First Report of Leaf Rust of Blueberry Caused by Thekopsora minima in Mexico

Blueberry (Vaccinium corymbosum L.) is becoming an important crop in the states of Jalisco and Michoacan in Mexico. Leaf rust, a disease causing extensive defoliation on plants with severe infections, was observed in the autumn of 2007 and it has become one of the most significant diseases of blueberry in these states. Symptoms on the upper surfaces of leaves appear as small, yellow spots that later turn necrotic as they enlarge and coalesce and eventually cover large areas of individual leaves. On the undersides of leaves, small flecks surrounded by small water-soaked halos appear, turn yellow, and produce powdery sori that are uredinia with urediniospores. Uredinia were hypophyllous, scattered to gregarious and at times superficially appearing confluent, up to about 300 μm in diameter, dome shaped and peridium hemispherical in cross section, orangish, becoming pulverulent, lacking obviously enlarged, well-differentiated ostiolar cells. Urediniospores were subglobose, obovate, oblong or ellipsoid, 17.6 to 27.2 × 12.8 to 17.6 μm, with hyaline, echinulate walls that are 1.2 to 1.8 μm thick, and with yellow-to-hyaline contents. Telia were not observed. On the basis of uredinial morphology (3,4), the rust was identified as Thekopsora minima P. Syd. & Syd. To distinguish this rust from other rust species causing disease on Vaccinium (2,3), a 1,414-bp region consisting of ITS2 and the 5' end of the 28S was amplified with primers Rust2inv/LR6 from uredinial lesions on infected leaves of V. corymbosum 'Biloxi' and sequenced (BPI 880580; GenBank Accession No. HM439777) (1). Results of a BLAST search of GenBank found 100% (1,414 of 1,414) identity to T. minima (GenBank Accession No. GU355675) from South Africa (3). Pathogenicity tests were completed as follows: (i) during the autumn of 2009, rusted leaves of cvs. Biloxi and Sharpblue were collected from the field; (ii) mature leaves from healthy plants of both blueberry cultivars were surface disinfested with 1% sodium hypochlorite for 2 min and rinsed with sterile distilled water; (iii) fresh urediniospores from rusted leaves were brushed directly onto the undersides of disinfested detached leaves; (iv) to avoid drying, wet cotton balls were placed on the petioles of inoculated leaves that were subsequently placed in resealable plastic bags; and (v) leaves were then incubated in a growth chamber at 22°C with a 12-h photoperiod. For each cultivar, 20 leaves were inoculated and five uninoculated leaves were included as controls and the test was repeated once. Yellow uredinia were observed 13 and 10 days after inoculation in cvs. Biloxi and Sharpblue, respectively. Leaf symptoms and uredinial characters were the same as observed previously in the field. To our knowledge, this is the first report of T. minima in Mexico. This report is significant for growers who need a diagnosis to control the disease and for breeders and plant pathologists who should consider developing more resistant cultivars. References: (1) M. C. Aime. Mycoscience 47:112, 2006. (2) F. L. Caruso and D. C. Ramsdell, eds. Compendium of Blueberry and Cranberry Diseases. The American Phytopathological Society, St. Paul, MN, 1995. (3) L. Mostert et al. Plant Dis. 94:478, 2010. (4) P. Sydow and H. Sydow. Monographia Uredinearum. Vol. III. Fratres Borntraeger, Leipzig, Germany, 1915.