Using either triazole or strobilurin fungicides to control powdery mildew in sugar beet can result in 5% yield increases, even in the absence of disease.

But it pays to wait until the canopy is fully expanded before applying a fungicide, otherwise the leaf expansion rate can be reduced, yields suffer and the crop becomes more disease susceptible.

Those are the conclusions from the trials conducted by Broom’s Barn’s Mike Asher, which investigated where the extra yield given by triazoles and strobilurins, but not from sulphur or a specific mildewicide, was coming from.

“We were looking at whether it was the lower leaves making a contribution, or if it was from an increase in photosynthetic efficiency.”

The results showed there was more chlorophyll in the leaves of treated plants, suggesting it was the latter, he says.

But applying the fungicides earlier did not give a greater yield response, even if the greening response was evident.

“We tried spraying at the beginning of June, and greening occurred within one week. But it had an inhibitory effect on the growth of the plant, and yields suffered.”

Where disease pressure is high, increases in yield of 13% are possible.

“Punch C remains the product of choice.

Opera will give very good disease control, but it costs more.”

The rhizomania resistant varieties, with their poor foliar disease ratings, may need a second spray if there is severe disease pressure.

“It will all depend on when the disease comes in.

Trials suggest it’s possible to get a 13-14 adjusted t/ha response from a two spray programme.”

fwarable@rbi.co.uk