Crisphead Lettuce Resistant to Corky Rot: Cultivars Glacier and Misty Day and 16 Resistant Breeding Lines

Abstract

Derived from a cross between the corky root (CR; Rhizomonas suberifaciens)-resistant cv. Green Lake (conferred by a single recessive gene cor cor) and Salinas, the Lactuca sativa cultivars Misty Day and Glacier (released in 1991) have dull, dark green outer leaves that are slightly paler than those of Salinas. Heads are firm to hard at maturity, larger than those of Salinas under moderate to severe CR stress and the same size at lower stress. Leaf texture is relatively soft and the leaf surface is moderately crinkled or blistered. Both cultivars have a taller stem than Salinas and may be more susceptible to bolting induced by high temperatures. In CR-infested soil, root damage to Misty Day and Glacier was slightly to considerably less severe than that observed for Salinas and similar to or slightly more severe than that of Green Lake. Misty Day and Glacier are also resistant to pathotype I of Bremia lactucae. The 16 breeding lines related to Misty Day and Glacier are all descended from the same F3 family (84-680) as the 2 cultivars and consist of 8 F5, 5 F6 and 3 F7 populations. Morphologically similar to the varieties, the lines vary primarily in leaf margin appearance, colour and texture and in extent of head cover.