Identification, Characterization, and Fungicide Sensitivity of Botryosphaeriaceae Fungi Associated with Avocado Branch Canker Disease in Southern California.

Abstract

Avocado branch canker disease, caused by Botryosphaeriaceae fungi, threatens avocado (Persea americana Mill.) production in California, reducing yield and orchard lifespan. To effectively manage avocado branch canker, it is imperative to identify the causal fungal species accurately and understand their biology and pathogenic potential. Over 500 fungal isolates were collected from symptomatic branches of avocados showing a general decline, including cankers and dieback of arms and twigs across several California orchards (n=23). Seventy-one representative isolates were selected for detailed analysis based on location, tissue type, colony morphology, and spore characteristics. Multi-locus phylogenetic analysis identified four Botryosphaeriaceae species: Neofusicoccum luteum (most prevalent, 66 isolates), Lasiodiplodia theobromae (3 isolates), Botryosphaeria dothidea (1 isolate), and Neofusicoccum australe (1 isolate). Pathogenicity tests revealed significant variability in aggressiveness, with L. theobromae (26.3 ± 3.77 mm) and N. luteum (23.6 ± 0.87 mm) causing the largest lesions. Additionally, a sporulation trial on an N. australe isolate revealed that acidified potato dextrose agar amended with sterilized pine needles at 22°C under constant light optimized its sporulation. In vitro fungicide assays identified fluazinam as the most effective fungicide across the species exhibiting the lowest EC₅₀ values. Metconazole and propiconazole also exhibited low EC₅₀, while azoxystrobin and boscalid showed the highest. Furthermore, L. theobromae showed reduced sensitivity to azoxystrobin (18.711 μg/mL), cyproconazole (6.381 μg/mL), flutriafol (4.151 μg/mL), and pyraclostrobin (5.477 μg/mL). These findings highlight the importance of species-specific identification and provide critical insights for integrated disease management strategies.