Fungal Diseases Associated with Robusta Coffee (Coffea canephora) in Southern Thailand: Symptom Diversity, Pathogen Characterization, and Environmental Drivers.

Robusta coffee (Coffea canephora) production in tropical regions is increasingly constrained by fungal diseases associated with complex pathogen interactions and favorable environmental conditions. This study investigated symptom diversity, pathogen composition, pathogenicity, and environmental drivers associated with fungal infections in Robusta coffee plantations in southern Thailand. Field surveys were conducted in three commercial plantations located in Thung Lan Subdistrict, Khlong Hoi Khong District, Songkhla Province, Thailand, during February-July 2025. A total of 90 coffee trees were assessed using a stratified sampling approach, and disease symptoms affecting leaves, branches, flowers, and fruits were recorded. Nineteen representative disease symptoms were identified and grouped into major disease categories, including dieback complex, fruit rot/lesion complex, and leaf spot complex. The dieback complex exhibited the highest disease incidence and severity (65.4 ± 5.2% incidence; 58.7 ± 4.3% severity), followed by fruit rot/lesion complex (42.7 ± 4.8% incidence; 36.5 ± 3.8% severity) and leaf spot complex (38.2 ± 3.9% incidence; 29.8 ± 2.7% severity). A total of 10 morphologically distinct fungal isolates were recovered from symptomatic tissues. Molecular analyses identified Fusarium falciforme (95.41%), Lasiodiplodia/Diplodia sp. (99.37%), Lasiodiplodia theobromae (99.79%), and Lasiodiplodia sp. (81.35%). Pathogenicity assays demonstrated that all representative isolates induced disease symptoms on detached Robusta coffee leaves. Among them, F. falciforme showed the highest aggressiveness, producing lesions up to 4.73 cm within 7 days after inoculation. Environmental monitoring further indicated that elevated humidity and moderate temperature during the rainy season favored fungal disease development. These findings demonstrate that fungal diseases in Robusta coffee are driven by multi-pathogen interactions shaped by humid tropical environments, providing an important basis for integrated disease management and sustainable coffee production under changing climatic conditions.