Studies on Pathogen Identification, Biological Characteristics and Fungicide Sensitivity of Impatiens hawkeri Leaf Spot Disease

<i>Impatiens hawkeri</i> W. Bull (<i>I. hawkeri</i>) is popular among consumers due to its diverse flower colors and year-round blooming. However, changes in ecological conditions, cultivation methods, and planting scale have led to increased disease incidence and diversity, particularly the widespread and destructive leaf spot disease. Currently, studies addressing the pathogen species and its biological characteristics remain limited. In this study, a highly pathogenic strain (IH-4) was selected from previously isolated fungi associated with leaf spot in <i>I. hawkeri</i>. Its taxonomic status was confirmed using upright fluorescence microscope analysis, internal transcribed spacer (ITS)/large subunit (LSU)/RNA polymerase II second largest subunit (<i>rpb2</i>)/β-tubulin (<i>tub2</i>) rRNA gene sequencing, and phylogenetic tree construction. Additionally, the biological characteristics of the pathogen and its sensitivity to 8 chemical fungicides were assessed. Strain IH-4 was identified as <i>Ectophoma multirostrata</i> (<i>E. multirostrata</i>) through combined morphological and molecular approaches. Optimal growth conditions included a temperature of 25 °C, a pH of 7, Potato Dextrose Agar (PDA) medium, fructose as the optimal carbon source, and urea as the optimal nitrogen source, with the fastest growth observed under a semi-light photoperiod (12 h light/12 h dark). Fungicide sensitivity assays indicated that 25% azoxystrobin exhibited the lowest half-maximal effective concentration (EC₅₀, 0.0724 μg/mL) and the steepest virulence regression slope (1.7), demonstrating the strongest inhibitory activity and highest sensitivity. Microscopic observations revealed that IH-4 hyphae penetrate <i>I. hawkeri</i> leaf tissues via stomata, colonize internally, and consequently cause host damage. This study provides a theoretical foundation for the timely and effective management of leaf spot disease in <i>I. hawkeri</i>.