First Report of Infection of Hop Cones by Alternaria alternata in Australia.

Abstract

The hop (Humulus lupulus L.) is a dioecious climbing plant, cultivated for its resins, which are produced in the cone, used primarily for the bittering of beer. In Australia, hops are grown in the states of Victoria and Tasmania. In late summer 2001, necrotic lesions were observed on the tips of bracts and bracteoles of developing cones at three hop farms in Tasmania. The necrotic area varied between 1 and 25% of the bracts and bracteoles, and in some cases progressed throughout the entire hop cone. Pieces of infected hop cones were surface sterilized for one minute in 2% sodium hypochlorite, plated on 2% water agar, and incubated at 22 ± 2°C. The most frequently isolated fungi (total number of isolations = 60) were transferred to 2% water agar and potato dextrose agar. In 90% of cases, the isolated fungus was Alternaria alternata (Fr.:Fr.) Keissl, as identified by M. Priest, NSW Agriculture, Australia. The pathogenicity of A. alternata was determined on detached, freshly picked cones of hop cultivar "Nugget." Cones (n = 25) were inoculated with a conidial suspension of the fungus (1,000 spores per ml) and incubated at room temperature in a sealed container on plastic mesh over tissue wetted with sterile distilled water. Symptoms first appeared three days after inoculation as necrotic tips of bracts and bracteoles, and within 10 days the entire cone had become necrotic. Symptoms were more severe in vitro compared to in the field. This was probably due to the maintenance of detached cones under constant high relative humidity. Disease symptoms did not appear on cones inoculated with sterile distilled water. The pathogen was reisolated from inoculated cones, completing Koch's postulates. The pathogenicity of A. alternata to hop cones was reported in the United Kingdom in 1988. To our knowledge, this is the first report of A. alternata on hop cones in Australia.