First report of white root rot caused by Rosellinia necatrix on Aronia melanocarpa in Korea

Aronia melanocarpa (Michx.) Elliott, black chokeberry, belonging to the Rosaceae, is cultivated worldwide for its popular fruits. In July 2015, black chokeberry (cv. Nero) shrubs exhibiting white root rot symptoms with approximately 5% disease incidence were sampled from an orchard in Gochang (35°25′58″N; 126°42′17″E), Korea. Symptoms included leaf yellowing and wilting of the aboveground plant part. White mycelia on the lower stems of affected shrubs and on adjacent soil surface were also witnessed under wet conditions. Ten symptomatic shrubs were dug from the field and the symptomatic tissues were used for isolating the potential causal agent. Small parts of symptomatic tissues excised from diseased roots of each shrub were surface-disinfested in a 1% sodium hypochlorite solution for 1 min, rinsed three times with sterilized water, and then placed on water agar at 25°C under a 12 h photoperiod for 3 days. The marginal segments of developing fungal colonies were transferred onto potato dextrose agar (PDA). The mycelia from 10 isolates showed pyriform swellings adjacent to the septa. The conidia were aseptate, hyaline, ellipsod to ovoid, and 3 to 5 × 2.0 to 2.5 µm. Based on the morphological characteristics, the fungus was identified as Rosellinia necatrix Berl. ex Prill. (Fetrini 1993). Fungal DNA was extracted with a DNeasy Plant Mini Kit (Qiagen Inc., Valencia, CA). The complete internal transcribed spacer (ITS) region of rDNA was amplified using primers ITS1/ITS4 (White et al. 1990) and sequenced. The resulting sequence of 573 bp was deposited in GenBank (accession no. KX538891). A BLAST search revealed that sequence of the Korean isolate shared 100% identity to those of other isolates of R. necatrix (e.g., EF592565, KF719201, EF592569, EF026117, EF592568, AB017658). For the pathogenicity testing, 2-year-old plantlets (cv. Nero) were inoculated by pouring inoculum of a 7-day-old culture on potato dextrose broth (PDB) onto the unwounded roots of five healthy plants. Another five plants inoculated with noncolonized PDB served as controls. The plants were maintained in a humid chamber (25 ± 2°C, RH 100%) for the first 48 h and then placed in a glasshouse with 80% relative humidity at 25 ± 2°C. After 4 weeks, all inoculated plants exhibited leaf yellowing, wilting, and root rot, whereas control plants remained symptomless. R. necatrix was consistently reisolated from the symptomatic tissue, thus fulfilling Koch’s postulates. A representative isolate was deposited in the Korean Agricultural Culture Collection (accession no. KACC48080). R. necatrix has been recorded throughout the world as a pathogen causing white root rot on many plants, particularly fruit trees such as almond, peach, plum, apple, pear, olive, cherry, and avocado (Farr and Rossman 2016; ten Hoopen and Kraus 2006). To our knowledge, this is the first report of white root rot caused by R. necatrix on A. melanocarpa globally as well as in Korea. Our continuous observations during summer season suggest that poorly drained soil conditions could increase the incidence and severity of white root rot caused by R. necatrix.