World: Rusts found 'unlikely' to develop strob resistance

Barley net blotch and cereal rust diseases appear unlikely to be able to develop the same type of total resistance to strobilurin fungicides as Septoria tritici, according to research carried out by the Universities of Basel and Turin in association with Syngenta.

The discovery should mean strobilurin fungicides, such as Amistar (azoxystrobin) continue to give good control of those diseases for the foreseeable future, Syngenta's David Ranner says.

The reason is to do with differences in the DNA make-up of each fungus's cytochrome b gene. Strobilurins block the fungus's mitochondrial respiration by binding to cytochrome b.

A mutation changing the DNA sequence from glycine to alanine at amino acid 143 (G143A) in the cytochrome b gene prevents strobilurins binding causing the fungi to become resistant to the chemical, Mr Ranner explains.

"The mutation has been reported in septoria and powdery mildew." The question is whether such a mutation could also occur in other fungi; is it just a matter of time before resistance develops, he says. To find out researchers sequenced the DNA code for cytochrome b in a wide range of fungal pathogens, and found it varied in size between even closely related fungi. "The cause is sequences of DNA, called introns, within the gene, that are not needed for cytochrome b protein function."

Normally these introns are removed when the DNA is translated into cytochrome b protein, with a particular sequence of DNA just before the intron sequence indicating what bits need to be removed, he says.

What the researchers discovered was in fungi such as rusts is that an intron starts immediately after the 143 position.

"It means if the mutation does occur it prevents the intron being removed [because the removal code is corrupted] so it can't make the cytochrome b protein properly and the fungus dies anyway, and of course, without the mutation the fungus continues to be susceptible to strobilurins. "It is a bit of a nightmare if you are a fungus."

So far, all the diseases that have become totally resistant to strobilurins do not have an intron immediately after the 143 position, while fungi, such as rusts, net blotch and early blight, which haven't succumbed to the G143A resistance, do, he says.

The discovery doesn't totally rule out resistance developing, he notes. "There are other mutations, such as F129L, but, so far, these only result in partial resistance."