Strob resistance gives chance to new mildewicide
Cereal mildew should prove
less troublesome with the
advent of new chemistry,
say two leading specialists.
Andrew Blake reports
HAD it not been for the strobilurins, mildewicide quinoxyfen might have been hailed as the most exciting cereal fungicide for years, says ADASs Bill Clark.
"It is the best mildew fungicide around, but like spiroxamine it has been overshadowed by the strobilurins."
With mildew resistance to strobs reported in Germany, these new materials should now get the recognition they deserve, he believes. Triazoles have rarely given satisfactory mildew control at normal doses, and morpholines often lead to growers chasing the disease all spring.
"In high risk areas such as the fens, quinoxyfen gets wheat growers out of the mess they often found themselves in when they did not get on soon enough and the disease got out of hand," he says.
Sold straight as Fortress, the fungicide has solely protectant action. But at the full 0.3 litres/ha dose it can keep mildew at bay throughout the season, he adds.
Alternatively, early treatments at lower rates can provide cover until true T1 timings at GS31/32. But users should remember that pustules present at spraying will remain. "They do not dry up, so you could be misled into thinking it is not working."
Competitively-priced spiroxamine (Torch) and morpholines such as Patrol (fenpropidin) and Corbel (fenpropimorph) will continue to have a useful role in T1 tank-mixes where mildew pressure is less severe, he believes.
Neither of the other main options, Unix (cyprodinil) and kresoxim-methyl-based Landmark can be considered front-line mildewicides, says Mr Clark.
Experience in Germany, and the prediction that strobilurins are more likely to encounter mildew resistance because of their mode of action, means kresoxim-methyl should be reserved for tackling higher league diseases like septoria and the rusts, he advises. "If you rely on a strob for mildew control you could be disappointed."
• Morpholines – May check crop.
• Triazoles – Weak at normal doses.
• Quinoxyfen – Mildew only but very persistent.
• Spiroxamine – Useful tank-mixer.
Early birds will achieve most efficient control
MILDEW in winter cereals is often worth tackling early.
That could be even more important this year, as late drilling tends to encourage the disease.
"It is always easier to control mildew early rather than waiting until the crop canopy closes in," says Morley Research Centres Doug Stevens.
Quinoxyfen in particular gives growers a chance to do that without risking loss of disease control later in the season, he says.
That the disease merits control is not in doubt. Morley trials in 1997 showed that an extra 1t/ha (0.4t/acre) yield of Shango wheat was entirely down to mildew control by a GS31/GS45 split quinoxyfen program. A split morpholine treatment lifted yield by 0.75t/ha (0.3t/acre).
The superior performance of quinoxyfen was no surprise, says Mr Stevens. It is a protectant and stops the fungus even starting to damage plants. And the morpholines have always been suspected of checking crops, he notes.
Another advantage is quinoxyfens persistence; depending on dose it can provide season-long protection. But it has no effect on other diseases and needs applying early, perhaps tank-mixed with a growth regulator or herbicide to get the best from it, he advises.
Spiroxamine is more akin to the morpholines, offering some control of rusts in wheat and net blotch and rhynchosporium in barley.
"It is slightly stronger than fenpropidin on pathogens other than mildew but not to the extent of fenpropimorph." Its most likely role is in tank mixes with triazoles or Amistar (azoxystrobin) which tend to be weak against mildew, he says.
Of the strobilurins, kresoxim-methyl is the only one with any significant benefit on foliar mildew, he maintains. Amistar appears to control mildew on the ear quite well. *
Be extra vigilant to beat eyespot – the stem base enigma
EYESPOT control can be a hit or miss affair, chemicals may be wasted while damaging infections go untreated.
This year growers will need to be extra vigilant, says ADAS. "The mild wet winter is likely to increase the incidence of eyespot this spring," says ADAS Rosemaund plant pathologist David Jones. "And a cool damp spring will promote rapid crop infection."
The fungus produces spores and spreads actively in wet conditions when the temperature is 3-14C (37-57F). It can infect just about any cereal crop over the autumn, winter and early spring period. Accurate and timely diagnosis is tricky.
"For good control of eyespot farmers need to identify the disease as early as possible. Fungicide activity on eyespot falls off after growth stage 32 as the spray cannot reach down the stem. The problem is that at this stage it is difficult to distinguish it from sharp eyespot, or fusarium foot rot," he notes.
In the early stages of true eyespot, lesions on the outer leaf sheath have a honey-brown colour, with a diffuse but sometimes more clearly defined border. Fusarium lesions are charcoal grey in colour, again with a diffuse margin, but are often seen at the junction with the leaf blade. They may also show as an elongated dark brown lesion on the margins of the leaf sheath.
