Caution Required in Distribution of Plants of Red Stele (Phytophthora fragaiae Hickman) -Resistant Strawberries

Abstract

Red stele of strawberry (Fragarin ×ananassa Duch.), caused by Phytophthora fragariae, was first observed in Scotland in 1921 (Wardlaw, 1927; Wynn, 1968). The causal agent was named and described by Hickman (1940). The pathogen usually enters through root tips or wounds and invades the stelar root tissue, progressing up to, but not into, the crown of the strawberry plants. Latera1 roots are usually infected first and die, giving a “rat tail” appearance to the root system (Scott et al., 1984). The plants are killed when severely infected by the pathogen. The pathogen spreads mainly via infected roots and possibly field equipment. The disease is of major importance in most countries where temperature and soil moisture conditions are favorable (10 to 15C, wet soil), while it has become a limiting factor in some areas. Pathogenic races of P. fragariae were first reported by Scott et al. (1950) and later by Hickman and English (1951). Subsequently, additional races of P. fragariae have been reported by other investigators (McKeen, 1958; Montgomerie, 1977; Scott et al., 1950), and the resistance of many cultivars to red stele has been evaluated (Khanizadeh et al., 1991, 1992; Maas, 1976; Maas and Galletta, 1989; Maas et al., 1989; Melville et al., 1980; Scott et al., 1984). One effective control is the breeding of red stele-resistant varieties. Plant breeders in most strawberry-growing countries are using sources of red stele resistance derived from known genotypes, e.g., ‘Sparkle’, ‘Aberdeen’, ‘MD683’, and ‘Stelemaster’. However, little is known of the genetic base of resistance to P. fragariae in strawberry. Van de Weg (1989a, 1989b) first identified individual resistance and virulence genes. He reported that the compatible and incompatible interactions between