First report of Armillaria gallica on highbush blueberry (Vaccinium corymbosum) in Italy

Since 2003 highbush blueberry plants (Vaccinium corymbosum) with atypical growth were found in the Trentino region (North-eastern Italy). At the beginning the plants were stunted and developed small leaves, that reddened prematurely in autumn. Later the roots rotted, several branches wilted and plants usually died within a few months. An Armillaria spp. was isolated on malt extract agar (MEA) medium from the roots of dead or stunted plants.

To identify the species of Armillaria, DNA was extracted from two isolates growing in the bark mulch used in the orchard and a further seven isolates infecting roots. The DNA was amplified by PCR using seven primer combinations designed on internal transcribed spacer and intergenic spacer regions of rDNA (Pérez-Sierra et al., 2000; Sicoli et al., 2003). To confirm the bands' identity, restriction fragment length polymorphism (RFLP) analysis with enzyme AluI and sequencing were done on amplicons obtained after amplification with the LR12R/O1 primer combination (Pérez-Sierra et al., 2000). Three isolates for A. borealis, one for A. mellea, A. gallica and A. cepistepes were used as references. All the isolates had the same restriction patterns and sequences (GenBank accession numbers: DQ336609 to DQ336617), as the reference for A. gallica.

From 2003 to 2005, the disease was slowly spreading in the orchard, with an average yearly increase that varied between 1% and 2.5%. Pathogenicity of two isolates; one isolated from the bark mulch and one from the root; was tested on blueberry plants. The inoculum was prepared using autoclaved apple wood pieces which were inoculated with the two isolates on MEA in Petri dishes (kept in the dark at 25°C). When the wood pieces were completely colonised, they were placed between roots of two-year-old potted blueberry plants. The infected plants were kept in a glasshouse (average daily temperature of 20±5°C, natural light). Ten months after the inoculation with the two isolates, the blueberry plants showed similar symptoms to those observed on plants with symptoms in the original orchard of isolation. Armillaria was re-isolated from the infected plants and re-identified as A. gallica using RFLP analysis and sequence comparison. Therefore Armillaria gallica is the causal agent of the highbush blueberry dieback detected in Trentino Region (Italy). Armillaria species induce root disease on a broad range of plants and cause economic losses, especially on fruit and forest trees. A. mellea and A. ostoyae have been reported on highbush blueberry in USA (Caruso, 1995). To our knowledge this is the first report of root rot caused by A. gallica on highbush blueberry.