Protect yield in new sugar beet varieties by controlling mildew

UK - July 12, 2006

Mildew control in the new high-yielding rhizomania-resistant sugar beet varieties is essential to protect yield and gross margin. 

The new rhizomania-resistant varieties make up some 25% of this year’s crop.  But poor resistance to mildew and late season rust threatens the high yield potential.  Mildew resistance ratings as low as 2 mean crops will be at risk from mid-July onwards unless growers protect against the disease.  

“Growers could see yield losses of up to 15 t/ha because mildew infections could hit 50-60% of the crop,” says Brooms Barn’s Dr Mike Asher. “With some varieties also susceptible to late infections of rust and ramularia, last year’s fungicide trials conclusively showed that a two spray fungicide programme was the best option to protect yield.” 

Dr Asher is confident that growers will see high returns on their fungicide investment on the new varieties that have potential for yields of 70 t/ha or more.  For long-lasting mildew control, Fortress (quinoxyfen) at 0.2 l/ha came top in fungicide trials at Brooms Barn last year.  It controlled mildew for over six weeks, from the third week in July through to mid-September. 

“Untreated plots showed 35% mildew infection by late August.  Despite all products controlling mildew until the end of the August, by mid-September all treatments apart from Fortress showed similar infection levels to those on the untreated plots.  Fortress was still showing up to 80% control,” says Dr Asher.  He advises growers to use Fortress in a tank-mix with either cyproconazole or carbendazim + flusilazole in fungicide programmes from mid-July on mildew susceptible varieties.   

The tank-mix gave the highest yield response and good returns as the mildew protection of Fortress was enhanced by the yield boosting physiological properties of triazoles or strobilurins. 

Dr Asher recommends a half rate triazole to take advantage of the physiological benefits, but a full rate Fortress is needed for the first application to protect against mildew until mid-September. A second fungicide application in mid-September may be justified on the latest lifted crops of susceptible varieties, using a half rate Fortress to prevent the late season development of mildew, plus a half rate cyproconazole to prevent late infections of rust or ramularia. 

“A two spray fungicide programme with a late application of cyproconazole for mildew and rust gave an additional 5 t/ha (adjusted yield) over a single spray programme.  This was clear evidence that, with a very susceptible variety and early mildew infection, a two spray programme may be cost effective for all but the earliest lifted crops,” says Dr Asher. 

Patrick Jarvis, British Sugar’s technical development manager advises that yield is king, but to achieve yield potential growers have to be prepared to apply a fungicide at least once a season, irrespective of variety. 

He recommends growers spray Fortress at half rate to prevent mildew development plus half rate cyproconazole by mid to late July before the cereal harvest.  “Be aware of harvest interval restrictions if using a second fungicide in mid September.  Fortress plus cyproconazole has a short four week harvest interval combination,” says Mr Jarvis.