Strobilurin Resistance in Septoria tritici 2003

With strobilurin resistance now an inevitable feature of septoria control in Ireland we must ask if the level at the start of the season has a bearing on the relative usefulness of strobilurins in a specific crop. Monitoring by Teagasc in 2003 has shown that resistance levels increased in plots even where no strobilurin selection pressure was applied in 2003. This is likely to mean that the base levels of resistance for 2004 will be higher than for 2003. This would further reduce the likelihood of strobilurins contributing to septoria control in 2004. In this article Brendan Dunne and Eugene O'Sullivan, Oak Park Research Centre, report on ongoing Teagasc research on strobilurin resistance.

Monitoring of European populations of Septoria tritici in 2001, including some from high-risk sites in Ireland, showed no trace of resistance to the strobilurin group of fungicides in this pathogen.

However, strobilurin resistance was reported in early 2003 in some leaf samples taken from crops in 2002.

Subsequently, Teagasc, Oak Park, in collaboration with Teagasc Advisory staff undertook a survey of S. tritici populations in winter wheat crops to determine the extent and frequency of resistance.

Samples of leaves infected with Septoria were taken from 21 crops in winter wheat-growing areas ranging from Donegal to Cork, during February and March 2003.

Resistance to strobilurins was found in Septoria populations in all crops sampled. The frequency of resistance ranged from 9% to 84%. It was greater than 30% in 16 of the 21 crops, greater than 50% in nine crops and greater than 70% in five crops.

The average level of resistance was 48%. The finding of resistance in all crops and the high frequencies with which it occurred was unexpected given that resistance had not been detected in 2001.

Opus Vs MBC

All isolates from all crops remained highly sensitive to epoxiconazole (Opus). Over 90% of isolates from all crops were resistant to MBC-generating fungicides.

This is surprising since MBC products have not been used extensively on winter wheat crops for many years. This also shows that there is not a fitness disadvantage associated with MBC resistance in Septoria.

Resistance in 2003

In summer 2003 Septoria populations in trial sites in counties Cork and Meath were studied to determine the influence of different fungicide programmes on the build-up of resistance.

At the Cork site, the level of resistance in the crop prior to laying down the trial was 22%. In late June, this had increased to 100% in plots that got three sprays of Modem and to 55% and 57% respectively in unsprayed plots and plots sprayed three times with Opus.

At Meath, the initial level of resistance was 57%. In July the level of resistance in plots receiving three sprays of Modem was 100%.

The levels of resistance in unsprayed plots, plots sprayed three times with Opus and plots sprayed with Opera at T1 followed by Opus at T2 and T3 were 89%, 85% and 94% respectively.

These differences in percentage resistance were also reflected in yield. These results are shown in Figure 1.

The 100% resistance following three sprays of Modem is not surprising since Modem would control sensitive strains of Septoria leaving the resistant strains to proliferate.

Resistance levels in plots not treated with strobilurins remained lower. However, they increased substantially above the levels initially detected.

The substantial increase in frequencies of resistance where there was no selective pressure from strobilurin use is difficult to explain.

The Septoria populations in these plots may have been influenced by the close proximity of strobilurin-treated plots, though the leaf samples were taken well away from the plot margins and therefore unlikely to be influenced by rain-splash of pycnidiospores from adjacent plots.

The role of air-borne ascospores in disease spread during the growing season is still unclear.

An interchange of ascospores between plots may account for the increases in levels of resistance where there was no direct influence of strobilurins.

There is also the possibility that resistance confers a fitness advantage on the pathogen though a fitness disadvantage is more often associated with resistance.

However, as already stated, there is clearly not a fitness disadvantage associated with MBC resistance in Septoria. The same may be true for strobilurin resistance in Septoria. All isolates of S. tritici taken from these trial sites remained highly sensitive to epoxiconazole, even where three sprays of Opus had been used.

Ongoing crop study

An extensive study of resistance in Septoria populations in commercial winter wheat crops is currently underway at Oak Park.

Samples were taken by Teagasc Advisers from 67 crops prior to the T2 fungicide spray to establish the initial levels of resistance.

Fifty two of these crops were again sampled three or four weeks post-T3 to determine the influence of the fungicide programmes used on resistance levels.

Using a ‘Real Time PCR' technique, resistance will be determined in both a qualitative and quantitative manner. This data will provide a valuable insight into the influence of fungicide programmes on resistance development.

DNA extraction from the more than 3,500 samples is taking place at present and analysis will be completed by the year-end.