Sharp eyespot lesions often have a shredded appearance and a clearly defined brown to purple border. A brown cushion of fungal growth on the surface of a lesion that can be rubbed away also distinguishes sharp eyespot from true eyespot.
To do damage, the eyespot fungus has to get through the leaf sheath into the stem. That is likely in cool, damp springs when leaf sheaths stick tight to the stem, allowing the fungus in. If the spring is dry leaf sheaths do not stay as tight and that slows disease development. "This all happens after growers have had to make a fungicide choice," says Dr Jones.
"At present our best course of action is to assess eyespot infection at GS31-32 and to apply a fungicide if more than 20% of tillers have eyespot lesions," he says.
That is only a guide and lesions penetrating through more than two living leaf sheaths, after dead outer leaf sheaths have been removed, may warrant a lower threshold.
"But there is a tendency to over-estimate the need to spray," he adds.
Spores are spread mainly by rain splash from diseased stubble on the soil surface. Cereal volunteers and couch will carry infection through a breakcrop, and partially rotted straw ploughed back up from a previous year can also harbour infection.
Over recent years R-type isolates have largely replaced W-types. The R-type develops later and more slowly than the W-type, making it doubly important for growers to keep looking for eyespot up to GS32, he warns. Which strain is present can affect fungicide choice.
Sportak (prochloraz) is effective on both types, but Sanction or Genie (flusilazole) only work well on the W-type. Unix (cyprodinil) is a reliable alternative to Sportak but lacks septoria activity. "And it is important to make this fungicide choice in the context of what other diseases are present," stresses Dr Jones.
"Where the number of lesions on tillers is high and other factors predispose the crop to high eyespot incidence, a full label rate of Sportak or Unix should be used. If lesion numbers are lower and the crop is at less risk, reduced rates could be tank-mixed, or another fungicide such as Opus or Landmark chosen to broaden the disease control spectrum." *
• Mild wet winter, cool damp spring.
• Early or deep drilling, high seed rates.
• Surface trash, sheltered sites.
• 2nd and subsequent cereals.
Pgr effect boosts OSR yield
Tebuconazoles plant growth
regulatory effect in oilseed
rape is much talked about,
but is it worthwhile?
Brian Lovelidge asked
the opinion of Peter Gladders
at ADAS Boxworth
OILSEED rape growers should see a handsome return on tebuconazole (Folicur) treatments, says ADASs Peter Gladders.
But it is not as a fungicide – March is too late for its main disease control benefits.
"The important thing is that tebuconazole stiffens the stems and keeps crops standing," he says.
Just how effective a pgr it is has become clear from trials over the past two years. Control of lodging and numerous side benefits increase yields by at least 0.5t/ha (4cwt/acre) on average. That is worth £65-70/ha, at a chemical cost of about £24/ha assuming full rate application of 1 litre/ha, he reckons.
"Last year we saw dramatic results from a Mar 19 spray, when stem growth was about 30cm (12in). The treated plots were still standing at harvest and the untreated ones were lodged."
Crops treated during rapid stem extension are usually reduced in height by 30-40%. But by the seasons end it may be only around 10%, or 10-15cm (4-6in).
"Crops do seem to bounce back, but the stem stiffening effect is retained," notes Bayer technical manager David Clark.
Not only is lodging reduced but the shorter canopy allows more light to get to the bottom of the crop. That seems to make pods lower in the canopy more robust, resulting in less shatter at harvest. Ripening tends to be more even and direct combining or swathing is made easier, says Dr Gladders.
Across six trials Bayer reports a 30% reduction in combining time in crops treated with 1 litre/ha of tebuconazole at stem extension. "And HGCA work last year showed that seed numbers in pods in the lower part of the canopy were increased by about 12% following tebuconazole treatment," he adds.
In other HGCA studies, a 1 litre/ha tebuconazole spray at stem extension increased yield by 30% compared with only 4-6% where other fungicides were used. This suggests much of tebuconazoles yield hike is from its pgr effect.
"Other triazoles do not appear to have this activity," he adds.
According to Mr Clark, anyone going down to half rate needs to mix in 1.5 litres/ha of 5C Cycocel. "That mixture costs around £10/ha and the latest it can be used is yellow buds stage so its timing is less flexible than straight tebuconazole. Cycocel alone is cheaper, but it is a pure shortener/stiffener."
Dr Gladders considers the tank mix is best for weaker stemmed varieties. Another option for an enhanced effect is two 0.75 litre/ha doses of tebuconazole, the first at early stem extension and the second at green bud stage. *
• 30-40% shortening at stem extension.
• Still 10% shorter at harvest.
• Average +0.5t/ha yield boost.
• 2 x 0.75 litre/ha doses, or Cycocel tank-mix.
Fungicides can have a valuable pgr effect on oilseed rape, boosting yield and easing harvest, says ADAS Boxworth plant pathologist Peter Gladders.