Histological observation of cucumber infected with Corynespora cassiicola

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In this study, we compared differences between cucumber cultivars D9320 (highly resistant) and D0401 (highly susceptible) after Corynespora cassiicola infection. There were no differences in conidial germination or penetration between these cultivars. The germ tube rapidly elongated within 24 h after inoculation and invaded the cucumber leaf tissue directly or via stomata 24 h after inoculation (hai). The hypersensitive response (HR) was observed using in vitro inoculation and the trypan blue staining method. Necrotic spots occurred only in the resistant cucumber cultivar D9320 in vitro inoculation. The water-soaked lesion appeared in D0401. The HR was observed earlier and stronger in D9320 than in D0401 in the light microscope. The phenomenon of hypersensitive cell death could be earlier observed by using light microscope. The accumulation of H2O2, lignin and callose was observed in D9320 and D0401 infected with C. cassiicola. The order of accumulation was H2O2, lignin, and then callose. H2O2, lignin and callose accumulation in D9320 occurred earlier and stronger than in D0401. The pattern of callose accumulation in D0401 was different from that of H2O2 and lignin. The intensity of the accumulation did not change over time in D0401. Consistent with lignin and callose accumulation, the expression levels of CsCALS and CsCCoAMT -increased earlier and were higher in the highly resistant D9320 cultivar than in the highly susceptible D0401 cultivar. The HR occurred after these accumulations. The amount of mycelium in D9320 was much lower than that in D0401 at 5 days post-infection (dpi). The HR plays a role in resisting infection of C. cassiicola.