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June 2024

A recent press release of Wageningen University mentioned important findings on the enigmatic potato bacterial disease. In the Netherlands blackleg and bacterial wet rot in seed pota­toes and seed flower bulbs only are responsible for several tens of millions Euros of damage. Chemical control is not available nor do resistant varieties exist.

Control takes place through samplin­g and assessing and certification of potato and bulbs and through the introduction of hygienic crop management practices that reduce the introduction and spread of the causal agent. The use of bacteria free minitubers and avoidance of wounding of tubers and assuring adequate drainage is part of such strategy. Dr Robert Czaikowski at Wageningen University discovered how the Dickeya (one of the previously called Erwinia) bacteria invade and then spread in the potato plant. The figure shows roots colonized by fluorescing Dickeya cells. Czaikowski showed how the bacterium can enter the roots of the plants and subsequently spreads through the vascular sys­tem to the stems, stolons and daughter tubers. If above ground parts of the tubers are infected daughter tubers can also be infested through the same system. The bulk of the bacteria are therefore found at the heel end of the tuber which is useful information for sampling purpose. Treating infected tubers to reduce the number of bacteria in the tubers is not really effec­tive. They may consist of radiation, hot air, hot water treat­­ments or baths of antibiotic and may reduce superficial bacteri­­al presence but will not affect the bulk of the bacteria inside the tuber tissue. Alas so far no bactericides are available that would kill the bacteria inside tubers. Endophytes are bacteria of fungi that live in plants but apparently do not cause any harm. Researchers have detected that the presence of some of such endophytes reduce the severity of certain bacterial of fun­gal disease. This may be because an endophyte occupies a part of the tissue of the plant and as such acts as a barrier and fends off the disease or – more interesting – the endophyte produces an antibiotic and eliminates the disease.

Czaikowski and colleagues isolated several bacteria species in rotting potato tubers and found that a species in particular was effective: isolate A30 with the scientific name Serratia plymuthica. The bacterium enters the plant the same way as Dickeya does through the roots. In greenhouse experiments it was found that A30 reduced blackleg development by hundred percent in the tubers and by 97 % in stems. The researchers still speculate on the responsible mechanisms of this reduction: competition for nutrients, induction of plant resistance to the black leg bacterium, the production of antibiotics and of siderophores. Antibiotic are poisoning bacteria and kill them and siderophores are a chemical compound that sequester iron ions and make them unavailable for bacterial growth – includ­ing blackleg bacteria. Treating black leg infected seed tubers with a solution of A30 strongly reduced the number of infected plants grown from them: from 55 % plants with black leg symptoms to 0 % both found in plants grown in compost or in field soil. The higher the density of A30 the more effective the strain is. Further study is needed to reduce the amount of the antagonist through formulations that make it stable, to opti­mize application procedures and longevity in the soil. It is also important to find out more about the mechanism as to be able to use the principle in other disease-host crop situations. The institute Plant Research International of Wageningen University and research Centre patented the A30 strain of Serratia plym­uthica and is looking for possibilities to market the product.

Anton Haverkort


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