Comparative Analysis of Morphological and Physiological Traits between Leaf- and Fruit-Derived Isolates of Alternaria leaf spot in Korea Hyoengeun An Major in Plant Pathology

다국어 초록 (Multilingual Abstract)

Alternaria alternata, the causative agent of apple Alternaria leaf spot and fruit spot rot, leads to significant economic losses in apple orchards. In this study, 68 isolates were obtained from symptomatic apple leaves and fruits in Gyeongsangbuk-do, Korea, to characterize their pathogenic potential and physiological traits. Cross-inoculation assays using detached ‘Fuji’ leaves and fruits revealed tissue-specific virulence: fruit-derived isolates produced larger lesions on fruits, whereas leaf-derived isolates induced more severe symptoms on leaves. Multi-locus phylogenetic analysis based on Internal Transcribed Spacer (ITS), Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), RNA polymerase II second largest subunit (RPB2), and Alternaria major allergen (Alt a 1) sequences showed that all isolates clustered within Alternaria section Alternaria, exhibiting limited nucleotide variation. Physiological characterization revealed that fruit-derived isolates exhibited faster mycelial growth on PDA at 10–25℃. On media with varying pH levels (4–10), fruit-derived isolates showed superior growth at pH 4–6, while leaf-derived isolates performed better at pH 7. Furthermore, fruit-derived isolates maintained higher conidial germination rates than leaf-derived isolates at 5, 10, 15, 35, and 40℃. While growth differences were negligible on fruit-extract medium, fruit-derived isolates grew faster on leaf-extract medium. Notably, when cultured on carboxymethyl cellulose (CMC) medium, leaf-derived isolates formed larger clear halos at temperatures ≥35℃, indicating stronger cellulolytic activity than fruit- derived isolates. These findings suggest that A. alternata populations in Korean apple orchards exhibit tissue-associated phenotypic differentiation rather than species-level divergence. This study provides fundamental insights into the ecological diversity of these pathogens and offers practical implications for managing leaf- and fruit- associated inoculum sources in apple orchards.

목차 (Table of Contents)

• INTRODUCTION 1
• MATERIALS AND METHODS 5
• 1. Fungal isolation and culture 5
• 2. Identification of fungal isolates 6
• 3. Differentiation of Isolates Using Targeted PCR Screening 8
• 4. Pathogenicity assays 10
• 5. Morphological characteristics of mycelia, colonies, and conidia 12
• 6. Effects of Temperature and pH on Spore Germination and Mycelial Growth of Alternaria alternata 13
• 7. Apple fruit and leaf extract agar for mycelial growth assay 15
• 8. Cellulolytic activity 16
• RESULTS 17
• 1. Isolation of Alternaria Strains from Gyeongsangbuk-do Region 17
• 2. DNA sequencing and phylogenetic analysis. 20
• 3. Comparative Pathogenicity of Fruit- and Leaf-Derived A. alternata Isolates in Apple 24
• 4. Colony and Spore Morphology of Alternaria Isolates 31
• 5. Mycelial Growth Curve 35
• 6. Effects of Temperature and pH on Mycelial Growth and Temperature- Dependent Spore Germination of Alternaria strain 37
• 7. Apple fruit and leaf extract agar for mycelial growth assay 41
• 8. Cellulolytic activity 44
• DISCUSSION 48
• REFERENCES 51
• ABSTRACT (KOREAN) 55