Epidemiology of fire blight of floricane fruiting red raspberry caused by Erwinia amylovora

Abstract

Current information on the epidemiology of fire blight of raspberry (Rubus idaeus), caused by Erwinia amylovora, is sparse and based on the assumption of a similarity to fire blight of apple (Malus ×domestica) and pear (Pyrus communis). To test this assumption, epidemiological studies of raspberry fire blight were conducted in controlled environment and in a planting of raspberry selection K81-6, which is highly susceptible to this disease. The bacteria moved down to and overwintered in canes, within 6–10 cm of the soil line, but not at the soil line or in roots. The initial inoculum, responsible for the disease cycle, originated from systemically infected fruiting laterals that develop on floricanes in the spring. These laterals produced bacterial ooze that attracted ants (Tetramorium caespitum) in search of nectar, which then transmitted the inoculum to healthy flowers. Also, tarnished plant bugs (Lygus lineolaris), meadow spittle bugs (Philaenus spumarius), and earwigs (Forficularia auricularia) were shown to vector the pathogen to healthy raspberry leaves and canes through feeding wounds. Infected canes produced copious amounts of bacterial ooze days before other symptoms of fire blight infection became evident. Flowers were infected primarily through the nectaries, and green fruit could be infected only through wound inoculations. Nearly ripe and ripe fruit were not susceptible to infection. The implications of subtle epidemiological differences between apple and raspberry fire blight on disease management strategies are discussed.