Identification and biological characterization of Nigrospora sphaerica, the causal agent of jackfruit leaf spot, in China

Abstract Jackfruit ( Artocarpus heterophyllus Lam.) is a valuable tropical fruit crop in southern China, yet little information is available regarding the pathogens affecting its foliage. In July 2023, typical leaf spot symptoms were observed on jackfruit plants in Nanning, Guangxi, China, which prompted an investigation into the causal agent. The colonies of the pathogen isolated from the symptomatic leaves appeared white initially, and gradually turned gray, the conidia were black, spherical to subspherical. Based on morphological observations and phylogenetic analyses, the pathogen was identified as Nigrospora sphaerica . The symptoms caused by inoculation of healthy leaves with N. sphaerica isolates were similar to those of jackfruit leaf spot observed in the field, and the pathogen was re-isolated from artificially inoculated symptomatic leaves. This was the first report of N. sphaerica causing leaf spot on jackfruit in China. Further biological characterization demonstrated that the pathogen achieved optimal mycelial growth at 28 °C on PDA medium, with maximal mycelial growth obtained at pH 8 and pH 9, which were not significantly different ( P < 0.05); among all tested media, malt extract agar was the optimum medium for mycelial growth. Soluble starch and beef extract powder served as the optimal carbon and nitrogen sources, respectively. In addition, 55 °C for 10 min was the lethal temperature for mycelial growth. These findings provide critical insights into the biology of N. sphaerica and its potential impact on jackfruit production. Understanding its optimal growth conditions may aid in developing management strategies to mitigate disease spread and reduce crop losses. Subsequent studies should explore disease epidemiology and potential control measures.