CROP MONITOR

Septoria tritici levels causing concern

Report, 15 June 2005

There are many crops of septoria-prone varieties such as Consort, Access, Equinox, Tanker and Savannah where control of Septoria tritici has been very disappointing this season. It is apparent in a number of commercial crops that disease is now established on leaf 3 and leaf 2, with occasional symptoms on the flag leaf. In the UK, such disease levels are unusual in treated crops at this time of year and are of concern because if symptoms continue to develop yield losses could be large. So why has this happened?

There are a number of co-incidental factors which may have resulted in this situation.

1. Strobilurin resistance.
In trials where resistance levels at the start of the season were 80-90% (common in the UK) there is no measurable control of the disease from strobilurin products. Consequently, they offer little or no additional control of septoria when mixed with triazoles.

2. Triazole doses.
Over the last 7-8 years many people have relied on the strobilurins to help triazoles in controlling septoria. Consequently in mixtures, the typical triazole doses used have gone down in recent years. Now that the strobilurins are not giving additional control of septoria, the doses of triazoles used are often too low to give good control of septoria.

3. Shift in triazole sensitiviy in Septoria tritici.
There is general acknowledgement that there has been a shift in the sensitivity of Septoria tritici to triazoles, although the effect on field performance remains uncertain. Analysis of HGCA-funded experiments over the last 10 years indicates a reduction in field performance in the triazoles - so even high doses are giving a lower level of control than previously achieved and the timing flexibility which the triazoles once provided may have been compromised.

4. The 2004 season.
In parts of the UK, the season has been favourable for development of Septoria tritici - particularly where crops were sown and emerged early. Heavy rainfall near to GS32 at the end of April ensured heavy infection of leaves 3 and 4 and prevented spraying in many crops. Consequently, for poorly timed T1 sprays, little control of septoria on leaves 3 and 4 was achieved. The flag-leaf and leaf 2 then emerged through a canopy of actively sporulating septoria on leaves 3 and 4. High temperatures in May and early June led to very short latent periods for septoria (10-12 days). Consequently, the effective eradicant activity of triazoles was reduced. This has meant in some cases, that even high doses of triazoles, timed only slightly late, have given poor control of the disease. The Defra-funded CropMonitor project is tracking disease progress 'live' at 15 trial sites across the country. CropMonitor is currently showing evidence of poor control of septoria at some sites. At sites where the T1 spray was applied within three days of leaf 3 emergence, disease control varied across the sites between 52% to 95% of untreated levels - the former despite a robust Opus + Bravo mix. At sites where the spray was up to 9 days early or 13 days late, levels of control were as low as 9%. Such timings are typical of many commercial crops, because spray days are limited. These observations are being confirmed in reports from commercial crops around the country, particularly from Eastern Counties.

At some of the poor control sites there is evidence that the flag-leaf sprays might be failing to give control on the flag-leaf and leaf 2. Some of this failure may be due to poor timing, but doses of triazoles have been very robust (sometimes full dose), which would normally negate such a timing delay. Once again, because temperatures have been high, the latent period for Septoria tritici has probably been very short - making timing for eradicant activity critical.

This has raised concerns over the future of high-risk septoria varieties - particularly in the high-risk parts of the UK. These varieties are clearly more difficult to manage in high-risk seasons and in traditional high-risk areas they could pose real difficulties for farmers. Consort in particular is still popular and continues to be grown in these areas. However, varieties with higher resistance to Septoria tritici are likely to be in demand. Observations on CropMonitor are demonstrating the value of inherent resistance to disease - www.cropmonitor.co.uk

Graph showing mean disease severity on leaf 3 across 15 CropMonitor sites in 2005

Overview of current situation
Whatever the final reason, the result is that the level of control of Septoria tritici is very variable around the country. At some sites there is little control of septoria even from high triazole inputs.

Opportunities to spray in the UK are often limited at key timings, so many commercial crops will be sprayed at + or - ten days outside optimum. There is growing evidence that we have lost the necessary flexibility of timing formerly given by triazoles and only part of that loss may be due high temperatures and disease pressure experienced this season.

Crops at lower risk from septoria such as Claire and Robigus continue to look much cleaner with septoria confined to lower down the canopy.

	delivered to the industry by HGCA This website was last updated on: 27/08/2008 © Crown Copyright Central Science Laboratory 2007
All risk predictions published by CropMonitor are provided in good faith and are NOT a substitute for rigorous fieldwalking in combination with advice from BASIS qualified persons. CropMonitor accepts no liability for crop loss or damage resulting from the use of CropMonitor.

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