Manufacturers septoria resistance testing disputes Irish findings

Bayer CropScience has tested Irish septoria samples that were said to be “less sensitive” to key wheat fungicide prothioconazole and found them to be well within the normal limits for UK septoria, according to the firm’s Alison Daniels.

Initial testing by researchers at Teagasc of septoria collected from field populations before any fungicide sprays were applied this spring found the fungus to be much less sensitive to prothioconazole, and somewhat less to epoxiconazole (Arable, 24 April).

But laboratory testing of four of the same septoria samples by Bayer suggested the isolates were actually more sensitive to the fungicide than the average UK septoria population had been over the past three years, Dr Daniels said. “They are right in the range of previous UK testing.”

The firm had also carried out parallel glasshouse experiments to test the fungicide response in wheat seedlings inoculated with the different septoria samples. “All of the samples were infective, but were controlled by label doses of prothioconazole. It is very encouraging, and suggests there is no reason to change field practices.”

As a result of its findings in April, Teagasc advised growers to avoid using prothioconazole more than once in a programme. “It wasn’t just the odd insensitive isolate we found,” said the organisation’s John Spink. “We tested 18 or 19 fields this spring and it occurred in all but one, and made up a large proportion of the septoria population in some fields. When you find that you start to worry.”

Teasgasc would stand by its results, he added. “Our results totally disagree – we’re talking orders of magnitude, not just a two to three-fold difference that might be due to laboratory differences – and it is difficult to know why.”

Detailed analysis of the less sensitive isolates by Teagasc scientists also suggested a previously rare mutation, S524T, might be contributing to the findings. “There is a good correlation between it, in combination with two other previously known mutations, and the lower sensitivity.”

He said Bayer’s results and early field assessments provided some reassurance the prothioconazole would not fail completely, but the acid test would be its final performance in the field, and what selection pressure had been exerted for resistant isolates, he said.

BASF testing of the Irish septoria against epoxiconazole largely confirmed the results of the Teagasc monitoring, the firm said.

“However, Teagasc isolates remained significantly more sensitive to Opus than UK reference isolates, which were effectively controlled by Opus,” it added. “Consequently, Teagasc now accepts recent findings do not represent a heightened risk of a reduction in field performance.”

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