A potato tuber rot disease of unknown cause, affecting 5 to 15% of the potato tuber, was observed at Gansu Province of China in March 2010. Sunken, round, oval, or irregular lesions formed at the umbilicus or buds of potato tubers after 30 days of storage at 4°C. These lesions gradually expanded to form khaki, lavender sunken lesions ranging from 1 to 3 cm. Small black bodies were observed in the center of the lesions after 45 days. Twenty-six diseased tubers were collected and surface sterilized with 75% alcohol. Diseased tissue was then directly transferred to potato dextrose agar (PDA) medium for isolation of pathogenic fungi. Eight fungal isolates from disease tubers were obtained and pathogenicity was evaluated. Conidial suspensions (106 CFU/ml) of per isolate were sprayed on 20 potato tubers, respectively. These potato tubers were stabbed about 20 times with five wounds in a row along the tuber and maximum distance between each row. Wounds were made 2 mm deep and 0.5 mm in diameter with a no. 4 insect needle. Control tubers received water without conidia. The inoculated tubers were put in an incubator at 15°C after 72 h with relative humidity 100%. Assays were repeated three times. Typical symptoms of the disease were observed 14 days after inoculation. Pycnidia sharing the characteristics of the inoculated isolates were retrieved from new lesions after 6 weeks, whereas symptoms did not occur on control tubers. Eight isolates were cultured on PDA medium for 7 days at 20°C and then at 5°C for approximately 30 days to determine cultural and morphological characteristics. Pycnidia were black brown, spherical or oblate, scattered or clustered, and ranged from 82 to 210 × 64 to 175 μm. Conidia were unicellular and colorless, and 2.1 to 4.4 × 5.8 to 11.5 μm. Chlamydospores were spherical and 27 to 81 × 18 to 63 μm. The fungi shared morphological characteristics of P. foveata described in the literature. On oat medium (OA), yellow-green, needle-like crystals were formed. The growth rate of the pathogen on MA and OA was 1.0 cm/day. The pathogens were identified as P, foveata based on the symptoms, morphology, and growth rate (1, 2, 3). Genomic DNA was extracted with UNIQ-10 column fungal genomic DNA extraction kit and ribosomal DNA was amplified with ITS1(TCCGTAGGTGAACCTGCGG) and ITS4 (TCCTCCGCTTATTGATATGC) primers. The nucleotide sequence of the 539-bp amplicon (GenBank Accession No. JQ804843) was 99% identical to the ITS sequence from P. foveata available from GenBank (GU237742). Management strategies for potato disease control must be adjusted for the presence and control of gangrene disease in Gansu Province.
References: (1) G. H. Boerema et al. Page 220 in: Phoma Identification Manual. CABI Publishing, Wallingford, UK, 2004. (2) EPPO. Quarantine pests for Europe University Press, Cambridge. 865, 1997. (3) W. R. Stevenson et al. Page 25 in: Compendium of Potato Diseases, 2nd Edition. APS Press, St. Paul, MN, 2004.
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