Quantitative Morphological Profiling and Isolate-Specific Insensitivity of Cacao Pathogens to Novel Bio-Based Phenolic Amides

Fungal pathogens, including Colletotrichum gloeosporioides and Pestalotiopsis spp., are significant threats to global cacao production. Understanding their varying responses to novel antifungal agents is crucial for developing sustainable plant protection strategies. This study investigated the quantitative morphological responses and isolate-specific sensitivity of three cacao pathogen isolates (one Pestalotiopsis sp. and two C. gloeosporioides) to four novel bio-based phenolic-branched fatty acids and their corresponding amides derived from renewable feedstocks. We observed a high degree of isolate-specific susceptibility. A phenol-branched soy oil-derived fatty amide (PhSOAM) proved most potent, significantly inhibiting the growth of Pestalotiopsis sp. and one C. gloeosporioides isolate. In contrast, the second C. gloeosporioides isolate displayed complete insensitivity to all tested compounds, highlighting significant intraspecific variation. Notably, quantitative image analysis revealed that PhSOAM uniquely altered fungal colony morphology by significantly increasing the length-to-width ratio, suggesting a mechanism of action involving the disruption of polarized growth. Multivariate analyses and machine learning models (R2 up to 0.74) effectively classified these responses, identifying the specific pathogen-compound pairing as the most critical determinant of the interaction outcome. This work not only highlights the potential of bio-based amides but also establishes a powerful analytical framework, combining morphological profiling with predictive modeling, to gain deeper insights into the complex, isolate-specific nature of fungal–antifungal interactions.