Tubakia iowensis was recently recognized and described as the causative agent of bur oak blight. The pathogen causes vein necrosis and early season leaf death on Quercus macrocarpa (bur oak). Tubakia sp. B, an undescribed species, was isolated from bur oak and other oaks and was found to be morphologically indistinguishable from T. iowensis. In chapter 2, T. iowensis and Tubakia sp. B differed in causing a greater amount of necrosis on inoculated leaf blade tissue and in causing less petiole death and fewer conidiomata (pustules) after inoculation of expanding shoots. Both Tubakia spp. caused vein necrosis, but Tubakia sp. B was better at causing leaf spots, and T. iowensis was better at infecting petioles and preventing petiole abscission. Thus, T. iowensis appears to have a narrower ecological niche.

Reports of bur oak blight appear to correspond with the geographic range of Q. macrocarpa var. oliviformis. However, even in the upland sites, there is substantial variation in symptom severity among trees within a stand. In chapter 3, seed sources were collected from within the geographic ranges of Q. macrocarpa var. oliviformis, Q. macrocarpa var. macrocarpa, and Q. macrocarpa var. depressa. Seedlings from blighted and healthy Q. macrocarpa trees also were inoculated to test the hypothesis that there is variation in resistance to bur oak blight. Greenhouse inoculations demonstrated that there was no significant difference in the number of symptomatic leaves or seedlings with petiole pustules among the different varieties of Q. macrocarpa. Seedlings from trees with bur oak blight showed no difference in production of petiole pustules when compared to seedlings from healthy trees in the same stand.