Bioactive Sesquiterpenoids from <i>Santolina chamaecyparissus</i> L. Flowers: Chemical Profiling and Antifungal Activity Against <i>Neocosmospora</i> Species

<i>Santolina chamaecyparissus</i> L. (cotton-lavender) is receiving increasing attention due to its potential for modern medicine and is considered both a functional food and nutraceutical. In this work, the phytochemical profile of its flower hydromethanolic extract was investigated by gas chromatography–mass spectrometry, and its applications as a biorational for crop protection were explored against <i>Neocosmospora</i> spp., both in vitro and in planta. The phytochemical profiling analysis identified several terpene groups. Among sesquiterpenoids, which constituted the major fraction (50.4%), compounds featuring cedrane skeleton (8-cedren-13-ol), aromadendrene skeleton (such as (−)-spathulenol, ledol, alloaromadendrene oxide, epiglobulol, and alloaromadendrene), hydroazulene skeleton (ledene oxide, isoledene, and 1,2,3,3a,8,8a-hexahydro-2,2,8-trimethyl-,(3a<i>α</i>,8<i>β</i>,8a<i>α</i>)-5,6-azulenedimethanol), or copaane skeleton (<i>cis</i>-α-copaene-8-ol) were predominant. Additional sesquiterpenoids included longiborneol and longifolene. The monoterpenoid fraction (1.51%) was represented by eucalyptol, (+)-4-carene, endoborneol, and 7-norbornenol. In vitro tests against <i>N. falciformis</i> and <i>N. keratoplastica</i>, two emerging soil phytopathogens, resulted in effective concentration EC₉₀ values of 984.4 and 728.6 μg·mL⁻¹, respectively. A higher dose (3000 μg·mL⁻¹) was nonetheless required to achieve full protection in the in planta tests conducted on zucchini (<i>Cucurbita pepo</i> L.) cv. ‘Diamant F1’ and tomato (<i>Solanum lycopersicum</i> L.) cv. ‘Optima F1’ plants inoculated with <i>N. falciformis</i> by root dipping. The reported data indicate an antimicrobial activity comparable to that of fosetyl-Al and higher than that of azoxystrobin conventional fungicides, thus making the flower extract a promising bioactive product for organic farming and expanding <i>S. chamaecyparissus</i> potential applications.