1 3 Theor Appl Genet (2013) 126:2197–2217 DOI 10.1007/s00122-013-2157-y REVIEW Genetics of tan spot resistance in wheat Justin D. Faris · Zhaohui Liu · Steven S. Xu Received: 22 February 2013 / Accepted: 12 July 2013 / Published online: 25 July 2013 © Springer-Verlag Berlin Heidelberg (outside the USA) 2013 P. tritici-repentis are likely to become increasingly signifi- cant under a changing global climate making it imperative to further characterize the wheat–P. tritici-repentis pathosystem and develop tan spot resistant wheat varieties. Introduction Tan spot, also known as yellow spot, is caused by the fun- gus Pyrenophora tritici-repentis (Died.) Drechs. (anamo- rph: Drechslera tritici-repentis (Died.) Shoem.) and affects all forms of cultivated wheat including both durum (Triti- cum turgidum L., 2n = 4x = 28, AABB genomes) and common wheat (T. aestivum L., 2n = 6x = 42, AABBDD genomes). Tan spot occurs worldwide and is a significant disease nearly everywhere wheat is cultivated. The fungus overwinters in stubble residue, thus the practice of retaining residue and no-till farming to reduce soil erosion has led to an increase in tan spot incidence. The fungus causes large, tan-colored lesions often surrounded by chlorotic haloes in susceptible genotypes, and in highly susceptible geno- types, the lesions tend to coalesce leading to large areas of dead leaf tissue (Fig. 1). Lesion development results in a decreased capacity for photosynthesis, which translates into plant stress and ultimately yield loss. Studies on yield losses attributed to tan spot have indi- cated that the highest yield reductions are observed when tan spot occurs on older plants, such as the boot and flow- ering stages, as opposed to when tan spot only occurs on juvenile plants (Rees and Platz 1983; Shabeer and Bockus 1988; De Wolf et al. 1998 for review). Tan spot-induced yield reductions are primarily attributed to reduction in kernel weight and the number of grains per head (Shabeer and Bockus 1988), and also through reductions in the num- ber of tillers, dry matter accumulation, leaf area index, and Abstract Tan spot is a devastating foliar disease of wheat caused by the necrotrophic fungal pathogen Pyrenophora tritici-repentis. Much has been learned during the past two decades about the genetics of wheat–P. tritici-repentis inter- actions. Research has shown that the fungus produces at least three host-selective toxins (HSTs), known as Ptr ToxA, Ptr ToxB, and Ptr ToxC, that interact directly or indirectly with the products of the dominant host genes Tsn1, Tsc2, and Tsc1, respectively. The recent cloning and characterization of Tsn1 provided strong evidence that the pathogen utilizes HSTs to subvert host resistance mechanisms to cause dis- ease. However, in addition to host–HST interactions, broad- spectrum, race non-specific resistance QTLs and recessively inherited qualitative ‘resistance’ genes have been identi- fied. Molecular markers suitable for marker-assisted selec- tion against HST sensitivity genes and for race non-specific resistance QTLs have been developed and used to generate adapted germplasm with good levels of tan spot resistance. Future research is needed to identify novel HSTs and corre- sponding host sensitivity genes, determine if the recessively inherited resistance genes are HST insensitivities, extend the current race classification system to account for new HSTs, and determine the molecular basis of race non-specific resist- ance QTLs and their relationships with host–HST interac- tions at the molecular level. Necrotrophic pathogens such as Communicated by R. K. Varshney. J. D. Faris (*) · S. S. Xu USDA-ARS Cereal Crops Research Unit, Northern Crop Science Laboratory, 1605 Albrecht BLVD, Fargo, ND 58102-2765, USA e-mail: justin.faris@ars.usda.gov Z. Liu Department of Plant Pathology, Walster Hall, North Dakota State University, Fargo, ND 58102, USA