TRADE AND MARKETING It’s not all Silver scurf that you see Old tubers and plant residue may contain Silver scurf spores and should not be in the store when the new harvest comes in. disease, it’s not difficult to confirm Black dot. Plant also infected by Black dot In order to know how to treat Black dot, it’s important to know how the fungus behaves. Elemans: ‘The first development is comparable to that of Rhizoctonia. When Black dot grows on the mother tuber or in the soil, it develops in the plant during the growing season by around 1 mm a day. The moment the plant starts ripening, the fungus creates sclerotia on dead plant parts. Especially at the bottom of the stem and in the roots, sclerotia develop into miniscule fungi. This process is fastest in a humid climate, and it produces a great many spores. If it starts raining, they’ll be flushed towards the tubers. With spores in the soil and a lot of moisture, infection can also start in the tubers. It’s important to know that the potato haulm can also carry Black dot spores. So, root residues are a source of infection, from waste heaps or haulm residue in the field. When the fungi have seriously infected the roots, the plant wilts quickly. You see this more in sensitive than in insusceptible varieties. Very few diagnoses of early wilting point to Black dot, yet it could well be the most important cause of potatoes dying off early.’ Healthy soil is the most important weapon An important weapon against Black dot is healthy soil, establishes Elemans. ‘This can be achieved by ensuring An important weapon against Black dot is healthy soil. healthy soil biodiversity, a good soil structure and organically balanced soil. If the soil life is balanced, there won’t be many Black dot problems; at least this is what the results of various studies have shown. What you can prevent, though, is Black dot developing further on the tuber after infection. Careful storage management is then very important. Black dot goes into storage together with the tubers. So it’s important to keep the temperature during storage as low as possible and avoid loss of moisture as much as possible. Also important is the underwater weight. Tubers with a low starch content show more damage than those with a high content. The starch content also depends on the variety; one variety is more sensitive than another.’ Keep generations apart Elemans also has recommendations for seed potato growers with high-quality planting stock. ‘For top material, it’s advisable to keep the generations apart. Ideally, you should handle and store all generations separately, but that’s not really feasible. At least, always keep top material such as young strains and invitro tubers separate from older generations. Don’t store them in the same space , use separate containers and take good care that all equipment is always Potato World 2015 • number 3 23 Pagina 22
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