Leaf Blight in Ilex verticillata Caused by Alternaria alternata: Mechanisms of Antioxidant Defense, Phytohormone Crosstalk, and Oxidative Stress Responses

<i>Ilex verticillata</i> (winterberry) is a valuable ornamental shrub increasingly threatened by leaf blight, a disease that compromises its aesthetic and economic value. While fungal pathogens like <i>Alternaria alternata</i> are known to cause leaf blight in horticultural crops, their role in <i>I. verticillata</i> and the host's defense mechanisms have not been fully characterized. Our study investigated the pathogen-host interaction by identifying the causal agent and examining the physiological and molecular defense mechanisms of <i>I. verticillata</i>. Through morphological and multi-locus molecular analyses (ITS, TEF1-α, G3PDH, RPB2), <i>A. alternata</i> was confirmed as the primary pathogen, fulfilling Koch's postulates. Pathogenicity assays revealed distinct disease progression stages, from necrotic lesions to tissue degradation. Transcriptomic profiling uncovered dynamic host responses, with early upregulation of pattern recognition receptors (PRRs) and transcripts encoding antioxidant enzymes (SOD, CAT), followed by downregulation of metabolic pathway genes. Phytohormone analysis highlighted intricate crosstalk, with salicylic acid (SA) peaking during mid-infection and jasmonic acid (JA) rebounding later, reflecting a coordinated defense strategy. Additionally, the oxidative stress marker malondialdehyde (MDA), an indicator of membrane lipid peroxidation, surged early, indicating membrane damage, while sustained induction of antioxidant enzymes suggested adaptive responses. The key finding was distinct phytohormone crosstalk, characterized by a mid-infection SA peak followed by a late JA rebound, alongside an early oxidative burst marked by MDA accumulation and sustained antioxidant enzyme activity. These findings provide a framework for understanding <i>I. verticillata's</i> defense mechanisms and offer insights for developing targeted disease management strategies, such as resistant cultivar breeding or hormone-mediated interventions.