The main diseases which reduce yield in wheat are highly weather dependent for their development.

Any control strategy works most effectively when the fungicides are applied prophylactically.

However, fungicide applications made earlier in the crops lifecycle (GS31) to protect it are often made without adequate knowledge of the future weather patterns.

Dr Dawkins says: A question I often used to be asked, ‘why do we see yield increases from certain fungicides when there appears to be no obvious disease? So, I decided to conduct some research to find out what was happening.

Recent field-based studies have shown that the yield of a wheat crop is often increased following fungicide use in the apparent absence of disease.

A ‘tonic effect has been proposed but the mechanism for this has not been fully understood.

A study conducted by Dr Dawkins at the University of Warwick investigated ‘tonic effects on the wheat crop from applying SDHI fungicides to the seed or to the plant at GS31.

The study investigated three of the factors that govern cereal yield; greening of plant tissues, use of water and growth of the plants, following fungicide use in the absence of disease.

Benzovindiflupyr, as in Elatus Plus, applied at GS31 to pot-grown wheat plants grown in the glasshouse resulted in a significant increase in yield (23%) over an untreated control in the absence of disease for the variety KWS Extase.

The increase in yield at maturity was attributable to a higher grain number per ear.

Measurements of greenness, using a SPAD meter, for leaf 4, which was present at the time of application at GS31, indicated that the fungicide preserved the green area of leaf 4 for longer than untreated plants and it is hypothesised this may supply assimilates for an increased duration, allowing maintenance of grain sites at the developing ear.

Water balance studies demonstrated some significant increases in water use, which probably contributed to the yield improvements found in the study, says Dr Dawkins.

Growth analysis of plants at various stages failed to demonstrate any substantial changes in growth or development.

Except where fungicidal SDHI seed treatment sedaxane (as in Vibrance Duo) was used as a seed treatment on the variety Costello, no significant increases in root mass or root length were detected.

Development The study with sedaxane on Costello demonstrated that early delivery of a fungicide at the seedling development stage significantly increased root development and early seedling leaf development compared to an untreated control, which resulted in a significant increase (27%) in grain yield per pot at maturity, compared to an untreated control.

It was shown, in this study, that increases in yield can occur from using SDHI fungicides, in the absence of disease.

The main mechanism of yield increase was through delaying lower leaf canopy senescence and reducing early grain site loss.

According to Frontier agronomist Jeremy Ruff, growers recognise the need for a managed fungicide strategy to control disease in wheat varieties offering high levels of genetic resistance, but are also aware of the yield responses that are often observed in the same varieties but in the absence of any significant disease.

Mr Ruff says: Weve seen yield responses in Frontiers 3DThinking trials in very low disease situations.

These responses require further investigation and explanation.

The work done by Dr Dawkins at Warwick is proving of real value in explaining to growers that if disease levels are low there can be significant responses in terms of yield and how they can arise.

It is interesting that applications of SDHI fungicides, made at GS31, can influence yield through non-disease control-related benefits, such as maintaining green leaf area of the lower canopy for longer.