Turf Fungicide longevity using GDD

I get asked this one a lot – How long will my turf 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
  • pH
  • Plant metabolism
  • Temperature

Lesson 2 – The climate is changing

A December now can easily give us the same temperatures a November from the 1990’s.

Prolonged periods of mild temperatures are challenging enough but couple them with long nights and the long periods of leaf moisture and it presents some real problems.

Lesson 3 – 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

We’ve also seen some great plant health benefits from Ryder when used with fungicides through this period of the year.

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.

And although we at Syngenta don’t wholeheartedly support this method we do recognise it is better than the monthly calendar applications we see so often.

I used to steer people towards using 130 GDD with a base temperature of 6c

I’d then cap it at 30 days but with time I’ve recognised the shortfalls of that method.

Most of this blog is taken from one I did a couple of years back which can be found here. Most of it is absolutely sound but I feel now feel GDD can be used better by selecting a base temperature of 0c to understand product longevity.

It was something I discovered whilst really digging into the Primo GDD data and you can read more about that here….

A base temperature of 6c is great for measuring grass growth but not great for product longevity.

So what would I do?

I’d use a figure of 170 GDD at base temperature of 0C to give me an indication of how much cover I was getting from my fungicide, when that was over I’d be evaluating the pressure and the weather and making a decision on my next application then.

As you can see from the above graph GDD is a good way of highlighting the temperature differences and helping to guide periods of fungicide cover.

The greencast GDD tool is great for this, you can set a threshold of 170 GDD at a base temp of 0c and it’ll send you an email when that is getting close to finishing.

Check it out here…..

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.

In Summary:

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.

With winters getting milder we have to be aware the periods of fungicide cover may get shorter.

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