Studies on downy mildew of hop plants

IV. Some observations on life cycle of the causal fungus (Pseudoperonospora humuli (Miy. et Tak.) Wilson), and especially on the primary source of infection in hop downy mildew

Abstract

This study was conducted at Furusato Hop Research Laboratory, Nagano Prefecture, by using Variety Shinshuwase grown in the field of this Laboratory. Three types of symptoms which were reported by Ware (1926) and Salmon & Ware (1927), i.e., basal, terminal, and lateral spikes, appear after the shoots began to elongate in late April to early May. Anatomical studies on the distribution of mycelium in each type of spikes revealed that almost all basal spikes and Type A of lateral ones carried mycelia in pith, whereas most of terminal ones and Type B of lateral ones contained mycelia in cortex. In basal spikes, mycelia grew upward within the pith as the shoot elongated and then shifted to the cortex. Thus they produced sporangia on the surface of stem and, also on the lower surface of leaves. The sporangia caused secondary infection and resulted in the formation of terminal and lateral spikes. Sometimes the combination of primary and secondary infections become in the origin of terminal and secondary spikes. Leaf infection was caused by both sporangia formed on these spikes and oospores survived in the debris. The optimun temperature for the germination of sporangia and oospores lies between 18 and 21°C. Fresh sporangia germinated within an hour. The oospores produced in the preceding year retained the ability to germinate, and the low temperature during their survival favored their longevity. Sporangia were produced in 4 to 5 days after zoospores were inoculated on the lower surface of leaves. In cones, bracts and bracteoles were most easily infected, causing a conspicuous decrease of resin content. Straps and rootstocks were successfully infected with the inoculation in late September and caused a considerable percentage of basal spikes in the following spring. Autumn foliar spray of Bordeaux mixture for consecutive four years decreased the percentage of basal spikes year by year. The present author emphasized the importance of wintered mycelium in rootstocks as the source of primary infection and the necessity to prevent the late infection.