24 May 2002

Bad mildew alert well worth heeding

As much as 64% of the

sugar beet crop is

expected to be hit by

mildew by the end of

August, according to this

years forecast from IACR-

Brooms Barn

THE mildew forecast, based on the number of ground frosts in February and March, indicates a very early and severe mildew epidemic, warns Dr Mike Asher of IACR-Brooms Barn.

"It means that crops in all regions, except possibly the far north, are likely to respond to treatment, with big benefits expected to be seen in many areas," he says.

His advice is to inspect crops weekly from mid-July, using the full recommended rate of fungicide applied within 7-10 days of finding the first mildew infected plants.

"Treatment up to early September will still be worthwhile, with the possible exception of the earliest lifted crops."

Fungicide trials over the past three years have shown that even in low disease years there is still an advantage from using triazoles, he says. "That was very apparent last year. Survey statistics show that only 25% of the crop area was infected with mildew by the end of August, and in trials very little disease was recorded on untreated plots. In previous years, colonisation by mildew had reached 50-60% of the leaf area by the same time."

Despite this, all treatments tested in 2001 gave healthy margins over product cost. Punch C (flusilazole + carbendazim), Alto 240 EC (cyproconazole) and Thiovit (sulphur) were applied once, at recommended rates, in late July/early August.

"During the disease assessments, it was apparent that there had been little mildew development in August and September," says Dr Asher. "So responses were less than in previous years, but still averaged 4t/ha, 3.3t/ha and 2.9t/ha from Punch C, Alto and Thiovit, respectively."

As in previous years, Punch C gave the most cost-effective return. "These trials are further proof that even in low mildew years and at C prices, significant returns can be achieved, especially from the triazoles.

"In high disease years, such as 1999 and 2000, Punch C gave about twice these returns," he says.

Soil-borne diseases were favoured by last years late drilling and persistent wet conditions, reports Dr Asher.

"There were reports of seedling losses from Aphanomyces blackleg. But it was largely controlled by tachigaren in the seed pellet."

Deformed roots were seen on plants which had survived the initial attack. "In some fields, these occurred in quite large patches of the crop."

Rhizoctonia was the other cause of patches of poor growth. "This disease mainly manifests itself as stunted plants with fangy roots and a proliferation of laterals. At present, there is no known control measure for the disease."

A record number of rhizomania contaminated fields were detected last year, bringing the total area up to 7323ha (18,095 acres).

"But despite the unprecedented number of new farms affected in 2001, the cumulative area is still only about 1% of our total beet growing land," notes Dr Asher. "Most of the outbreaks were on light sandy soils in East Anglia."

Most crops were drilled a month later than normal last year, into soils that were already warm. "So rhizomania was predicted to show up more widely. And the aerial survey by DEFRA in August was helped by crops remaining green."

Future management of the disease is under discussion, as the UKs Protected Zone Status for rhizomania lapsed in March 2002. "So the disease is no longer under statutory control." &#42


Cost(£) Ave yield increase Margin (£/ha) Margin (£/ha)

(adj t/ha) (A/B quota) (C quota)

Punch 16.0 4.0 96 32

Alto 13.5 3.3 79 26

Sulphur 6.0 2.9 75 29

The longer diseases can be kept out of the crop the sweeter the final outcome. Mildew is set to be the main target this season.

1Mildew forecast 64% of crop area will be affected by end of August.

2Action Inspect crops weekly from mid-July onwards.

3Treatment Apply full fungicide rates 7-10 days after finding infected plants.

4Fungicide trials Significant returns from fungicides, especially triazoles, even at C prices.

5 Punch C Gave highest returns in 1999, 2000 and 2001 trials.

6Root diseases Favoured by last years late drilling and wet conditions.

7Aphanomyces blackleg Caused seedling loss and deformed roots where not controlled.

8Rhizoctonia Symptoms are stunted plants with fangy roots and many laterals. No control measures.

9Rhizomania Record number of contaminated fields detected last year.

10Future policy Under discussion now that rhizomania not under statutory control.

Fortress benefits

Brooms Barn trialled the fungicide quinoxyfen, newly approved as Fortress for use in sugar beet to control powdery mildew, in 1998 and 1999, notes Dr Asher.

"Unfortunately there wasnt a lot of mildew in those trials in either season. We know quinoxyfen is mildew specific and it produced a yield equivalent to other fungicides.

"What we dont know is whether it provides the same physiological benefits as triazoles into late autumn."