Tan spot

Pyrenophora tritici repentis

Pyrenophora tritici-repentis is an ascomyecetous fungus, order Pleosporales. The ascocarps - ellipsoidal or globoid pseudothecia (250-700 µm) with central beak (50-120 x 100-150 µm, ostiole 25-50 µm) and setae (40-250 x 5-12 µm) - are reported from overwintered culms, sheaths, leaves and stubble. Bitunicate asci (claviforme, then cylindrical, 120-200 x 35-55 µm, with 4-8 overlapping, linearly biseriate spores) are produced among numerous pseudoparaphyses (2-4 µm wide). The 3-septate ascospores are cylindrical, ellipsoidal, straight, 40-60 x 15-25 µm, transversely, yellowish-brown, and surrounded by a gelatinous sheath 2-4 µm thick.

Tan spot of wheat includes two distinct symptoms: necrosis (tan) and chlorosis. The first symptoms on wheat leaves are well defined, yellow, elliptical, 0.5-2.0 cm x 2-4 mm, spots that soon turn tan or gray in the centre. This is followed by leaf necrosis progressing from the tip. Elongated chlorosis can occur on some cultivars with some strains. The expression of necrosis and chlorosis in response to infection by P. tritici-repentis is the result of specific interactions between individual isolates and host genotypes.
On ears, the pathogen may cause pink grain, red smudge or - in association with other fungi - black point, however, glumes are not infected.

A pathotype classification system was established on the basis of the induction of necrosis (nec) and/or chlorosis (chl) by individual isolates. Necrotic 'tan' lesions may be due to the production of a host selective proteinaceous toxin (Ptr ToxA); nec- isolates lack the Ptr ToxA encoding gene. Ptr ToxB causes chlorosis in wheat genotypes, which develop chlorosis to race 5 isolates; the activity of the toxin is light-dependent. Both toxins are required for the establishment of compatible interactions with susceptible wheat genotypes.

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