Abstract
In previous papers (19622), 19653) the author emphasized the importance of wintered mycelium in rootstocks as the primary infection source of hop downy mildew and also the importance of penetration through stomata and lenticels in the secondary infection. In the present experiments it was confirmed that the infection by zoospores can also take place through wounds on straps and rootstocks. When a piece of the strap, 10cm long, was cut in half longitudinally and inoculated with zoospore suspension, sporangiophores were produced on cortex and phloem of the cut surface but not on pith, after 6-days incubation at 20-23°C and 90% relative humidity. Sporangiophores were produced more abundantly on rough surface made by tearing the strap than on the smooth surface made by cutting with a sharp knife, and on the upper portion of the strap than on the lower portion. When eyes on the strap were cut and the strap was inoculated with zoospore suspension, sporangiophores were produced in a ring surrounding the cut surface. when buds were pulled out, sporangiophores were produced only within the scars. These facts suggests that the infection occurs not only through stomata and lenticels but also through the wounds which are expected to occur by the cutting and prunning in early spring.
