Abstract:The pathologic changes between susceptible cotton variety and its resistant descendants obtained by directional selection in disease plot were studied. The results showed that the differences in macroscopical symptoms is the susceptible cotton varieties showed symptom after inoculated for 7 days, and became severe after 25 days. However, there had only slight symptom or no symptom in the resistant descendants at the meantime. Furthermore, all the vascular tissues turned brown in the susceptible cotton varieties, but in the resistant descendants, the brown regions were only restricted below the hypocotyls. As to the cytology and cell ultrastructure of host, in the invading prophase, the cell wall of the invaded part of resistant descendants was congregated with wall apposition. In the invading metaphase, the hyphae of the pathogen can invade the cortical area of cells of susceptible varieties, but only can invade the outer cortex cells of resistant descendants. In the invading anaphase, more abundant aggregated deposits and secretion were emerged to surround the fungal cells in resistant descendants so as to prevent the hyphae to spread. In general, resistant descendants have more ability in resistant pathogen infection and spread than the susceptible variety.