a Toluidine blue-stained semithin section, showing the paraphyses region at the interface between sterile and fertile veins, where significant TUNEL positivity has been detected. b, c Examples of intact sterile hyphae, with normal nuclei; d sterile hyphae showing cytoplasmic emptying and vacuolization. e, f Altered nuclei with irregular condensed chromatin masses and dilated nuclear envelope lumen (arrow). a asci, n nuclei, pa paraphyses, sv sterile vein. Bars = 50 µm in a; 4 µm in b, d; 2 µm in c; 1 µm in e, f. TEM transmission electron microscopy