A bacterium feared by farmers is apparently also infecting onions in addition to sugar beets and potatoes, researchers have discovered. They were able to isolate the pathogen’s DNA from onions that had already been harvested and had previously shown unusual signs of rotting. However, it is still unclear how much the bacterium could affect onion cultivation.
Onions are one of the most commonly grown vegetables in the world and in Germany around 15,000 hectares of arable land are dedicated to them. But now farmers in Lampertheim, Hesse, have noticed unusual symptoms on some of their “Red Baron 1” onions. While the onions looked healthy at the time of harvest, they became glassy after four months of storage and then showed signs of rot that could not be attributed to any known storage disease. What had happened?
A bacterial infection as the cause?
The affected farmers’ field is located in a growing area that is severely affected by the so-called “Syndrome Basses Richesses” (SBR). This “syndrome of low sugar content” is a bacterial disease that affects sugar beet and potatoes. Infected vegetables show, for example, yellowing, deformed leaves and necrotic tissue changes. They also contain less sugar than regular crops. Farmers with infected fields therefore regularly experience severe losses in quality and yield.
But could the Hessian onions really be affected by SBR? This would make them the first known case among onions. To find out, researchers led by Eva Therhaag from the Julius Kühn Institute in Dossenheim looked for signs of the SBR-causing bacterium “Candidatus Arsenophonus phytopathogenicus” in the DNA of the affected onions. And indeed: five of the 69 onions examined carried the dreaded proteobacterium. Three of them showed slight to moderate signs of rotting, two still appeared healthy on the outside.
Impact on onion harvest still unclear
“The finding of ‘Ca. A. phytopathogenicus’ in onions is a novelty and could indicate an expansion of the host spectrum of the vector and pathogen in the regional crop rotation,” report Therhaag and her colleagues. However, whether it was really the bacterium that caused the symptoms of the Hessian onions still needs to be investigated more closely. Accordingly, it is currently not possible to estimate how and whether the pathogen could affect the German onion harvest in the future.
It is also still unclear whether the bacteria got into the interior of the plant in the way that is currently known. In the case of sugar beet and potatoes, it is transmitted by the feeding of the reed glasswing leafhopper (Pentastiridius leporinus), which is spreading more and more in this country. If the leafhopper ingests the bacteria while sucking on infected plants, it can transmit them to other plants. The leafhopper cannot be controlled with conventional insecticides and farmers do not yet have any other effective control options available to them.
Source: Julius Kühn Institute, Federal Research Institute for Cultivated Plants; Technical article: Plant Disease Note, doi: 10.1094/PDIS-03-24-0526-PDN