Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2011
Publication Date: 5/29/2011
Citation: Ramirez-Mendoza, M.R., Rebollar-Alviter, A., Minnis, A.M., Dixon, L.J., Castlebury, L.A., Valdovinos-Ponce, G., Silva-Rojas, H.V. 2011. First report of leaf rust of blueberry caused by Thekopsora minima in Mexico. Plant Disease. 95:772. Interpretive Summary: Rust fungi are parasites that cause considerable damage to crop plants throughout the world. As more countries grow new crops, new diseases are appearing. Mexico is now producing blueberries using cultivars developed in the United States. Recently it was discovered that these cultivars when grow in Mexico are susceptible to a disease referred to as blueberry rust. Studies of the unique structures and the genes of this rust were used to determine the accurate identification of the organism causing this disease. Although known in the United States, this rust has not been reported previously in Mexico. Plant pathologists will use knowledge of this fungus to determine how to breed for resistance and control this disease of blueberry.
Technical Abstract: Blueberry (Vaccinium corymbosum L.) is becoming an important crop in the states of Jalisco and Michoacan in Mexico. As the area under blueberry cultivation increases, new diseases causing severe losses are appearing. Leaf rust is one of the most destructive diseases of blueberry in Mexico. Sori on the undersides of leaves have been observed for more than three years, but the causal agent was unknown. Symptoms on the upper surfaces of leaves appear as small yellow spots that become necrotic while developing into numerous large, discrete spots that cover large areas of individual leaves. On the underside of leaves, small flecks surrounded by small water-soaked halos appear that turn yellow and produce powdery sori that proved to be uredinia with urediniospores. Extensive defoliation has been observed on plants with severe infections. Uredinia are hypophyllous, scattered to gregarious and at times appearing confluent, up to about 300 µm diam., dome-shaped and peridium hemispherical in cross section, orangish, becoming pulverulent, lacking obviously enlarged, well-differentiated ostiolar cells. Urediniospores are 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 with yellow to hyaline contents. Telia were not observed. Based on uredinial morphology the rust was identified as Thekopsora minima P. Syd. & Syd. Since this rust may be easily confused with other rust species on Vaccinium, the ITS2 region and 5’ end of the 28S rDNA consisting of 1414 bp were sequenced from infected leaves of V. corymbosum ‘Biloxi’ (BPI 880580) with the resulting sequence (GenBank Accession No. HM439777) identical to T. minima (GenBank Accession No. GU355675) from South Africa. 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 breeders who should consider more resistant cultivars.