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Emergence of a new disease, bur oak blight, with recent increases in spring rainfall
T. C. HARRINGTON (1), D. McNew (1). (1) Iowa State University, Ames, IA, U.S.A.

Bur oak blight (BOB) appeared on native bur oak (<i>Quercus macrocarpa</i>) in the 1990s and early 2000s in Minnesota, Iowa and neighboring states. Symptoms included mid- to late-season necrosis of leaf veins and dead leaves attached to twigs. Initially, conidiomata of the European leaf spot pathogen <i>Tubakia dryina</i> were associated with the necrotic leaf veins, but closer study led to the description of a new species, <i>T. iowensis</i>. Surveys of natural groves have shown a steady annual increase in disease levels, but variation in DNA sequences suggested that <i>T. iowensis</i> was indigenous. Further, BOB was found to be widespread throughout the range of <i>Q. macrocarpa</i> var. <i>oliviformis</i>, which is adapted to upland, savannah sites. Other <i>Tubakia</i> spp. were associated with veinal necrosis or leaf spots, but only <i>T. iowensis</i> formed overwintering conidiomata on petioles attached to twigs. Infection of emerging shoots during wet periods in late April and May resulted in a 2 mo or more latent phase, with petiole necrosis in July and non-abscising leaves in fall, providing the overwintering inoculum. Consecutive springs of average or above rainfall appear to lead to build up of BOB in individual trees. Spring rainfall has increased 50% in recent years in Iowa, which has not had two consecutive springs of below-average rainfall since 1989. Mature bur oak established on these former savannah sites in an epoch of drier springs, without selection for the resistance needed in today’s climate.

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