The disinfection of potato seed tubers with formalin, organic and inorganic mercury compounds is useless against silver scurf of potatoes caused by Helminthosporium atrovirens. Although generally good surface disinfectants, these fungicides cannot reach the mycelium of the fungus growing underneath the skin. When conditions are again favorable, the pathogen will grow conidiophores that will bear conidia, the germination of which is not impaired by the previous treatments.Cultural practices such as fallow, crop rotation as well as soil treatments with Lanstan (soil fungicide) or Vapam (soil fumigant) did not reduce the percentage of infected tubers at harvest when used in conjunction with apparently disease-free potato seed tubers.Besides showing the ineffectiveness of disinfecting potato seed tubers, these results indicate that evaluating the true health condition of seed-potatoes according to whether or not the fungus is present on the skin is not to be relied upon, that sorting out visibly infected tubers does not suffice to free a "seed" from the disease, and that cultural practices and chemical soil treatments are useless if the seed-potatoes are already infected, thereby laying emphasis on the infected potato seed tubers as the main source of inoculum for silver scurf.
Influence of abiotic factors, inoculum source, and cultivar susceptibility on the potato tuber blemish diseases black dot (
Colletotrichum coccodes
) and silver scurf (
Helminthosporium solani
)