| Summary: | 碩士 === 國立中興大學 === 植物病理學系所 === 101 === Black root rot of carrot (Daucus carota L.) caused by Thielaviopsis basicola (Berk. & Broome) Ferraris (syn. Chalara elegans Nag Raj & Kendrick) is one of the most important diseases occurred in fresh carrot markets in Taiwan. The typical symptom shows black mold patches on carrot roots which are infected by the pathogen. When infection is severe, mold-like lesions coalesce to cover whole surface of carrot root, resulting in death of the root tuber. This disease was observed on the commercial carrots in Taichung during summer season in 2011. Three isolates CT01, CT02 and CT03 were obtained from diseased carrot roots. Inoculation tests were conducted to confirm their pathogenicity. Three isolates were able to cause root rot and stunting of tobacco seedlings and also made commercial carrot root tubers black rot. On V8 juice agar, colonies of the pathogen are olivaceous or blackish brown. It can form two types of spores. Conidia endogenous catenulate in a linear series from phialides, cylindrical, truncate at both ends, 8.15-34.79 × 3.33-7.37 μm. Chlamydospores elliptical, rounded at the tip, truncate at both ends, 4–8 celled, deep brown to dark, each cell 6.44–15.55 μm length, 8.77–12.49 μm wide. Based on ribosomal DNA internal transcribed spacer (ITS) sequencing analysis with specific primers, sequences of the isolates CT01, CT02, and CT03 were similar to T. basicola, and sequences identity between them were 99, 98 and 99% respectively. According to their pathogenicity test, morphological and molecular characteristics, three isolates CT01, CT02 and CT03 were identified as T. basicola. To survey the distribution of T. basicola in Taiwan, a carrot disk baiting technique was used to determine its prevalence in carrot-growing fields. Among 89 soil samplings from Tainan, Yunlin, and Changhua, the pathogen could only be detected from two carrot cultivated fields. The optimal temperature for mycelial growth and conidial germination of T. basicola was at 24℃. pH values from 6 to 8 were most suitable for conidial germination. Inoculation tests were conducted to clarify the factors which influence the disease occurrence of carrot black root rot disease. It was found that the optimal temperatures for infection of carrot root tubers by T. basicola were from 20℃ to 24℃. Colonization of the pathogen could be enhanced by wounds on carrot root surface. Carrot root with wounds could be infected by the pathogen even with one spore. Relative humidity (RH) affecting the occurrence of black root rot was quite remarkable. Disease did not occur when RH was below 85%. Lesions started to show up at RH 93%, and expanded rapidly with RH over 97.5%. Lesion sizes were positively correlated (r = 0.98) with the number of conidial sporulation on the lesion. Influence of the occurrence of carrot black root rot by the wash methods for cleaning carrot root was proved. The most severe symptoms appeared when carrot roots were wiped by scrub sponge which could cause the surface injury of carrot root tubers. Our results indicated that inoculum level, wound, temperature and relative humidity were the major factors affecting the occurrence of carrot black root rot. In addition, the abiotic or biotic materials were screened for controlling of carrot black root rot disease. Among all the materials we used, acetic acid, ClO2, and Pichia kudriavzevii were significantly effective in reducing the conidial germination of T. basicola and development of the disease lesions compared to distilled water treatment as a control.
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