The influence of stomatal opening on the infection of hop leaves by Pseudoperonospora humuli

Abstract

In experiments using both attached hop leaves and leaf disks floating on water, a general correlation was found between the number of stomata per unit area open at the time of inoculation with Pseudoperonospora humuli and the amount of subsequent infection, assessed as area infected and degree of sporulation after 7 days. Whether stomatal opening was varied by light or darkness, chemicals or different soil moisture régimes, a similar result was consistently obtained, irrespective of leaf age or cultivar. Microscopic examination of the leaf surface after inoculation in the light with a large number of open stomata, revealed that the majority of zoospores settled and encysted singly on stomata which were then penetrated by germ tubes. In contrast, in darkness, when stomata were mostly closed, zoospores settled between stomata and few germ tubes encountered and penetrated stomata. This differential response to stomata occurred on hop leaves of various ages and cultivars and on leaves of certain non-hosts. A statistical test, based on sequential analysis, allowed zoospore-stoma associations to be evaluated with the low zoospore numbers used. In reciprocal inoculations of hop and vine leaves zoospore behaviour in Pseudoperonospora humuli was compared with that in Plasmopara viticola. Marked differences in the number of zoospores settling on single stomata occurred which appeared to be due to both stomatal and zoospore factors. The results are discussed in relation to downy mildew in the field, resistance to the disease and the possible mechanism of the zoospore response.