TRADE AND MARKETING It’s not all Silver scurf that you see In sensitive varieties, which react strongly to Black dot, you sometimes see an accumulation of mycelium that form black dots that also look a bit like Rhizoctonia. this for sure from close by, Elemans knows from experience. ‘In Black dot, the picture can be quite different, especially in different varieties. Sometimes you only see a small, dark film. In sensitive varieties, which react strongly, you sometimes see an accumulation of mycelium that forms black dots that also look a bit like Rhizoctonia. These infected spots don’t have a fixed pattern. Only under the microscope can you really see whether you’re looking at Black dot with its characteristic treeshaped structures. That picture is quite different in Silver scurf. You then see Only under the microscope can you really see whether you’re looking at Silver scurf with its characteristic tree-shaped structures. silvery spots that are usually centrical in shape and the circles can overlap and form one patch. Silver scurf is also sometimes confused with bronze discolouration. Bronze discolouration is not a disease, but a natural, silvery gleam of the potato skin. The difference between Silver scurf and bronze discolouration can be shown by keeping the tubers in a humid environment for a period of time. Silver scurf then shows its spore formation.’ Start with clean seed ‘And then, Black dot prevention. As Measures against Silver scurf and Black dot summarised Handle and store top planting stock separately Choose non-susceptible varieties Start with clean, disease-free seed Treat tubers preventatively before planting Grow in healthy, disease-free soil Go for an ample crop rotation scheme Ensure good soil structure and drainage Avoid storing host plants Avoid plant stress Avoid tuber damage Store in clean storehouse without old plant or tuber residue Carefully dry seed and table potatoes after storage Grade as soon as possible after storage Avoid condensation Store at a low and constant temperature With Silver scurf, you see silvery spots that are usually centrical in shape and the circles can overlap and form one patch. (Silver scurf and Black dot) (Silver scurf and Black dot) (Silver scurf and Black dot) (Silver scurf and Black dot*) (Black dot) (Black dot) (Black dot) (Black dot) (Black dot) (Silver scurf) (Silver scurf) (Silver scurf) (Silver scurf) (Silver scurf and Black dot**) (Black dot and Silver scurf) Avoid condensation formation on delivery and during transport (Silver scurf) * with a high soil infection, the effect seems small ** if Silver scurf spreads, Black dot infections can also increase with Silver scurf, it’s important to start cultivation with healthy seed. With a strict storage regime, good health and, possibly, a preservative, you can control Silver scurf quite well. Black dot is different. The problem is that Black dot spores can survive in the soil as long as eight years. Weather and climate also play an important role in Black dot, according to Elemans. ‘The biggest chance of infection of the potato tubers is during the period when the plants have died off, followed by a wet period. The spores are then flushed from the stolons and stem to the tubers. That’s where you find the sporophores of Black dot. If the period after haulm killing is dry, you won’t see many problems. This is shown by tests in which tents were placed over the plants after the haulm is killed. The results showed that there was a big difference between the dry and the wet sections of the plot. The influence of the weather was quite clear in France last season. Last year, some fields were quite wet during the ripening and haulm killing periods. This can be seen by many more lots with Black dot. The French haven’t had many problems in other years’, Elemans says. ‘Black dot can also hit a crop long before haulm killing though. This is shown by early haulm dying. It’s not so easy to recognise, partly because not many farmers know that Black dot can be the cause and they point to drought or Verticillium wilt as the culprit. Moreover, the symptoms of the disease can differ a lot. Once you recognise the 22 Potato World 2015 • number 3 Pagina 21
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