Emerging fungal pathogens have always been an issue of concern in southeastern U.S. strawberry production. In 2023, an unusual outbreak of Gnomonia leaf blotch occurred at one North Carolina and multiple South Carolina (SC) strawberry farms and marked the first reports of its occurrence in SC. Molecular identification and phylogenetic analysis of isolates from multiple locations identified the fungus Gnomoniopsis fructicola as the causal agent. In vitro germination of G. fructicola progressed slowly and remained less than 40% even after 24 h of incubation. Similarly, germ tube growth was slow compared with other pathogens. Slow symptom development on strawberry leaves of young strawberry plants grown in the greenhouse started 5 weeks after inoculation. Once the pathogen established on greenhouse plants, leaf necrosis forming blotches was observed. The baseline sensitivity of G. fructicola isolates to commonly used chemical classes of fungicides was assessed. Propiconazole, cyprodinil, pyraclostrobin, and fludioxonil were highly effective in mycelial growth assays with effective concentration of fungicide required for 50% growth inhibition (EC50) values ≤0.01 μg/ml. Iprodione and thiophanate-methyl were also effective, with EC50 values ranging from 0.05 to 1.38 and 2.01 to 23.96 μg/ml, respectively. Fluopyram and fenhexamid were ineffective. with EC50 values >100 μg/ml. Based on conversations with the producers, the disease outbreak was linked to transplants from the same nursery source. This study reports for the first time the presence of Gnomonia leaf blotch in SC and provides valuable insights into chemical management options for G. fructicola.
Keywords: Gnomonia comari; Gnomonia fragariae; Gnomonia leaf blotch; Gnomoniopsis fructicola; Zythia fragariae; Zythia fruit rot; fungicide sensitivity; strawberry.