16 April 1999

Debate intensifies on strob

dose advice

Debate over FRACs

strobilurin guidelines on

doses is growing.

Andrew Blake reports

MUCH of the controversy over FRACs guidelines hinges on the nature of the resistance involved, according to Derek Hollomon of IACR-Long Ashton.

All the signs are that for strobilurins it is what pathologists call "qualitative". That means it is controlled by a single gene in the fungus or only a few working together.

"Quantitative" resistance, found where triazole and morpholine fungicides are involved, differs in being controlled by several genes. Reducing doses allows the more resistant fungus strains, which constantly develop through mutation in any population, to survive. That accounts for fluctuating levels of control often encountered in the field.

With qualitative resistance, as to strobilurins, lowering the dose may reduce the selection pressure and so slow the rate of change within a population, Dr Hollomon suggests. But doing so goes against FRAC guidelines.

So far the only sign of resistance to strobilurins is in German mildew. NIAB has found some shift in sensitivity to triazoles by yellow rust, but it is not yet of practical significance, he believes.

More worrying would be the development of resistance in septoria where spray timings are much trickier. "But there is no evidence of that happening. The problems in Germany probably arose because they were using epoxiconazole as a mixture partner to control mildew and that is not robust enough."

That begs the question of why growers should stick to the FRAC guidelines on doses when mildew can easily be controlled by morpholines and newer actives quinoxyfen and spiroxamine. There is only one real answer and it has nothing to do with resistance.

Dr Hollomon believes the guidelines must recognise that strobilurins are not just fungicides but offer benefits unconnected with disease control. "We do not fully understand these physiological effects, but you wont get them if you start lowering doses." FRAC advice on restricting the number of strobilurin applications to two per crop is sound, according to ADAS pathologist David Jones.

So, too, are recommendations to use mixture partners, albeit the correct ones. "But you could still have problems," he warns. "If you choose too low a partner dose and its control soon falls away, you would be left relying on the strobilurin. The partner has got to be robust both on product and dose."

FRACs guidelines are scientifically sound and farmers should follow them, maintains geneticist James Brown of the John Innes Centre.

"There is still little understanding of how dose rate relates to the development of resistance. But we know that with strobilurins it is a single step from being completely susceptible to being totally resistant. So it is important to kill as many strains as possible in one go."

Dick Neale for agchem distributor Hutchinson is fully behind FRAC and emphasises that full and recommended doses are not the same thing. "The full dose for Amistar, Landmark or Mantra is 1 litre/ha. For an early application with a good protective triazole we would be happy in most circumstances to use 0.6 litres/ha of Amistar or 0.5 of Landmark or Mantra.

"Using a little-and-often approach is irresponsible. You may still end up with 0.7 litres/ha. But the problem is that you will have selected out any resistant population.

"It must be appreciated that the mechanism of resistance is different to anything we have had with triazoles or morpholines. Once it has happened here we will get no second chances. I am adamant that we will select for resistance if we use continuous low doses."

Notts farmer Robert Sutton is unconvinced. "Organisms do not adapt in some pseudo-intelligent way to chemical attacks on their metabolism. They either live or die."

Only those that live can reproduce and create copies of their genes in the next generation. "So it follows that the more effective the fungicide, i.e. the fuller the dose, the greater the shift in the population towards resistance."

His key point is that without fungicide the susceptible strain dominates. "In the absence of sustained attack by the same chemical the susceptible strain would probably reassert its dominance."

Resistant strains are not inevitable, he adds. "There are many examples of fungicides not losing their effectiveness after years of repeat usage, particularly in the treatment of yellow rust.

"By the same logic a mixture of two chemicals may not avoid the problem. Only in the unlikely event that two strains of the fungus exist, each of which is susceptible to one of the ingredients but resistant to the other, will this strategy provide a long term solution."

FRAC strob guidelines

&#8226 Stick to recommended doses.

&#8226 Maximum two sprays a crop.

&#8226 Bad mildew merits effective partner.

&#8226 Apply before disease established.

FRACs Steve Heaney says it is important to remember that resistance to strobilurins is caused by mutation of one of the many energy-producing mitochondria within any fungal cell of spore. It differs from other types of resistance which arise from gene changes in the cell nuclei.

A single fungicide-resistant mitochondrion cannot provide enough energy for the cell to survive and reproduce. It relies on energy from other susceptible ones to do so, he explains. "The only way resistance can develop is if sensitive mitochondria also survive to keep the cell functional."

FRAC argues that these sensitive but resistance-inducing mitochondria have a better change of survival at a lower strobilurin dose. "The probability of control of sensitive mitochondria being delivered must be higher with the higher dose," says Mr Heaney.

As a fungicide manufacturer Dow Agrosciences is not involved with strobilurins. But cross-resistance studies on its quinoxyfen mildewicide confirm that the German mildew overcomes strobilurins through single gene resistance. "It is what we call disruptive resistance like we had with benomyl in the 1970s. Products very quickly do not work," says the firms Chris Longhurst.

"That is different to the directional resistance you have with triazoles and morpholines where you may get poor performance but few cases of absolute control failure."

Mr Longhurst views quinoxyfen (Fortress), with its completely different mode of action, as an ideal strob partner in an anti-resistance strategy.

