Wheat blast or ‘brusone’, caused by the ascomycetous fungus Magnaporthe oryzae B.C. Couch (synonym Pyricularia oryzae Cavara), was first identified in 1985 in Brazil. Magnaporthe oryzae is composed of a range of morphologically identical but genetically different host-specific pathotypes that are specialized for infecting rice (Oryza pathotype), wheat (Triticum pathotype - MoT), perennial and annual ryegrass (Lolium pathotype), foxtail millet (Setaria pathotype) and many other graminaceous hosts. Isolates from different hosts are genetically distinct, although cross infection occurs to some extent. Wheat blast has become a serious biotic constraint to wheat (Triticum aestivum L) production in parts of the warmer wheat growing areas of the Southern Cone region of South America causing yield losses of 10 to 100% in recent years (Duveiller et al. 2016). Wheat blast was observed for the first time outside of South America during the 2015-16 cropping season in the districts of Kushtia, Meherpur, Chuadanga, Jhe...

The ToxA–Tsn1 system is an example of an inverse gene-for-gene relationship. The gene ToxA encodes a host-selective toxin (HST) which functions as a necrotrophic effector and is often responsible for the virulence of the pathogen. The genomes of several fungal pathogens (e.g., Pyrenophora tritici-repentis, Parastagonospora nodorum, and Bipolaris sorokiniana) have been shown to carry the ToxA gene. Tsn1 is a sensitivity gene in the host, whose presence generally helps a ToxA-positive pathogen to cause spot blotch in wheat. Cultivars lacking Tsn1 are generally resistant to spot blotch; this resistance is attributed to a number of other known genes which impart resistance in the absence of Tsn1. In the present study, 110 isolates of B. sorokiniana strains, collected from the ME₅A and ME₄C megaenvironments of India, were screened for the presence of the ToxA gene; 77 (70%) were found to be ToxA positive. Similarly, 220 Indian wheat cultivars were screened for the presence of the Tsn1 gene; 81 (36.8%) were found to be Tsn1 positive. When 20 wheat cultivars (11 with Tsn1 and 9 with tsn1) were inoculated with ToxA-positive isolates, seedlings of only those carrying the Tsn1 allele (not tsn1) developed necrotic spots surrounded by a chlorotic halo. No such distinction between Tsn1 and tsn1 carriers was observed when adult plants were inoculated. This study suggests that the absence of Tsn1 facilitated resistance against spot blotch of wheat. Therefore, the selection of wheat genotypes for the absence of the Tsn1 allele can improve resistance to spot blotch.