Two organisms cause the diseases referred to as powdery mildew: Leveillula taurina and Oidium neolycop-ersici. These diseases are primarily found in greenhouse production sites, but can also be found in the field on a wide range of crops. Symptoms begin on the upper leaf surfaces as yellow lesions which later develop into bright yellow irregular patches. From the lesions, necrotic areas develop, engaging the entire leaf. The two fungal species can be distinguished by the way in which the mycelium grows on the leaf (Lindhout et al. 1994). A dominant resistance gene to L. taurica, Lv, was identified in S. chilense (Yordanov et al. 1975) and mapped to chromosome 12 between CT121 and CT129. Proprietary markers are being developed to assist incorporating this resistance gene into cultivars (Scott and Gardner 2007). Several resistance genes have been found to the Oidium species (Lindhout et al. 1994; Mieslerova et al. 2000). Two incompletely dominant genes, Ol-l and Ol-3, located on chromosome 6 near the Mi gene were introduced from two S. habrochaites accessions and SCAR markers are available (Huang et al. 2000). Chromosome 4 carries the recessive gene, ol-2, and CAPS and AFLP markers are available for MAS (De Giovanni et al. 2004). Two additional genes Ol-4 and Ol-5 were identified on chromosome 6 (Bai et al. 2003; Bai 2004b). Three QTLs linked to quantitative genes have been identified by Bai et al. (2003). One was located in the same region as Ol-1 and Ol-3 on chromosome 6 and the other two on chromosome 12 near Lv.
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