Pyrenophora tritici-repentis

Scientific Name Pyrenophora tritici-repentis (Died.) Drechsler [teleomorph=sexual form], Drechslera tritici-repentis (Died.) Shoemaker [anamorph=asexual form] = DTR
Synonyms P. sarcocystis (Berk. & M.A. Curtis) Ravenel [syn.], P. trichostoma (Fr.) Sacc. [syn.], P. tritici-vulgaris J.G. Dicks [syn.], Pleospora culmorum (Cooke) Sacc. [syn.], P. sarcocystis (Berk. & M.A. Curtis) Sacc. [syn.], P. trichostoma (Fr.) Fuckel [syn.], P. tritici-repentis Died. [syn.], D. tritici-vulgaris (Y. Nisik.) S. Ito [syn.], D. tritici-vulgaris (Y. Nisik.) S. Ito ex S. Hughes [syn.], Helminthosporium gramineum f.sp. tritici-repentis Died. [syn.], H. tritici-repentis Died. [syn.], H. tritici-vulgaris Y. Nisik. [syn.]
Common Names English: tan spot, yellow leaf spot of wheat; Spanish: Mancha amarilla; German: Weizenblattdürre; French: Tache auréolée
Description 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.
The Drechslera stage produces cylindrical, straight or slightly bent conidia (75-250 x 13-19 µm); thin septa (5-13) separate subequal cells 15-20 µm long; basal cell longer than wide, resembling a snake's head. The condiophores are formed on necroses single or in small groups; they are erect, straight, flexuous or geniculate, simple, light yellowish-brown with an often enlarged base, 10-16 µm wide; < 250 µm long.

Biology

Damage

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.

Lifecycle

P. tritici-repentis survives saprohytically on stems, sheaths and leaves of wheat. Ascospore formation starts in winter; after maturation they are forcibly discharged in spring and are responsible for infection in the lower canopy (primary inoculum in GS 22-33). Conidia are formed on the exterior of empty ascomata, on old stubble and leaf lesions and cause spreading of the disease to upper levels in the growth season (secondary inoculum). Production of conidia is linked to necrotic tissue and RH >95%. Secondary infection is favored by leaf wetness, high RH and temperature >10 °C for 2 days; the latent period is 6-7 days.

Cause

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.

Occurrence

Additional Crop Information

Pyrenophora tritici-repentis infects all cereal crops with Triticum species being most susceptible. The fungus is also pathogenic in Agropyron, Agrostis, Bromus, Calamagrostis, Cynodon, Elymus, Phalaris.

Agricultural Importance

Wheat kernels are infected through the glume, lemma and palea; kernels are highly susceptible from GS 75-85. In endemic regions, seed recovery rate of P. tritici-repentis commonly is 1 %.

Control

Integrated Crop Management

Reduced tillage, especially direct-seeding promotes the incidence and severity of tan spot by increasing the proportion of stubble on the soil surface. The number of ascomata is significantly reduced on straw with close contact to the soil and the number of ascomata increases with the height of standing stubble. The risk of infection decreases with the level of stubble decomposition which may be cultivar-specific. Stubble burning is the most effective control followed by incorporation of crop debris in the soil.
Major emphasis for control has been put on breeding for resistant cultivars. Decision support systems have been developed for optimum timing of fungicide applications. Sensors that measure moisture retention in infected straw are used to predict ascospore maturation.

Chemical Control

Azole and strobilurin-containing fungicides are effective in reducing including tan spot. Spray application of products containing azoles such as tebuconazole, fluquinconazole or prothioconazole as well as strobilurins fungicides such as trifloxystrobin and fluoxastrobin are suited to control the disease.
Due to the partly seed-borne origin of the disease also azole containing seed treatments show some effect.

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