Judging 'appropriate' fungicide dose in cereals

To control input costs and manage fungicide resistance threats, many farmers want to follow lower-input fungicide programmes. To achieve this, it is important to know how to limit the total dose without compromising disease control in wheat and barley crops.

In terms of fungicide resistance and production costs, it makes sense to use the minimum fungicide dose needed for effective disease control.

In theory, limiting the total dose is simple. It can be achieved by:

• Limiting the dose at each treatment
• Limiting the number of treatments

Disease control considerations are far more complex.  

The extent to which disease develops in a crop is a balance between several factors, such as disease pressure and the ability of a crop to resist or avoid infection.

There are several ways to influence this balance, such as by growing disease resistant varieties and changing the approach to rotation, cultivation and drilling.

In seasons with low disease pressures, less-intensive fungicide programmes can be followed.

A good example is in 2020 when a group of winter wheat growers cut fungicide costs without unacceptable hits on yield. In this season, septoria tritici pressures were relatively low.

• The cost of a fungicide application rises as dose increases
• Yield loss is proportional to the amount of disease present
• The appropriate fungicide dose is the point at which margin is maximised
• Doses lower than the appropriate dose reduce profit through inadequate disease control
• Doses higher than the appropriate dose reduce profit because the cost of additional fungicide outweighs the value of the extra yield
• Selection for fungicide resistance is also more likely with increasing dose – resistance management guidance states to use the minimum dose required to effectively control the target disease
• Appropriate dose can be estimated by considering disease risk factors (e.g. weather, varietal resistance, drilling date and previous cropping), grain price, fungicide cost and efficacy
• The appropriate dose varies more with disease pressure and varietal resistance than with grain price

It is also important to understand the relative efficacy of fungicides. 

The AHDB fungicide performance project provides high-quality, independent information on the relative efficacy of fungicides against key diseases in wheat, barley and oilseed rape.

The visible disease present (and yield response) at a range of doses can be illustrated on dose-response curves. Charts are published for major diseases and key products on the fungicide performance page.

Note: Dose information on fungicide labels is usually based on many years of trials in diverse locations.

A well-designed rotation and an appropriate variety will lay a solid foundation and reduce the requirement for chemistry.

On the Recommended Lists for cereals and oilseed (RL), varieties with ratings of 4 or less can be interpreted as susceptible, and varieties with ratings of 8 or 9 as having high levels of resistance.

Yield responses to fungicides can vary as much as 3 t/ha between the most susceptible and most resistant varieties.

Disease-resistant varieties also offer greater flexibility within a spray programme, including the omission of sprays, especially where disease pressure is low.

In wheat, for example, a T0 spray can help insure against a weather-delayed T1 for septoria tritici control.

However, there is considerably less need for this practice in a resistant variety because disease progression is much slower.

At the T1 timing, varieties with a high septoria tritici disease resistance rating are also unlikely to need a succinate dehydrogenase inhibitor (SDHI) fungicide, especially for later-sown crops (i.e. after late October).

Omitting the T0 altogether and the SDHI at T1 not only reduces costs, but also lessens the resistance selection pressure on fungicides.

The Fungicide Resistance Action Group (FRAG) produces guidance on pesticide resistance issues.

Hosted by AHDB, this information can be used to help protect crops and the long-term efficacy of fungicides.