RE SEARCH Bacteria project completed? project group’s research, no increased infection of seed was found after haulm pulling of plants infected with Erwinia. Less puddling was caused by haulm pulling that was spray killed, however, which means that pulling spray-killed haulm results in less risk of spreading than pulling green haulm. One hundred percent proof was not yet found for this assertion, according to the researchers, and that’s why they recommend studying further the role of haulm killing through puddling and looking into the various ways of haulm destruction. (photo 004) Mechanical lifting causes puddling (photo 005) Another study carried out by the project group has shown that, when lifting with mechanical means, infected tubers are puddled over a lot for more than 10 metres. When a potato digger is used and the potatoes are then manually lifted, no puddling is found at all. This information is being considered as well-known in the field. According to Velvis, some of the issues that were examined during the Erwinia study have been mentioned time and again by growers, but these allegations have never before been corroborated. This also applies to the puddling during lifting, says Velvis. He thinks it a big advantage of this study that there is now proof of the extent of puddling during lifting. ‘As far as I know, this has never been studied before in this particular way.’ In the same vein, the length of time the Erwinia bacteria can survive in puddled tuber waste was studied. According to trial results, this can be at least as long as one month, even after drying, and that is longer than was known so far. The project group is now recommending a further study into the question of how puddling can be minimised. The project group also recommended finding methods to deactivate the Erwinia in the lifted contaminated tubers and tuber waste as quickly as possible after lifting. For example, by accelerated desiccation or a decontamination method. Drying better than disinfection (figure 02) The project group has already checked some of the disinfection methods. Researcher Henk Velvis and trainee Gijs Gunnewijk from HZPC-R&D have studied this subject extensively. They studied five standard disinfectants plus Plunoran, which contains copper oxide. They also looked at the effect of ozone, electrolysed water and UV on external tuber infection. They checked out whether these factors had a growth inhibiting or killing action on Erwinia chrysanthemi (stem wet rot and aerial stem rot). They made a few extraordinary discoveries, starting with the standard disinfectants. A few of them were able to inhibit the growth of Erwinia chrysanthemi starting with one/fifth to one/tenth of the recommended dose. Maximum killing was only successful when the disinfectants were brought into immediate contact with Erwinia. This was only studied for Halamid and Jet5, which – of all studied products – gave the strongest growth inhibition. If the Erwinia had the form of thick paste, a disinfectant spray did not have any effect. In such cases, drying was Haulm pulling appears to encourage the pulling apart and disappearance of the mother tubers. Infected tubers can puddle a lot over more than 10 metres during mechanical lifting. 20 Potato World 2010 • number 1 Pagina 19
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