The difficulty in controlling cereal pathogens should not be underestimated. These organisms are complex and found ways to survive in the wild long before we decided we didn't want them in our cultivated fields.
Part of any pathogen's survival mechanisms is to change in response to an outside influence that stops it developing and reproducing. These outside influences could be cold, wet conditions, change of cultivar or the application of a fungicide.
Fungicides, once discovered and introduced, have a defined life of practical use before the pathogen develops ways around the fungicide's mode of action. The usefulness of any fungicide is dependent on how often it is used and how it controls the pathogen, ie a fungicide relying on one mode of action will have a shorter life than one which controls the pathogen in several different ways (Strobilurins versus Chlorothalonil).
As an example, if a fungicide had a life cycle of, say, 40 applications, then if it was applied on average three times a year, its useful life in the field would be around 13-14 years.
Septoria continues as a moving target and it appears the pathogen has shifted again this year. Oak Park duo Dr Stephen Kildea and Dr Eugene O'Sullivan have been monitoring changes in septoria populations for the past few years and they were among the first scientists to spot changes and identify trends in septoria development.
The genetic trends that occurred over this period established that septoria had developed mechanisms to tolerate more fungicide (before, the fungicide controlled them) and these strains have been classed as mutations (S524T, V136A, Y461S).
The mutations make these septoria strains harder to kill (or insensitive) by prothioconazole and epoxiconazole, therefore both these fungicides are grouped into Group 1-type triazoles. However, these septoria strains are easily controlled with both tebuconazole and metconazole (Group 2).
The development of these mutations have not, so far, resulted in poor disease control but the most likely evidence growers will see in the field is reduced persistence, ie fungicides not controlling the disease for as long as it normally did.
Septoria populations monitoring last year have shown that about half were of the insensitive type (or less easily killed by fungicide). Therefore, these populations are in every field and each grower must apply best practice to minimise the impact of this septoria. In a high disease year, a grower who only uses a one fungicide type (Proline, Opus, Rubric, etc) is highly likely to lose yield in comparison to using alternative groups or adding SDHI-based chemistry to the mix.
Last year turned out to be a low disease year, thus fungicides were not put under pressure and, in general, fungicides worked well.
The Teagasc advice for controlling septoria and managing insensitive strains in winter wheat is outlined in table 4 (right). The strategy is based on keeping populations of insensitive septoria to a minimum through the season.
Additional actives may be needed for other diseases.
Triazole grouping is based on sensitivity to various Septoria strains. SDHI: Boscalid (Venture Extra), Bixafen (Aviator), Isopyrazam