First report of crater rot caused by Rhizoctonia carotae on carrots (Daucus carota L.) in Turkey

Crater rot was observed on carrots growing in fields in the eastern Mediterranean region of Turkey; one of the country's main production areas. Relatively high humidity and cool conditions of the 2002 and 2003 growing seasons may have been factors that influenced the severe disease incidences that were observed (47.6 and 53.2% respectively). This resulted in 55 to 70% yield reductions in the fields inspected.

Primary symptoms seen were a band of dark brown necrosis around the carrot crown and horizontal brown canker-like lesions mostly on the crown and upper roots (Fig. 1). Small pits subsequently developed beneath the lesions, that enlarged into sunken brown crater lines with a white, flocculent mycelium produced under high humidity. Sections cut from infected carrot roots were surface-sterilised in 1% NaOCl for 2 min, plated on 2.0% water agar medium supplemented with 50 mg/L streptomycin sulphate, and incubated for 7 to 10 days at 24 ± 2°C. Hyphal tips from the margin of each developing Rhizoctonia colony were subcultured on potato dextrose agar (PDA). Fungal colonies were initially white and turned brown with age. Dark brown to black sclerotia were occasionally produced (Fig. 2A). Hyphae occasionally had long, multinucleate swollen cells. Hyphal branches arose at right angles and contained a cross-wall near the branch point. Clamp connections were observed in the hyphae (Fig. 2B). The causal organism was identified as Rhizoctonia carotae based on these morphological characteristics (Rader, 1948; Jensen, 1971).

Koch's postulates were completed by inoculating wounded carrot roots with 5mm-PDA plugs, bearing both mycelium and sclerotia of the isolated pathogen. Inoculated carrots were kept in a moist chamber (100% RH) at 23°C for 14 days. Control carrots were sterilised with 2% NaOCl and treated with uninoculated PDA plugs in a similar manner. Small brownish sunken lesions, identical to those observed in naturally-infected carrots, became visible 5-7 days after inoculation. No lesions were observed on control carrots. The pathogen was successfully re-isolated from inoculated carrots, thereby completing Koch' s postulates.

R. carotae has previously been reported from USA (Rader, 1948; Punja, 1987), Denmark, Norway, Sweden, Russia (Jensen, 1971) and the UK (Derbyshire & Crisp, 1978). This is the first report of R. carotae on carrots in Turkey.