I get asked this one a lot - How long will my fungicide last?
When you apply a fungicide there is a number of things going on:
- Reducing spore population around the plant
- Reducing spore population on the plant
- Defending and protecting the plant on the outside from new pathogen
- Getting into the plant and killing the pathogen
- Getting into the plant and defending from new infection
How much of each of the above is happening, and how long it goes on happening, doing depends on:
- Timing of application / stage of disease
- Active ingredient applied
- Disease pressure
- Growth conditions
In its simplest form, experience indicates you can rest easy and expect the fungicide to perform for:
- August - 10 days
- September - 14 days
- October - 21 days
- November - 30 days
After that point it's time think carefully about what's next. Hopefully that's been enough to get you through the pressure period and you've moved into a better window with some cool temperatures, high winds or low humidity.
However if you're still in those horrible humid conditions that you know result in Microdochium pressure on your site, you'll know it's time for action. Furthermore, these figures will vary a bit up and down the country as they are so climate related, so maybe we need to look at something better.
Lesson 1 - Fungicides do not necessarily last for 30 days
The effectiveness of a fungicide will reduce over time and the speed of that breakdown is related to:
- Exposure to light
- Plant absorption
- Microbial activity
- Plant metabolism
Lesson 2 - How can I make it last longer?
Fungicide longevity can be improved by:
- Better applications
- Reducing drift
- Choosing the correct water volume
- Selecting the right nozzles
- Ensuring good plant health
- Preventative applications rather than curative
- Good management afterwards
So how do I time my next application?
Growing Degree Days for decision support
There is a growing trend towards using GDD to help guide you with your fungicide timings.
I know several people who are using 130 GDD to guide their applications. It's a much better system than a monthly calendar approach, but it's only a guide and you have to be aware of the limitations.
The challenge with using GDD is that according to the results calculated above applications made at the end of September in some parts of the country would take around 2 months to reach 130 GDD, and by early November that could be 3 months! Yorkshire in November can be a very high period of disease pressure and to expect 3 months control from an application is just crazy.
GDD is a great tool to work out anything temperature related, but with disease pressure and fungicide longevity temperature is only a part of the problem. There are many other factors to take into account:
- Damage (golfers, maintenance, etc)
- Periods of leaf wetness
None of these factors are taken into account in a simple GDD model.
However if you wish to use a GDD model (and I do think this is better than a calendar) then cap it at 30 days.
If we do that, you can see in the above chart how someone in a warmer area such as Poole on the South Coast really doesn't typically get to monthly applications until Mid October, whereas someone in Harrogate may get there around the end of September.
GDD lessons learned - but does it tell us more?
The GDD method does give us some insights into how long a fungicide will protect the plant.
During the earlier part of the microdochium outbreak season, when GDD-based intervals are typically less, longevity can be shorter:
- We are mowing lots
- Temperature is high
- We have lots of light
- Microbial activity is high
As the season, and GDD intervals extend, we see factors where fungicide longevity extends correspondingly:
- Mowing reduces
- Temperature reduces
- Light reduces
- Microbial activity reduces
Part of the success of this GDD strategy is the compressed applications early in the season when pressure is highest.
By getting control of the disease population early we allow the later applications to be widened as the climatic conditions become less suitable for microdochium. The disease population would have been kept low thanks to the work done to reduce the challenge earlier in the season.
When this is achieved then good ITM strategies can be enough to keep the disease presence minimal. But that's assuming the weather plays ball.
Get control early in the season. But take your breaks when they come - keep an eye on the weather and if you can stretch the applications out when disease pressure is low then do so.
That's the skill of greenkeeping - know your site and the pressure you are under.
If you can keep pressure disease at bay until early October, then you can move into some decent periods of longevity.