Figure 1: Typical crater rot caused by R. caratoe on carrot roots. Note dark brown decay (arrows) around the crown and upper parts of the root.

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).

Figure 2: A Typical dark brown sclerotia (*) and swollen hyphae (arrow) of the pathogen. B Hyphal clamp connection (arrow).