Resistance message is reaffirmed

21 April 2000




Resistance message is reaffirmed

MILDEW control with strobilurins will be a problem within this growing season, warns a leading Scottish researcher.

But that does not mean growers should abandon anti-resistance strategies.

Protecting efficacy against key diseases such as septoria or net blotch is crucial, says SAC plant pathologist Fiona Burnett.

"The manufacturers are resigned to the fact wheat mildew control has gone and that barley mildew will go down the same road. The resistance risk is not so much about mildew as the other diseases."

Dr Burnett accepts that in practice mildew resistance management may not persuade growers to mix in a specific mildewicide. But by flag leaf, or possibly ear spray in Scotland, growers can expect to see reduced mildew control from the strobilurins.

At these timings, eradicants such as fenpropidin (as in Patrol) or spiroxamine (as in Torch) should be added where mildew is active on the upper leaves, not just on the stem, she says.

More immediately, at T1, protection of susceptible varieties with Fortress (quinoxyfen) is an ideal anti-resistance strategy, providing mildew is not already established. A half to three-quarter dose of morpholine or spiroxamine should be used on crops already infected. &#42

Quinoxyfen contrast

Mildew resistant to quinoxyfen has been known to exist since the products development stages, but in contrast to the strobilurin resistant strain of the disease, the quinoxyfen resistant isolate is a poor competitor. "It is very, very unfit, and has never taken off in the population, which is a relief because the quinoxyfen is there for a long time," says Dr Burnett. There is no evidence of cross-resistance to both strobilurins and quinoxyfen in any strains of the disease and Fortress (quinoxyfen) as a protectant pre-T1 treatment remains a good anti-resistance strategy for mildew, she adds.


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