Discovery of novel pathogens causing black spots on Torreya grandis cv. Merrillii nut and fungicide effectiveness during post-ripening

Torreya grandis, a highly nutritious nut endemic to South China, undergoes a critical postharvest stage for nutrient conversion, aroma synthesis, and removal of astringency. Black spots disease is a common and serious post-harvest disease that severely affects nut quality, with black spots appearing on the hard shell and internal seed coat, accompanied by a peculiar odour. In this study, pathogens mediating for infection were annotated alongside profiling of volatiles via gas chromatography-mass spectroscopy (GC-MS) in the infected nuts. Additionally, the effectiveness of tetramycin against black spots disease was evaluated. Lasiodiplodia theobromae TGBHS1 was identified as the primary causal pathogen for black spots using morphological and molecular phylogenetic analysis. Furthermore, the aroma compounds of TGBHS1 infected nuts notably increased compared to the control group, including alcohols, aldehydes, esters, and ketones, especially with the unpleasant smell compounds terpinen-4-ol and Bicyclo [3.1.1]hept-3-en-2-one, 4,6,6-trimethyl-, (1S). Optimal conditions for mycelial growth of TGBHS1 were at 30 °C and pH 5.0, with sucrose serving as the preferred carbon source. Among the tested concentrations of tetramycin, 3.0 × 10⁻³ g L⁻¹ was found to be highly effective in mitigating black spots incidence prior to the postharvest stage. Such findings offer brand-new understanding on pathogen management and postharvest optimization in T. grandis nut leading to improved quality and safety.