Narcissus smoulder : cause, epidemiology and host resistance

Abstract

The cause and epidemiology of narcissus smoulder and the mechanisms by which daffodil bulbs resist infection by Botrytis were investigated. Isolates of B. narcissicola and B. cinerea were not easily distinguished by conidiophore or conidial morphology but were readily identified by sclerotia produced on potato dextrose agar. They were also distinguished by their pathogenicity to narcissus from mycelial inocula. The majority of isolates collected from field-grown narcissus were B. narcissicola. Following inoculation with conidia in sterile water, both species typically failed to colonise narcissus. However, damaging tissue or adding nutrients allowed B. narcissicola to infect. It was concluded that B. narcissicola is the major cause of smoulder. Healthy bulbs inoculated with B. narcissicola, or grown in soil containing botrytis sclerotia, developed lesions in the shoot (primary symptoms). Botrytis narcissicola was commonly isolated from the bulb neck of plants with primary symptoms; it is suggested that infected bulbs are the major source of smoulder outbreaks. Secondary infection by conidia was enhanced by damaging leaves, and open stalk ends left after flower picking were found to provide an important site for infection development. Botrytis narcissicola was isolated from bulb necks when plants with symptoms of secondary infection had died down. The effects of prolonged storage, planting depth and soil type on the production of primaries from infected bulbs are discussed. Following conidial inoculations, lignification and phytoalexin accumulation were detected in bulb scales. Three out of twelve phytoalexins were identified, as closely related hydroxyflavans. Botrytis narcissicola was found to be as sensitive to the inhibitors as non- pathogenic Botrytis species. From the inability to detect phytoalexins or extensive lignification in or around spreading lesions, it was concluded that pathogenicity of B. narcissicola depends on its ability to suppress the host's resistance mechanisms. A range of flavonoid compounds related to the hydroxyflavan phytoalexins were assayed in an attempt to define the structural requirements for antifungal activity.