TRADE AND MARKETING It’s not all Silver scurf that you see Silver scurf has had a lot of bad press recently as a problem disease. However, it’s not always Silver scurf that you see. ‘Diseases that are attributed to Silver scurf are often cases of Black dot’, points out Maries Elemans during his work as field staff member of the Dutch Potato Organisation (NAO). With Black dot, the infected spots don’t have a fixed pattern. The disease patterns can vary a great deal, especially between varieties. ‘W elcome to the complex world of Silver scurf (Helminthosporum solani, ed.) and Black dot (Colletotrichum coccodes, ed.)’, Elemans begins with a big smile. That smile disappears, though, when he starts explaining the seriousness of the problem. ‘As a result of its soil-bound nature, Black dot is a more complex problem than Silver scurf, which is currently drawing all the attention. Especially when you think that examiners often identify an infection as Silver scurf while it isn’t that at all, but Black dot. Even in pamphlets and media communications, you see the wrong pictures for Silver scurf. It’s usually Black dot or a combination of dot and Silver scurf. You know the difference for certain when you put them under the microscope. In Silver scurf you can see the black sporophores with their typical bifurcation. Cosmetic tuber problem ‘There’s a great deal of literature available about both diseases, which can also be consulted in libraries and on the internet’, Elemans continues. ‘A big difference between Silver scurf and Black dot is that Silver scurf is mainly a question of quality. Besides cosmetic tuber problems, Black dot also causes plant disorders and consequently yield losses. So, Silver scurf only relates to the tuber and Black dot to plant and tuber. The transmission of the Silver scurf fungus takes place primarily via the tubers. It travels via spores in the soil from the mother tuber to the daughter tuber. Spores are hardly ever found in the soil naturally. Research, among others by the Louis Bolk Institute, has shown that 99 percent of Silver scurf spores are broken down in the soil. So, there’s hardly any chance of infection from the soil in subsequent crops. Tackling Silver scurf in the storehouse It is possible to control Silver scurf in storehouses though, Elemans continues his lecture. ‘Silver scurf can spread rapidly in storehouses. A strict storage regime and farm hygiene are therefore important weapons against fungi. Old tubers and plant residue may contain Silver scurf spores and should not be in the store unit when the new harvest comes in. The units must always be as clean as a whistle. One infected potato tuber in the storehouse can explode into millions of new spores. With ventilation you can infect an entire, healthy lot in no time. A diseased and/or old tuber in the unit can cause more problems than a bit of contamination in the harvested daughter tubers. For example, the risk of transmitting Silver scurf in the field from mother to daughter is many times smaller than the transmission of spores from infected tubers during storage and processing. There’s much more potential danger for infection in the storehouse. It’s the ventilation that encourages rapid dissemination. So, before storage, mother tubers should be removed from the seed and table potatoes as much as possible. Furthermore, you need to keep the temperature in the storehouse as stable as possible. This means that temperature fluctuations in the store units and the tubers should be avoided. Extra moisture means new opportunities for Silver scurf to develop and form new spores. Careful drying after storing also helps limit the spread of fungi.’ Microscope shows Black dot From a distance, most gleaming spots on a skin can already easily be identified as Silver scurf. But you only know Potato World 2015 • number 3 21 Pagina 20
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