However, over time, a minor disease strain, resistant to the fungicide, but only present at only very low levels, will build up its population.
Eventually that population becomes the principal strain and we have disease resistance and reduced yield. It takes nine to 10 years for leaf blotch diseases, including septoria, from the introduction of a new mode of action to the first detection of resistance.
Progress of that resistance is very much dependent on how we manage crops and use fungicides.
There are number of factors to slow resistance to fungicides: These include:
- Use only when required. Unnecessary application increases both costs (€ ) and perhaps more importantly resistance pressure. Fungicides provide insurance and given the unreliability of our weather there is a strong temptation to spray "in case...". However, spraying too early can result in a gap in spray cover later in the season necessitating additional fungicide application.
- Use the lowest dose required. Higher rates will clean out all of the susceptible strains leaving a clean leaf for the resistant strains who now do not have to compete for survival.
- Mix different modes of action - two way mixes of triazole + Strob or SDHI, three way mixes of triazole + strob +SDHI.
- Do not mix two triazoles for septoria control except at T3. Mixtures of two triazoles result in a long exposure period to individual modes of action when one or both of them are applied later. This increases risk of resistance.
- Rotate different modes of action. The longer a disease organism is exposed to a particular mode of action the greater the risk of resistance. Hence the recommendation that you do not use a triazole at T0 in wheat. The triazole component of SDHI mixes should be alternated between Group 1 epoxiconazole & prothioconazole and group 2 products containing metconazole & tebuconazole at T1 and T2. For early rust control, if necessary, in wheat use products containing morpholine and strob.
- Multisite fungicides provide protection to partner products. Use Chlorothalonil at T0, T1 and T2 in wheat.
- Timing of application is the most critical factor. Apply T1 to wheat between 3rd leaf emerged and fully emerged. I expect that early-sown crops will be fit for T1 this week but that most crops will not be at leaf 3 until next week. Like most things in farming, the calender is no replacement for inspection and in-field analysis.
Winter barley has generally remained very clean and while mildew is present in most crops it has not moved on to new foliage. The first critical spray timing is now - g.s. 31 - 33. The fungicide will consist of a triazole plus an SDHI or strob and +/- a morpholine. Bontima/Cebara provides the option of not using a triazole. Combine fungicide use with a growth regulator where appropriate.
Mildew is present in most crops of winter oats where a morpholine has not been used. Talius is an excellent product to prevent mildew but has no knockdown effect. The T2 may consist of Cielex +/- morpholine, Elatus Era, Jenton, Tocata.
Finally, read the labels carefully and adhere by buffer zones. Keep records of all pesticide usage, products, application rate and date.
For those of you with smart phones, a photograph of the field followed by a photograph of the pesticide containers could well be the easiest way to keep records until such time as you can sit down to paperwork.
Keep safety a priority - use appropriate clothing, gloves, masks etc. Make sure that children do not touch your protective clothing until such time as it is washed after use.
PJ Phelan is a tillage advisor based in Tipperary and is a member of the ACA and ITCA