The Irish climate provides the ideal conditions to produce high-yielding cereal crops. This high yield has been observed in winter barley in recent times, although the area decreased in 2017 and 2018. However, an estimated 75,000ha are planted to winter barley for the coming season. While factors such as straw price and the three-crop rule may have influenced this increase, recent high yields are also likely to be a factor.

These high yields have been due to improved agronomy and the introduction of high-yield potential two-row and hybrid six-row varieties to the market. The management of two-row barley is now well established, although the relatively recent introduction of high-yield potential hybrid six-row varieties poses important management questions and specifically disease management questions.

The question of disease

While our climate may be ideal for producing high-yielding crops, it also provides perfect conditions for fungal pathogens to grow and spread within crops. In this regard, things such as resistant varieties, rotation and other methods must be conducted as part of an integrated pest management strategy to help reduce or control these fungal pathogens.

That said, most winter barley will still require application of fungicide, as wet weather disease, such as rhynchosporium, net blotch, rust, ramularia and mildew, pose a threat to yield. The development of resistance due to overuse, as well as the need to control costs, means that fungicides must only be applied at stages in crop development when the crop is most at risk to yield loss due to infection.

Two- versus six-row varieties

Previous research on two-row barley has shown that the period when yield is most at risk is when grain number and storage capacity are being determined. Therefore, it is not surprising that research carried out by Teagasc across eight site seasons from 2010 to 2013 showed that the timings which had the greatest effect on yield were mid- to late-tillering, GS31/32 and GS49, with no benefit from applications in the autumn or post-GS49 observed.

Currently, these fungicide timings are also being applied to hybrid six-row varieties, although it is not known if yield is limited by the number and storage capacity of the grains or if the crop needs to be kept greener for longer to help fill the grains.

Six-row varieties have dramatically different yield components compared with their conventional two-row counterparts. They produce more grains per ear, but fewer ears per m2, which combine to produce more grains per m2 but the average grain weight is lower.

The grains per m2 produced by six-row varieties are similar to that of a wheat crop, as shown in Table 1. In wheat, fungicide timing is focused on the protection of the top three leaves of the canopy to ensure the canopy is kept clean into grain fill. This begs the question – should a six-row variety be treated like a two-row variety or possibly more like a wheat crop?

Fungicide timing trial

To answer this question, a field experiment was conducted to investigate the effect of five different fungicide programmes, ranging from an untreated to a four-spray programme, which included an ear spray on both a conventional two-row (KWS Tower) and hybrid six-row (Volume) variety grown at the standard recommended seed rate and nitrogen fertiliser application rate and a higher (+25%) than recommended rate. This experiment was carried out across two sites, Teagasc Oak Park and SRUC Edinburgh, and three growing seasons – 2014/15, 2015/16 and 2016/17.

The results of this experiment indicated that, despite the differences in yield components between two- and six-row varieties, they both responded similarly to the fungicide programmes. However, surprisingly, there was a significant response to an ear spray in both varieties. This is something which had not been observed in the research conducted from 2010 to 2013 and it raised a number of questions.

In those experiments, the application at GS49 was a 60% rate of an SDHI, azole and strobilurin mix without chlorothalonil (CTL). However, CTL was included in the ear spray. Since the 2010 to 2013 timing experiments, ramularia had become an increasing problem in winter barley, with the disease developing resistance to all major fungicide groups with the exception of CTL. So the question had to be asked – would the response to the ear spray have occurred if CTL was included at GS49 as is currently recommended?

Importance of the GS49 spray

An additional experiment was carried out in 2018 to look at the inclusion, or not, of CTL in these later timings at two sites, Oak Park in Carlow and Kildalton in Kilkenny. Two additional varieties, a conventional two-row (KWS Cassia) and a conventional six-row (KWS Kosmos), were added to the two previously used to test if the response was due to variety.

Fungicide treatment consisted of an over-spray of all treatments of a 50% rate azole plus morpholine at mid- to late-tillering and a 60% rate of an SDHI, azole and strobilurin mixture at GS31/32.

There were six fungicide treatments used in this experiment, but the main comparisons were between a 60% rate of an SDHI, azole and strobilurin mixture, with and without CTL at 1.0l/ha at GS49. Each of these treatments were tested with and without a follow-up ear spray of a 50% rate of a straight azole plus CTL to check if there is a benefit from an ear spray where CTL was included at GS49.

CTL timing was critical

The results of this experiment showed no difference in yield between all four varieties, while the disease assessment carried out at GS75 (mid-grain fill) showed no difference in how the disease levels in the four varieties responded to fungicide treatment. At GS75, the main disease present in all varieties was ramularia, with the level of infection assessed on the top two leaves and averaged across these leaves.

The SDHI, azole and strobilurin mixture without CTL had an average disease level across all varieties of 18% and 26% in Oak Park and Kildalton respectively, while the addition of CTL to this reduced disease levels to 2% and 1% for the same sites (Figure 1).

Yield data was not available from the Kildalton site due to an equipment failure, although the difference in disease control translated into yield at Oak Park, with yields averaged over the four varieties of 8.4 t/ha and 9.3 t/ha with and without CTL at GS49 respectively (Figure 2). Also, in the treatments where CTL was included at GS49, there was no benefit to yield or disease control from the addition of an ear spray in any of the four varieties.

These results show that if CTL or an effective product for ramularia control is included at GS49, there is no further requirement for fungicide treatment. However, with CTL soon to be withdrawn from the market by the EU commission, it will be a real challenge to find an equal alternative or a combination of influences that will act to contain this disease over time.