How to choose partner

ALL strobilurins need a triazole partner on wheat, says ADASs Bill Clark. But which and at what rate depends on diseases present and eradicant activity required.

At T1 timings septoria eradication is the main objective and growers should opt for one of the top four triazoles; epoxiconazole (Opus), tebuconazole (Folicur), cyproconazole (Alto), or new active from AgrEvo fluquinconazole (Foil).

"Good eradicant products are essential at T1 to clean up leaf four and the emerging leaf three. The rate depends on how long leaf four has been out, but I wouldnt go below half-rate," says Mr Clark.

Older triazoles like propiconazole (Tilt) do not have enough activity to be sure of eliminating leaf four infection. That can infect the all important flag leaf later.

The top triazoles are also best on rusts and there is little to choose between them.

Eyespot can merit a departure. "If eyespot is an issue, growers may choose flusilazole. It has the bonus of some eyespot activity. Where eyespot is serious go to Unix. But an Amistar/Unix mix will still need a triazole added to eradicate septoria."

Landmark (epoxiconazole + kresoxim-methyl) has some eyespot activity and a role where growers are concerned about stem base browning. "If they are unsure whether browning is eyespot, then they havent got a real problem. If eyespot is severe it is quite obvious."

Mildew merits a low dose morpholine, preferably fenpropidin, to clear it up. Mantra (kresoxim-methyl + epoxiconazole + fenpropimorph) might avoid a three-way tank-mix. "None of the triazoles are good enough on their own."

T2 strategies should be based on a strobilurin plus triazole, Mr Clark advises. Used at early flag-leaf emergence the top four triazoles are virtually indistinguishable, and growers may even manage with some older molecules where only protectant activity is needed. But delays require full rates of the most effective ingredients.

"In protectant-only mode a half rate is as good as a full dose with the top triazoles. But higher rates are needed where eradication is required. Dont forget you are targeting leaf two as well as the flag leaf at T2. It will have emerged 10 days earlier and growers must ask if the weather has been wet? If they are confident there is no latent septoria, then they can drop the dose. If not, a higher rate, and/or stronger product, is needed.

"That is when the skill comes in, balancing dose rate needed with the price of each partner product," he concludes.

[PANEL]

IN barley azoxystrobin acts as a genuine fungicide with outstanding activity on net blotch, says Mr Clark. But mixers are necessary to combat other diseases, he stresses.

At T1 Unix (cyprodinil) broadens the spectrum considerably, tackling severe eyespot and mildew effectively. Flusilazole is a cheaper compromise where stem base browning is only moderate. Where rhynchosporium is the main concern, reverting to morpholines or a triazole, probably Opus (epoxiconazole), is the only option.

T1 principles apply to T2 awns emerging sprays too. Net blotch in a wet season easily justifies the expense of azoxystrobin at this stage. A triazole may be added to eradicate established disease, or a morpholine, preferably fenpropimorph, where mildew is a threat. But that shouldnt be needed after T1 Unix, he notes.

[POINTS BOX]

STROBILURIN MIXERS

Top triazoles to eradicate septoria.

Unix or flusilazole for eyespot at T1.

Morpholine mixer for mildew.

Match rate to eradication needed.

In barley azoxystrobin acts as a genuine fungicide with outstanding activity on net blotch, says Mr Clark. But mixers are necessary to combat other diseases, he stresses.

At T1 Unix (cyprodinil) broadens the spectrum considerably, tackling severe eyespot and mildew effectively. Flusilazole is a cheaper compromise where stem base browning is only moderate. Where rhynchosporium is the main concern, reverting to morpholines or a triazole, probably Opus (epoxiconazole), is the only option.

T1 principles apply to T2 awns – emerging sprays too. Net blotch in a wet season easily justifies the expense of azoxystrobin at this stage. A triazole may be added to eradicate established disease, or a morpholine, preferably fenpropimorph, where mildew is a threat. But that shouldnt be needed after T1 Unix, he notes.

Strob resistance a different mechanism

FRACs Steve Heaney says it is important to remember that resistance to strobilurins is caused by mutation of one of the many energy-producing mitochondria within any fungal cell or spore. It differs from other types of resistance which arise from gene changes in the cell nuclei.

A single fungicide-resistant mitochondrion cannot provide enough energy for the cell to survive and reproduce. It relies on energy from other susceptible ones to do so, he explains. "The only way resistance can develop is if sensitive mitochondria also survive to keep the cell functional."

FRAC argues that these sensitive but resistance-inducing mitochondria have a better change of survival at a lower strobilurin dose. "The probability of control of sensitive mitochondria being delivered must be higher with the higher dose," says Mr Heaney.

German mildew disruptive resistance

As a fungicide manufacturer Dow Agrosciences is not involved with strobilurins. But cross-resistance studies on its quinoxyfen mildewicide confirm that the German mildew overcomes strobilurins through single gene resistance. "It is what we call disruptive resistance like we had with benomyl in the 1970s. Products very quickly do not work," says the firms Chris Longhurst.

"That is different to the directional resistance you have with triazoles and morpholines where you may get poor performance but few cases of absolute control failure."

Mr Longhurst views quinoxyfen (Fortress), with its completely different mode of action, as an ideal strob partner in an anti-resistance strategy.