Ash Yellows Occurrence and Association with Slow Growth of Green Ash in Iowa and Wisconsin Cities

Abstract

Green ash in nine cities in Iowa and Wisconsin were surveyed in August and September 1994 for occurrence and impact of ash yellows (AshY) phytoplasma infection. In each city, the survey included 12 arbitrarily selected trees in each of three crown condition categories: less than 10% crown dieback, 11 to 30% crown dieback, and more than 30% crown dieback. Up to four trees per community with witches'-brooms were also sampled. Occurrence of phytoplasmal infection in sampled trees lacking witches'-brooms ranged from 3% to 19% among the cities in the survey. The number of trees that tested positive for phytoplasmas, or that exhibited epicormic sprouts, deliquescent branching, basal bark cracks, or basal shoots, did not differ significantly among the crown dieback classes sampled. Trees that displayed witches'-brooms or basal shoots were significantly (P<0.005) more likely to be infected with phytoplasmas (54%) than trees lacking these symptoms (13%). The presence of epicormic sprouts, deliquescent branching, or basal bark cracks was not significantly associated with presence of phytoplasmas. Trees infected with phytoplasmas had significantly (P<0.0001) less radial growth than noninfected trees from 1973 to 1994.