Phytophthora porri in leek : epidemiology and resistance

In winter, Phytophthora porri is an important pathogen of leek ( Allium porrum L.) in the Netherlands. The fungus survives the crop-free period in summer by oospores in soil, and infects the leaves in autumn. Field studies indicated that dispersal by rain splash is crucial for initiation of an epidemic. Disease foci expanded at a rate of ca. 3 cm.d ^-1in the first month after artificial inoculation. At temperatures below 5°C the fungus is still able to grow in leaf tissue. Temperatures above 45°C are detrimental to all fungal structures, including oospores. Cultivars differed in partial resistance, both in naturally inoculated field tests and in zoospore-inoculated glasshouse tests. Within cultivars, a broad variation range for partial resistance was demonstrated by means of clone tests. High levels of partial resistance were observed in several landraces. Two landraces were crossed and backcrossed to cultivars.