Fight against pathogens

Breeders at KWS are successfully working on the development of Cercospora-tolerant sugarbeet varieties that offer high yields

The spots are just millimeters in size, light gray and have reddish-brown margins – these are all the initial signs of an infection. At first, it only appears on the leaves, but eventually moves on to the leafstalks. In the case of severe infection, it spreads all over the plant and completely destroys the entire leaf.

The leaf disease Cercospora beticola is one of the world’s most harmful diseases for sugarbeets: The fungus can lead to yield losses of up to 50 percent1. Around two thirds of the world’s cultivated areas are facing moderate to severe incidences of the disease.

Using traditional breeding techniques, KWS has now succeeded in bringing sugarbeet varieties with improved Cercospora tolerance onto the market. Compared to other Cercospora-tolerant varieties available until now, they also exhibit significantly higher yields – both on areas that are free of the disease and on acreage that has already been infected.

The new Cercospora-tolerant varieties are already being cultivated in the United States, Italy, Spain as well as Austria and will soon be available in other markets. Moreover, the German Plant Variety Office recently approved two CR+ varieties for Germany.

The new Cercospora-tolerant varieties offer increased protection throughout the entire vegetation period. This could potentially reduce the use of fungicides and is therefore an important factor for achieving more sustainable farming.

Cercospora: climate change facilitates the spread

Optimal conditions for the infection and spread of Cercospora are high humidity of over 95 percent and temperatures between 25 and 30 degrees Celsius. The leaf spot disease usually appears at the beginning of July, when there is a higher amount of crop moisture, and the leaves come into contact with each other.

Leaves already infected with Cercospora initially only exhibit a few small spots. These increase in terms of the size and amount, until they cover nearly the entire leaf surface, causing the leaves to wither and die over time. The beet can then form new leaves; however, these, too, become infected and also die off.

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    Werner Beyer, Head of Resistance Breeding, inspects young sugarbeet plants that have recently been pricked out. This step is important to enable the plants an optimal growth.

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    As part of a series of tests in the greenhouse, Werner Beyer and the technical assistant Britta Ahlswede are analyzing Cercospora infection of sugarbeet plants.

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    Werner Beyer, Britta Ahlswede und Harald Keunecke, from phytopathology, during the inspection of the test series.

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    The infestation with the leaf disease Cercospora is clearly visible on the leaves.

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    In the laboratory, Britta Ahlswede und Harald Keunecke are closely examining the Cercospora fungus and other pathogens.

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    The pathogens in the Petri dish can be clearly differentiated visually as well. Here you can see Cercospora-isolates are being propagated on petridishes in the lab for greenhouse testing.

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    Sugarbeet plants are not only susceptible to fungal leaf diseases, such as Cercospora, but also to diverse yellowing viruses and other pathogens.

Climate change is contributing to both the spread and increasingly severe manifestation of Cercospora. Mild winters and warm and humid summers promote the infection, due to rising temperatures this has been migrating to the north, and occurring more severely and frequently in classic Cercospora areas.

Cercospora can survive on the vestiges of infected leaves for several years, thus enabling it to survive in the ground until the next sugarbeet season. Another source of infection are wild plants that are afflicted, given the right humid and wet weather conditions, their spores can be transmitted by rain, wind or insects to plants located on the field. The spores germinate on the leaf surfaces and invade plant cells by way of the stomata.

1 Wolf, P. F. J., Kraft, R., and Verreet, J. A. (1998). Schadrelevanz von Cercospora beticola (Sacc.) in Zuckerrüben als Grundlage einer Verlustprognose. (Relevance of damage caused by Cercospora beticola (Sacc.) in sugarbeets as the basis for a loss forecast.) J. Plant Dis. Prot. 105, 462–474.

What about weather extremes?

The next sowing is due in the coming weeks. "We will have to deal with more extreme weather conditions," says farmer Flögel. But sugar beet has great potential, he said, because "it can cope with warmer temperatures". He will therefore continue to cultivate it. Asked about country lore, Flögel says: "It's a leap year in 2020. And it's actually "leap year, cold year." Sugar beet does not tolerate cold well. It remains to be seen whether country lore holds true in the 21st century. One way or another: Flögel is well prepared – and well advised.

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