EYEING UP THE LOSS
Dont underestimate eyespot. David Millar reports on the latest thinking on this stem-base disease.
SO you think you know all about eyespot? Well, what about its synergy with take-all? Or the big losses in 1996 when there was very little tell-tale lodging to be seen?
Common perceptions that eyespot only matters when it leads to lodging are not borne out in ADAS surveys. Indeed, the high-yielding, lodging-free 1996 harvest might have been even more productive if many growers had prevented the widespread appearance of whiteheads in their wheat crops.
Bill Clark, of ADAS, which carried out the eyespot incidence surveys for Novartis, says many growers may be unaware that 1996 was the worst eyespot season for about 20 years. Because of the high yields generally prevailing, they were prepared to overlook the high numbers of whiteheads in some crops.
By monitoring 50 East Anglian fields per year three times a season, ADAS found losses of up to 35% of yield in 1996, averaging out at 15% due to uncontrolled eyespot. In the following year there were very few visible whiteheads, no lodging, but still an average 7% depression in yield.
Last year, there were even fewer signs of eyespot; few growers thought it a problem; yield losses averaged about 2%; but a more serious 10-15% of potential yield disappeared on farms where Mr Clark found eyespot cosying up to take-all.
Quite simply, he says, most growers underestimate the potential effect of eyespot. Combined with an inability to spot the disease in its early stages, let alone treat for it, eyespot is more of a threat than expected. Over the three years from 1996-98, the average yield loss was a costly 8%, yet the surveys showed how inaccurate the on-farm assessments can be.
ADAS did not interfere with disease management on the farms surveyed so was able to come up with an accurate assessment of how each grower or his adviser fared. In 1996, half the crops were over the spray threshold for controlling eyespot, yet only 7% of crops were treated. Come 1997, just 18% of crops were over the trigger threshold and 18% were treated – mostly the wrong 18%, adds Mr Clark.
And, in 1998 ADAS found 22% of the survey fields should have been sprayed but only 6% were treated.
What went wrong in 1996? Although a severe eyespot year with varieties such as Brigadier and Soissons particularly affected, inspections at GS30-31 showed around 20% over threshold but this had leapt to 50% just after GS32. Coupled with the farmers ability to spot the much more visible yellow rust or mildew, there was sufficient scope for eyespot to be overlooked in favour of these other diseases.
Mr Clark concedes it is difficult to identify eyespot and difficult to assess how seriously it should be taken. "Farmers need to be aware of whether or not it is a penetrating lesion they are examining," he points out. "If it is penetrating, it is likely to be eyespot; fusarium causes stem browning but will rarely penetrate."
Most fields have a mixed population of the W (wheat) and R (rye) strains of eyespot. Unix is equally effective against either strains, as is Sportak, but Sanction and Landmark are more effective on the W-types.
Mr Clark recommends regular inspections for eyespot, especially just before GS32 when the canopy thickens.