Occurrence of Thekopsora minima on highbush blueberries in Lower Saxony 2015–2016: prevalence, susceptibility of varieties and some aspects of infection biology

Abstract

The leaf rust pathogen Thekopsora minima of highbush blueberries (Vaccinium corymbosum) was detected for the first time in Germany in 2015, in two nurseries in Lower Saxony. One introduction was related to recent import of plants from the USA, but the source of the other outbreak was unclear. In 2016, intensive surveillance was conducted of garden centres, tree nurseries, blueberry plantings, neglected plantings and volunteer blueberries growing in close proximity to commercial plantings. T. minima infections were found only on wild plants or neglected plants, in three different rural districts of Lower Saxony. In addition, proof was found of T. minima being present in Lower Saxony since at least 2011. Uredinia and urediniospores were the only developmental stages observed, providing strong evidence of subsequent infection cycles of urediniospores of T. minima able to survive German winter conditions without a host change. Experiments revealed urediniospore infections of the outer bud leaves or pherophylls only, with no disease symptoms on the innermost leaves. Removing diseased leaves accompanied by fungicide treatment was an effective measure to eliminate T. minima. Susceptibility of 14 V. corymbosum varieties under natural conditions was assessed during surveillance. ‘Bluecrop’, ‘Blue Bayou’, ‘Goldtraube’, ‘Poppins’ and ‘Reka’ were found to develop severe symptoms, while ‘Cipria’, ‘Denise Blue’, ‘Liberty’ and ‘Pink Icing’ showed intermediate symptoms and ‘Aurora’, ‘Blueberry Glaze’, ‘Duke’, ‘Jelly Bean’ and ‘Peach Sorbet’ remained symptomless. Possible routes of introduction of the pathogen, its survival and the role of volunteer blueberries in prevalence of T. minima are discussed.