Rhizomania defences show weaknesses

Since rhizomania (root madness) first hit the UK in 1987, plant breeders have worked hard to create varieties that resist the virus causing them to yield well irrespective of its presence.

The first such varieties performed relatively poorly on clean land. But breeders’ progress has been such that about 85% of this season’s crop will be sown with resistant varieties – up from only 51% last year.

Until now the sole source of that resistance, which Broom’s Barn’s Mark Stevens stresses is only partial, has been the so-called Holly gene, also known as Rz1; and the discovery of more aggressive rhizo strains that can overcome it, albeit apparently only in local hot spots, has set breeders and some growers fresh challenges.

One such strain, the P-type (named after the Pithiviers area south of Paris where it is widespread), was first found in the UK in 2001 in Norfolk.

“It cropped up again in in two places, near Norwich and Thetford, in 2007, but we haven’t seen it since,” says Dr Stevens.

More worrying, he suggests, is the AYPR strain first identified near Woodbridge, Suffolk in 2007. “We’ve seen it every year since, and it was confirmed on eight more fields last year including one for the first time in Norfolk.”

A total of 22 fields are now AYPR-infected with root yields slashed by up to 60% and sugar contents down 2% in the affected parts of the fields, he notes.

“Although the current area affected is very small, less than 100ha, I think it could be a lot bigger than that as we rely on people sending in suspicious samples for analysis.”

Ideally, varieties combining resistance to the virus plus the organism that carries it into the crop, the soil-borne Polymyxa betae, should offer the most durable defence, he believes. “But resistance to the vector is a complex trait and there’s nothing on the immediate horizon.”

Several companies are working to develop AYPR-resistant varieties. They include KWS, SESVanderHave and Strube.

Field tests last year showed that a new source of resistance, the C48 gene known as Rz2, in the variety Diana from KWS, gave marked yield improvements over Holly gene material on AYPR-infected land, says the firm’s Simon Witheford.

Dr Stevens confirms that. Grown alongside Bullfinch (see picture), which derives it resistance from Rz1, Diana gave 15-32% more raw root weight and up to 1% extra sugar.

“We sampled on 6 December. We could still see symptoms of rhizomania in the roots, but it’s a step in the right direction.”

Looking further ahead KWS has Isabella, combining both Rz1 and Rz2. “But so far we don’t have any results on how it performs on AYPR-infected soil,” says Mr Witheford.

“Trials in France in particular have shown the double resistant varieties have improved performance under medium to high levels of rhizo infestation irrelevant of the strain of rhizo. There will be work in the UK this season to try to demonstrate this here.”

All viruses evolve and rhizomania is no different, stresses Ian Munnery of Elsoms which trials and markets varieties for SESVanderHave & Strube.

“We must establish whether the AYPR strain is evolution or a more benign type triggered by its environment.”

The affected fields represent less than 1% of the UK sugar beet area, he notes. “And even within these the infections are less than 1% of the area – representing under 0.01% of the national area.

“This is no consolation if you’re affected, nor reason for complacency amongst growers outside that area. Vigilance and breeding solutions are essential.”

Dr Stevens comments that he is aware of two fields that are 60% infected.

Elsoms’ Tandem-tagged varieties harness Rz1 and a proprietary source of Rz2 to deliver a more robust resistance mechanism than either single resistance in isolation, says the firm’s Richard Robinson.

“Data shown at the BBRO winter conferences mirror our results. For the past three years we’ve trialled tandem and RZ2 genetics and the tandem variety Magistral has shown the greatest promise – 30-50% better than other varieties under AYPR.”

Most modern varieties contain the Rz1 or Holly gene providing excellent yields, bolting resistance and tolerance to most UK rhizo strains, notes Mr Robinson.

“By contrast the older, susceptible varieties are not only prone to the disease, but build up the inoculum 700% each year they’re grown, increasing pressure on the Rz1 gene.”


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