The ongoing application of fungicides to crops is resulting in changing pathogen populations and this, in turn, results in the changing performance of individual products. In this regard, one could be forgiven for being confused and information overload can result in one not being able to see the wood from the trees when it comes to disease control strategies. However, taking time to stand back and asking what we are trying to achieve with disease control in tillage crops can help to bring some clarity.

Remember:

  • We are not aiming for complete disease control, but to maximise yield for the minimum spend – we are not growing fruit or vegetable crops where a blemish caused by disease can make the crop unmarketable.
  • We are aiming to apply fungicides at the correct stage in the pathogen’s lifecycle: when they are susceptible to the product. In the case of septoria and rhyncho, this is before you can see a visible disease symptom – once you can see a lesion no amount of fungicide will kill it.
  • We are only aiming to control disease on the plant organs that are critical to yield formation; controlling disease on plant parts that are not contributing directly to yield provides no return, but it will still cost money. In the case of foliar disease on wheat, the critical organs are the top three leaves, while in barley it is the formation and maintenance of ear-bearing tillers that is critical to yield formation.
  • Once we have these principles in perspective, it is clear that correct timing of sprays is the most important thing to get right. The best product applied at the wrong time will not give you a return on investment.

    Wheat

    Results from over nine experiments conducted by Teagasc in recent years, which compared a range of products applied at the T0 timing (leaf four fully emerged), showed no significant yield benefit from this treatment. In the same trials, a correctly timed T1 spray on leaf three gave an average yield benefit of 0.5 t/ha.

    CONTEXT: Spending €60/ha on the first spray on 50ha of wheat would cost the grower €3,000 and give no return if it was applied too early to leaf four. But it would provide an average return of around €4,000 if applied correctly to leaf three, assuming a wheat value of €160/t. And this response will be even greater in high disease-pressure crops.

    This may not seem like a great return on investment, but this spray is delaying the development of the epidemic up the extending stem. It does this by protecting leaf two, which is already partially emerged at that stage, and by providing the basis for the all-important flag leaf spray.

    The flag leaf spray, which should be applied when the flag leaf is fully emerged, is by far the most important single treatment, giving on average a 1.7t/ha response. This spray timing protects the second half of leaf two, which had not yet emerged when the previous spray was applied, as well as protecting the flag leaf. The cost of this spray is usually in the order of €70/ha, so the €3,500 spent on our 50ha of wheat would give an average payback of €13,600.

    The ear spray is primarily to provide protection against head blight, but it also provides a top-up of foliar disease control programme. The average response to the T3 is about 0.5t/ha, but a portion of this response is coming from head blight control, so the response to foliar disease control is less than from the leaf three timing.

    The ear spray is primarily to provide protection against head blight.

    Because the main aim of the T3 is to control head blight, this treatment should be applied during flowering when the florets are open, as this is the only time when some control of fusarium can be achieved. Clearly, the small response to foliar disease control at this timing shows that any sprays applied later are unlikely to give an economic return on spend.

    These three key spray timings –

  • Leaf three fully emerged.
  • Flag leaf emerged.
  • Flowering.
  • – were established in experiments in the late 1980s and early 1990s, when the products available included Benlate and Tilt. These products were inherently weaker septoria products than we have available today, even allowing for the development of insensitivity in recent years.

    Therefore, there is no economic reason to adjust these spray timings and neither is there any necessity for additional sprays. From an anti-resistance perspective, adding additional spray timings is one of the most effective ways of ensuring that product resistance will develop quickly.

    It is also worth noting that the first fungicide should be applied when the third-last leaf is fully emerged. We often think of this timing as being when the second node is detectable (GS32), but this is only a rule of thumb. In early drilled crops, or when there has been a very warm autumn and winter, leaf three might not be fully emerged until the third node is detectable (GS33). Conversely, in late-drilled crops, the third-last leaf may be emerging at first node (GS31).

    The most important timing on winter barley over the eight experiments was at early stem extension (GS31/32).

    Therefore, it is important to monitor leaf emergence in crops to help get the timing right. This might seem like a chore, but when you consider the cost of the products you are applying and the implications of getting the timing right to help get a return on your investment, this assessment can be a very well-paid few minutes’ work.

    Barley is different

    As indicated previously, the principles of yield generation are quite different in wheat and barley, despite their similar appearance, and they also differ in terms of how they respond to disease control.

    In barley, yield reduction generally comes about when early disease reduces the head count, either through restricted tiller production or increased tiller death.

    The crop is able to fill more grains than it can produce, as there are far fewer grains per ear than in wheat.

