Owing to their relative disease resistance and showy spring flowers, cultivars of Callery pear (Pyrus calleryana) are commonly planted ornamental and streetside trees in New York state and elsewhere in the U.S. Over the past 2 years, we collected a leaf rust on P. calleryana ‘Bradford’ or ‘Chanticleer’ (also known as ‘Cleveland Select’) from Hempstead (Nassau County), East Moriches and Riverhead (Suffolk Co.), Rochester (Monroe Co.), and Staten Island (Richmond Co.), NY. Leaf samples were collected in June and August 2010 and 2011; adaxial surface lesions resembled infection by fungi in the genus Gymnosporangium (Pucciniales). Lesions were yellow- to red-orange with irregular red to purple margins, 1 to 3.5 cm in any one dimension, and contained 20 to 45 black, subepidermal spermogonia. Hypertrophied plant tissue was evident on the abaxial surface below the spermagonia, but aecia were absent. Genomic DNA was extracted from rust-infected tissue on P. calleryana ‘Bradford’ (East Moriches, Hempstead, Rochester, and Staten Island) and ‘Chanticleer’ (Riverhead), and the D1/D2 domain of the 28s ribosomal DNA was PCR-amplified using primers 4 and 11 (3) and sequenced. Partial 28s rDNA sequences (GenBank Accession Nos. JN969962 to JN969966) were either identical or nearly so (99%) to that of the trellis rust fungus, G. sabinae (AF426209 and AY512845). G. sabinae produces aecia with conspicuous balanoid peridia from August to November or until leaf drop; both characteristics are unique to the genus (1). We therefore monitored rust-infected Callery pears in Riverhead as well as a common pear (P. communis) in Ithaca, NY, for the appearance of aecia in 2011. The telial state of G. sabinae on taxa in the genus Juniperus sect. Sabina (e.g., J. sabina and J. virginiana) was not observed within the vicinity of affected pears. In late September, aecia of G. sabinae with morphological features identical to those described by Kerns (1) and Yun et al. (4) were collected from a P. calleryana in Riverhead (Cornell Plant Pathology Herbarium; CUP-067943) and a P. communis in Ithaca (CUP-067943): aecia roestelioid, hypophyllous; periderium balanoid, apex intact and conic, cancellate along the sides; aeciospores brown, globoid to broadly ellipsoid, 22.5 to 28.2 × 25.1 to 32.4 μm, walls 3.3 to 4.7 μm thick. A native of Eurasia and North Africa, G. sabinae was first identified in North America on common pear in 1960 in British Columbia and California (2). Thereafter, the fungus was collected on Callery pear in northern Washington in 1988 and recently, in Michigan in 2009 (4). To our knowledge, this is the first report of G. sabinae on P. calleryana ‘Bradford’ and ‘Chanticleer’ as well as P. communis in New York and the new records represent a large (>800 km) eastward expansion of the distribution of the pathogen in the U.S. The geographic locations of affected trees described herein suggest that the trellis rust fungus is well-established across New York, and has spread undetected since its introduction. Given the widespread planting of Callery pear and occurrence of susceptible Juniperus spp. in urban landscapes of New York, G. sabinae has the potential to become a perennial problem where epidemiological conditions permit host alternation.
References: (2) F. Kerns. A revised taxonomic account of Gymnosporangium, 1973. (4) A. McCain. Plant Dis. Rep. 45:151, 1961. (1) G. Van der Auwera et al. FEBS Lett. 338:133, 1994. (3) H. Yun et al. Plant Dis. 93:841, 2009.