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Title: First report of Juneberry rust caused by Gymnosporangium nelsonii on Juneberry in Michigan

item Castlebury, Lisa

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2011
Publication Date: 5/29/2011
Citation: Schilder, M., Lizotte, E., Yun, H., Dixon, L., Castlebury, L.A. 2011. First report of Juneberry rust caused by Gymnosporangium nelsonii on Juneberry in Michigan. Plant Disease. 95(6):770.

Interpretive Summary: Rust fungi are parasites that cause significant damage to crop plants in the United States. Juneberry is a new crop that is increasingly grown for its edible fruits. A rust fungus was discovered to cause significant damage to the fruits of commercially grown juneberry plants in Michigan. This fungus was identified as a rust species that has not been reported before in Michigan and is described and illustrated in this publication. This research provides a description and illustration of this new disease. Plant pathologists will use this research to monitor the juneberry crop for symptoms and signs and to develop fungicides to control this disease.

Technical Abstract: Amelanchier alnifolia, commonly known as juneberry or Saskatoon serviceberry, was historically a prairie fruit native to the Northern Great Plains, southern Yukon and Northwest Territories. While juneberry is an important fruit crop in the prairie provinces of Canada, small commercial plantings also occur throughout the northern United States (2), including Michigan. On July 18, 2009, severe rust symptoms were observed on a 2-year-old planting of 1,000 A. alnifolia ‘Smoky’ plants in Northport, Michigan. The plants had been sourced as seedlings from a nursery in Canada. Symptoms were present on fruits and leaves on virtually all of the plants. Symptomatic fruit were still immature, and, on average, more than 70% of the fruit surface was covered with horn-like white aecia with conspicuous orange aeciospores. Portions of twigs also showed fusiform swellings (1 to 4 cm long) covered with aecia. Aecia were hypophyllous, fructicolous and caulicolous, roestelioid, and 2–4 mm high. The peridium was cornuted, lacerating at the apex, and whitish to brownish. Peridial cells were linear rhomboidal, 50 to 105 µm long, hyaline to brownish, outer walls smooth, inner walls with small papillae and side walls rugose or moderately rugose. Aeciospores were globoid, 20 to 35 × 25 to 38 µm (average 30.7 × 32.5 µm), orange to cinnamon brown, and densely verrucose with walls 2.5 to 3.5 µm thick. Based on fungal morphology and the host on which the rust was found, the pathogen was identified as Gymnosporangium nelsonii Arthur. A DNA sequence of the ITS region was 99.81% similar to an unpublished sequence of Gymnosporangium nelsonii. The sequence has been submitted to GenBank and is the first sequence of G. nelsonii in GenBank. The specimen has been deposited at the U.S. National Fungus Collections. The infection is likely caused by basidiospores originating from telia on Juniperus spp. in the area surrounding of the field. However, no telial horns were found on junipers in the immediate vicinity. This is the first report of G. nelsonii on juneberry in Michigan. Due to the devastating effect of this disease on fruit quality, fungicide programs have been devised for control. Juneberry growers in the Midwest need to be aware of this disease and monitor their crop carefully for symptoms and signs.