Many growers could be spraying final leaf four rather than leaf three when applying T1 wheat fungicide sprays, according to research by Central Science Laboratory.
Analysis of when each leaf layer was uppermost at 15 CSL live monitoring trial sites over the past two seasons has pinpointed leaf three emerges later than when many growers would believe, with full leaf three emergence ranging from 5-15 May.
But the average T1 timing in commercial crops over the past 15 years is around 23 April, CSL plant pathologist Judith Turner points out.
“That’s a clear indication the current T1 timing is hitting final leaf four rather than the intended target.”
It leaves room for improvements in disease control, she believes.
Analysis of the final septoria levels in the 300 randomly selected farms in the CSL national disease survey suggests disease levels are higher on farms where T1 sprays were applied before the end of April compared with 1 May onwards in the past four years.
“In the high epidemic year of 2002 there was a particularly clear benefit of a later T1 spray.”
However, Dr Turner recognises there may be practical difficulties for growers to leave all their T1 sprays until the beginning of May.
“Growers will always have to maintain some sort of safety margin.
I’m not saying growers should go later, but it suggests disease control could be improved if we can find some mechanism to help growers hit leaf three.”
But hitting leaf four may not matter as much if you’re using Proline (prothioconazole), Velcourt technical director Keith Norman believes.
In trials, Proline gave significantly better control of septoria in July than Opus on leaves that had not emerged at the T1 application date.
“We made the inference that there was some internal translocation of Proline into leaves not emerged at application.”
CSL is also questioning the value of T0 sprays, which have increased from being sprayed in 10% of wheat crops during the 90s to over 30% in the past three years.
“They’re attempting to mop up any inoculum that developed during the winter and early spring,” CSL’s Steve Parker says.
“But it is difficult to know truly what disease pressure is there, and what leaves you are actually targeting.”
T0 fungicides are usually protectant products, which will not give any control of established infections, or low rates of triazole fungicides that are unlikely to be robust enough to take out latent infections, he suggests.
“The jury is still out on T0s. They can work really well, but equally can be really ineffectual, which is what industry is finding.”