Occurrence of strawberry root and crown rot in Western Australia

Root and crown rots are important diseases of commercial strawberry crops. Several fungi have been reported as causal agents of strawberry crown and root rot, and they cause considerable yield reduction worldwide. The fungi Phytophthora spp., Verticillium spp., Fusarium spp. Gnomonia fragariae and Colletotrichum spp. are known to be important pathogens (D’Ercole et al. 1989; Tezuka and Makino 1991; Freeman et al. 1997; Maas 1998; Morocko et al. 2006). Strawberries are a high-value export crop that are grown in the temperate areas of Western Australia. Soilborne fungal pathogens are routinely controlled by soil fumigation, but plant death has recently been extremely high in some fumigated and non-fumigated strawberry fields.

During the surveys conducted in 2005 and 2006, a high incidence of strawberry death was observed in coastal districts up to 50 km north of Perth areas (Fig. 1). Mortality of Camarosa and Gaviota varieties of strawberry (Fragaria × ananassa) was between 0 and 60% in some strawberry fields. A total of 50 affected and asymptomatic plants and soil samples surrounding their roots was collected randomly from five fields. Roots were carefully washed under running tap water and the crown of each plant was dissected lengthwise. Vascular discolouration of the crown and root rot was observed on 75% of the samples collected. Crowns and roots of affected and asymptomatic plants were surface-sterilised by immersion in a 1.25% aqueous solution of sodium hypochlorite for 1 min, rinsed in sterile water and dried in a laminar flow cabinet. Pieces of root and crown were then separately placed on potato dextrose agar (PDA), water agar and selective media (P10VPH and P10VP) (Tsao and Guy 1977) and then incubated at 22 ± 3°C. Emerged fungal colonies were sub-cultured on carnation leaf agar, PDA and V-8 juice agar and incubated at 25°C with a 12 h dark and light cycle. The soil samples were baited with cotyledons of Eucalyptus sieberi for isolation of Pythium and Phytophthora species (Marks and Kassaby 1974). Growth rate, colony morphology and morphological characteristics of the fungi isolated were determined.

The pathogenicity of Fusarium isolates was tested on Fragaria × ananassa cv. Camarosa, Lycopersicon lycopersicum cv. Petula and Cucumis sativus (Lebanese cucumber) in a glasshouse experiment. Strawberry runners and 4-week-old seedlings of tomato and cucumber were inoculated by dipping the roots in a spore suspension (10⁵ spores/mL) before planting. Controls were dipped in tap water using the method of Stall and Walter (1965). Fusarium isolates used were pathogenic on the strawberry runners but non-pathogenic on the tomato and cucumber plants tested. Pathogenicity of the Phytophthora cactorum isolates was tested by inoculating wounded and non-wounded strawberry crowns and roots with 5-mm mycelial plugs from the 7-day-old V-8 juice agar culture. Strawberry runners were planted in the sterilised potting mix and four mycelial plugs per pot were in direct contact with the crown and roots for each treatment. Control plants were tested correspondingly, but with sterile agar plugs. Root rot and vascular discolouration of the crown tissues were evident on the inoculated plants and not on the controls after 3 weeks. Pathogenicity, symptoms and morphological characteristics of the reisolated fungi from strawberry runners were confirmed following Koch’s postulates.

During the survey, a high incidence of crown rot with typical infection by Fusarium spp. discolouration was observed. Fusarium oxysporum f. sp. fragariae was consistently isolated from the samples tested (Fig. 2). This fungus has been reported in Queensland and Japan as an important pathogen of strawberry (Winks and Williams 1965; Maas 1998). Phytophthora cactorum was detected from crown, root and soil samples (Figs 3 and 4). Phytophthora cactorum has not been reported causing crown or root rot on strawberry in Western Australia. However, there is a record of this fungus on the other hosts in the culture collection of Department of Agriculture and Food Western Australia (WAC). Phytophthora cactorum causes lather rot of fruit and crown rot of strawberry in Australia, the USA, Europe and parts of Asia and Africa (Washington et al. 1999; Eikemo et al. 2004). In this study, F. oxysporum f. sp. fragariae and P. cactorum were often isolated independently or in combination with Pythium spp., Rhizoctonia spp., Colletotrichum spp. or Macrophomina spp.

Cultures of F. oxysporum f. sp. fragariae and P. cactorum have been deposited in the WAC as WAC 12708 and WAC 12984, respectively.