point in trying to target disease down in the canopy with the final spray.
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    Special focus on sprays: crops and grassland