Resistance breeding for sugarbeets always needs a new ace up its sleeve.

Rhizomania test in the greenhouse: Britta Ahlswede and Werner Beyer test breeding lines in rhizomania-infected soil.

A paradise for harmful organisms

The reason: Sugarbeet is a real paradise for harmful organisms: At all developmental stages, from sowing to harvest, the sugar-rich root offers plentiful food for fungi, viruses, nematodes and insects. Resistance breeders therefore have their hands full protecting seeds and the resulting plants from infestation. Finally, one of the primary goals of KWS is to provide the best high-yield seeds for farmers. The plants should remain healthy and resistant over the entire period of growth—usually 180 to 220 days. “Plant protection, for example in the form of pesticides applied via pill coating, and the natural resistance of the plants, complement each other in an ideal way,” explained Beyer.

Strong against pathogens

The main task of resistance breeding is to equip future KWS varieties for the respective cultivation regions with the most important resistance genes. The varieties should be able to protect themselves, as far as possible, against pathogens or pests. Then less pesticides need to be used. Resistance breeding is particularly important in those diseases for which no chemical or biological plant protection measures can be taken.

Such a disease—possibly the most important disease of the sugarbeet worldwide—is the virulent beet necrotic yellow vein virus (rhizomania). “Beet necrotic yellow vein virus” (BNYVV) nests in young seedling plants. The aim of the virus is not to kill the beet. Rather, the pathogen wants to multiply as a parasite inside it. It was resistance breeding that made it possible for sugarbeet to be cultivated under heavy infestation—for example, in Italy or southern Germany, but also in California and many other regions. Currently, rhizomania-resistant varieties are needed on a large area of the global acreage.

The evolution of the virus is progressing

Today, most sugarbeet varieties carry resistance genes that protect plants against a virus attack and allow only a small amount of virus reproduction. Resistance to rhizomania is one of the basic features of every variety of KWS sugarbeet. But the evolution of viruses is progressing naturally. They can change their properties through mutation and then attack the previously resistant beets again. This is called overcoming resistance, or resistance breaks. In fact, virus strains that can overcome Rz1 (Holly)-resistance of sugarbeets and multiply in supposedly resistant plants have been known for some years now. “Fields in the Netherlands in particular have been affected,” explained Beyer.

The breeding goal: to be faster than the pathogens

However, resistance breeders at KWS already had an alternative when the Rz1 gene was overcome for the first time: Since about the year 2000, further resistance from wild beets Beta maritima has been gradually crossed into the KWS gene pool. In some markets, this is marketed in combination with Rz1 under the name “RZ2.0” and offers particularly good protection against heavy rhizomania infestation.

“It is important that breeders do not attempt to conjure up an alternative resistance out of a hat until a resistance is already broken, but are prepared for such a situation as early as possible. It can take ten years or longer until a new source of resistance has been crossed into marketable varieties,” said Beyer. Breeders are therefore looking for alternative sources of resistance to protect existing defense sources. All in all, it helps that KWS, which has been family-run for more than 160 years, is financially independent and able to think and act in periods of decades. Therefore, in the next few decades, farmers can expect well-adapted, resistant varieties.

Breeding under time pressure

However, it is often only a matter of time until new resistance is also overcome. Some resistance, such as the Rz1 resistance gene against rhizomania, has held firm for many years and the spread of resistant strains of the virus is very slow. In fungi, which often multiply very fast and strong and produce several generations of offspring per year, the process can also be very fast and often take only a few years. The time factor also plays a significant role in competition with other breeding companies. The faster and more efficient KWS can cross a new resistance, the more farmers can benefit from new, innovative products.

Where can new sources of resistance be found?

For many diseases, there are well-known sources of resistance. In addition, breeders often test wild beets from gene banks looking for new resistance genes. Such a process sometimes takes many years to genetically fix a new resistance and analyze its inheritance. Sometimes resistance genes are found in the classic base material, such as resistance to powdery mildew, beet rust and many other harmful fungi.

Above all, breeding is always teamwork: The interaction of several breeding programs and the combination of different resistances make up the varieties that come onto the market. All of this makes it necessary for KWS experts to always be on the lookout for new trial fields with heavy infestation. The leases for the experimental fields around Pithiviers will probably have to be extended several times.

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