Pyrenophora tritici-repentis

Pyrenophora tritici-repentis (teleomorph) and Drechslera tritici-repentis (anamorph) is a necrotrophic plant pathogen of fungal origin, phylum Ascomycota. The pathogen causes a disease originally named yellow spot but now commonly called tan spot, yellow leaf spot, yellow leaf blotch or helminthosporiosis. At least eight races of the pathogen are known to occur based on their virulence on a wheat differential set. The tan (yellow) spot fungus was first described by Nisikado in 1923 in Japan. and was later identified in Europe, Australia and the US, in the mid 1900s. The disease is one of the most important fungal disease on wheat and the fungal pathogen is found to infect in all parts of the world wherever wheat and other susceptible host crops are found. P. tritici-repentis overwinters on stubble, and due to recent heavily no-till/residue retention cultural practices, increased incidence and yield loss of up to 49% has been witnessed if ideal conditions occur. It forms characteristic, dark, oval-shaped spots of necrotic tissue surrounded by a yellow ring. It is responsible for losses that account for up to 30% of the crop, due to its effects reducing photosynthesis. Pathogenesis and toxicity in P. tritici-repentis is controlled by a single gene, transformations of this gene cause the pathogen to become benign when interacting with wheat. This has major implications for those in agriculture seeking to combat the effects of this fungus.

from Wikipedia