Plan your fungicide strategy

That was one of the fundamental principles put across to growers last week by the Foundation of Arable Research during one of three Focus on Fungicides training courses.

The half-day sessions, run in Timaru, Rolleston and Palmerston North, took growers on a step-by-step journey through cereal crop development and yield, disease development, fungicide strategies, applications, modes of action, and resistance management.

“The first thing to point out is fungicides do not create yield,” stressed research coordinator Nick Poole to the Timaru workshop. “They only protect something you will inherently get from your crop depending on the water and nutrients available to it.”

That said, as Poole later pointed out, some fungicides, notably the azoles such as Opus (epoxiconazole) or Proline (prothiaconazole), do have some curative or “kick-back” action, in that they are able to knock-out disease already present in the plant.

But such action is limited and most yield response from fungicide treatments comes from protecting leaves from further infection.

“There is no substitute for applying fungicide to the leaf you wish to protect,” stressed Poole.

Doing that requires an understanding of crop development, and workshop delegates got to cut up wheat and barley plants to determine which leaves were emerging, and how that correlates with growth stage (GS).

As a rule in wheat leaf 4 emerges at GS 30-31; leaf 3 at GS32; leaf 2 with GS33, and flag leaf with GS39. “But this doesn’t work for very late sown, rapidly developing crops,” warned Poole.

“Leaves and nodes of these tend to get out of synch. In a June-July sown crop at GS32 you’ll find you’ve got leaf two coming out. That needs care.”

To be sure, leaves should be dissected back to the embryo ear to identify exactly which is the most recently emerged: no easy task as with leaf three emerging, the flag leaf is just a vestigial flake and even leaf 2 is still little more than a centimetre long.

In wheats, flag leaf contributes most to yield, consequently it’s the most important leaf to protect. However, how much it contributes to yield depends on crop situation, affecting the weighting of fungicide applications.

“The point is it’s not fixed in stone,” stressed Poole. “When you think about your fungicide strategies you need to think about what leaves you are trying to put that fungicide on.”

Knowing wheat leaves typically take 110-120 day degrees* to emerge, whereas barleys come out in 95-100 day degrees, will help anticipate application timings. Consequently barleys have a shorter gap between the start of stem elongation and ear emergence.

“This is why we traditionally talk about growth stage 30-31 for barley T1s whereas we talk about growth stage 31-32 for wheat.”

Another key factor in the fungicide strategy is a cultivar’s disease resistance ratings, and the likely pathogens you are trying to prevent. Then there’s the persistence of the product to consider, and whether one higher rate, or two or even three split lower doses would work better.

When it comes to multiple doses the long-term efficacy of products must be considered too: more than two applications of a strobilurin or SDHI product in the same season is considered too many due to the risk of promoting fungicide resistant strains of disease. 

* Sum of max plus min temp each day, divided by two.