ABSTRACTThe rust fungusPuccinia substriatavar.indicaproduced germ tubes, appressoria, and substomatal vesicles on seedling and mature leaves of susceptible and resistant pearl millet as well as two nonhost plants, corn and peanut. Lack of directional germ tube growth on peanut leaves appeared to affect the ability of germ tubes to successfully locate stomates. In seedling leaves of the susceptible cultivar Tift 23DB, the fungus produced an extensive system of intercellular hyphae and sporulated abundantly. In seedling leaves of the moderately resistant cultivar 86–8770, necrosis of host cells in response to fungal infection occurred by 2 days post-inoculation. Colonies gradually expanded in size and were associated with macroscopic necrotic flecks with sporulation occurring at 15% of the infection sites. In seedling leaves of the highly resistant cultivar Tift 85DB, fungal colonies were extremely limited in growth and did not sporulate. They were associated with intensely autofluorescent, necrotic host cells. Mature leaves of each cultivar were more resistant to fungal colonization than seedling leaves. Some colonies in mature leaves of Tifit 23DB were completely encompassed by necrotic cells and did not expand further. No sporulation was noted in mature leaves of 86–8770 or Tift 85DB. Fungal colonies were not successfully established in either nonhost. Thickening of plant cell walls at sites of attempted infection was noted in corn. In peanut, guard cells underneath fungal appressoria became autofluorescent.