    There is a wide range of good fungicide-active ingredients available for use in barley. This means that we can easily choose mixtures and sequences of different fungicide families to reduce the risk of resistance development, while still achieving good disease control. However, these fungicides are all primarily protectant rather than eradicant in their activity.

    The principles of yield generation are quite different in wheat and barley.

    The need to protect tiller production and survival, combined with the protectant nature of the products, means an early start to disease protection is needed. If disease is coming into the crop early, as will often happen in the more southern counties, then a spray during late tillering (pre-stem extension) is often required.

    In a series of recent winter barley fungicide timing experiments, carried out over four years (2010 to 2013) in both Carlow and Cork, a spray during late tillering gave an economic response in three of the four years in Cork, but only one year in four in Carlow.

    This need to time the start of the disease control programme when disease is coming into the crop was demonstrated by an autumn fungicide treatment giving an economic response in two years out of four in Cork where there was early disease pressure.

    The most important timing on winter barley over the eight experiments was at early stem extension (GS31/32). This coincides with the period of tiller survival in barley, when loss of green area to disease can result in increased tiller death and reduced head count.

    As stated previously, barley has the capacity to fill more grains than it can produce, so there is little need for late disease control to protect the canopy late into grain filling. The final spray should be timed at when the awns are emerging. This protects the crop when final grain number per ear is being set and can help to increase grain number and potential grain size.

    Delaying the final spray until the ear is fully emerged means that it is too late to affect grain number per ear, and across the series of experiments there was no significant response to fungicides applied at this time.

    In winter barley where there is high and early disease pressure, a three-spray programme will be needed, starting at late tillering, with a second spray at early stem extension and a final spray as the awns are emerging. Where disease pressure is lower and where it comes into the crop later, a two-spray programme timed at early stem extension and awns emerging should be sufficient.

    Deciding on whether to opt for a three- or a two-spray programme requires that the crop is monitored carefully from early spring to check its disease status. If there is early disease present, act quickly.

    The principles of disease control in spring barley are exactly the same as for winter barley. The main difference is the faster rate at which spring barley grows and develops.

    In a similar series of experiments to those described previously on winter barley, the most responsive spray times were late tillering and awns emerging.

    There was no benefit from an additional spray at early stem extension and, as with winter barley, there was no response to a spray applied to the fully emerged ear.

    Oilseed rape decisions

    The three main diseases of oilseed rape are light leaf spot, phoma and sclerotinia.

    Light leaf spot reduces yield by killing the developing flower buds as they emerge. Therefore, it needs to be controlled early in the life of the crop, in the autumn or late winter period and long before the green bud stage.

    The three main diseases of oilseed rape are light leaf spot, phoma and sclerotinia.

    Both phoma and sclerotinia reduce yield by damaging the stem and thus reducing water and nutrient uptake during pod filling. In severe cases, it will cause complete stem failure and loss of the top of the plant. As with light leaf spot, phoma must be controlled early in the life of the crop in the autumn and late winter.

    The disease infects the leaves and grows down the leaf petiole into the stem. Once in the stem, fungicides are ineffective, even though there will be a long delay until the tell-tale phoma stem cankers appear.

    Sclerotinia has the potential to be the most damaging disease, but generally the risk is low.

    The disease carries from year to year as sclerotia (which look like rat droppings) in the soil, so for there to be any risk of infection, these must be present in the field or in neighbouring fields. If sclerotinia has not been previously found in the locality, then the risk of infection will be low.

    When present, sclerotia have to germinate to produce apothecia (small orange or brown trumpet-shaped mushrooms about 1cm in diameter) when the crop is flowering and this requires warm, moist soils. The weather then needs to be dry for the apothecia to release spores which land on the petals. The weather then needs to turn damp to allow the petals to stick to the stems when they fall.

    The fungus then uses the petal as a food source to allow it to germinate and grow down into the stem. Once infection has taken place, the growing fungus causes the stem to bleach and die and further sclerotia are produced in the stem.

    The risk of sclerotinia infection is generally low, because it requires this very specific sequence of weather conditions in order to develop an epidemic.

    However, when an epidemic does develop, the effects can be devastating. If there is a history of sclerotinia close to an oilseed rape crop, a single spray at early flowering should provide sufficient protection.

    Conclusion

    As explained earlier, knowing the disease target and the correct timing for effective control is essential. Proper timing is essential in order to make the most of the investment in disease control. Even the best fungicides will not pay if applied at the wrong time. Time spent monitoring the disease.

    Key points

  • Disease control in winter wheat should begin after the third last leaf is fully developed.
  • Depending on conditions, disease control in winter barley might either begin during late tillering or at early stem extension.
  • Early spray treatments are essential where light leaf spot or phoma occur on oilseed rape.
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