Disease Name: Tan spot or yellow spot.

Pathogen:
Pyrenophora triticirepentis (Died.) Drechs.
(anamorph Drechslera tritic -repentis (Died.) Shoemaker).
The fungus is an homothallic ascomycete.

Symptoms:

• On susceptible wheat leaves the tan spot fungus produces oval or diamond-shaped to elongate irregular lesions (spots) that enlarge and develop a tan color with a yellow border and a small dark brown spot near the center.
• Individual isolates of P. tritici-repentis could induce in specific wheat lines or cultivars, either necrosis (death of tissue) or extensive chlorosis (yellowing) symptom independently or together.
• Based on the production of necrosis and or chlorosis associated with the production of at least three host-specific toxins up to 8 races were identified using on a set of differentials.
• Initially the fungus develops yellow-tan flecks, 0.5-2.0 cm x 2-4 mm in size on both upper and lower leaf surfaces.
• These lesions later expand into lens-shaped tan blotches up to 12 mm long, containing a small dark brown spot near the center, surrounded by a yellow border.
• As a lesion matures the centre becomes darker due to the production of air-borne olive-brown conidia which can infect other wheat plants, specifically the leaves, stems and seeds.
• The tan spot fungus can infect the spike and cause discoloration of wheat glumes and seeds.
• Tan spot disease is sometimes associated with other fungi (C. sativus causing spot blotch) in wheat.

Confirmation:

• Similarities between young lesions caused by spot blotch, tan spot and glume blotches (Stagonospora nodorum) in wheat may cause confusion. Septoria trtici blotch is easily recognized with a hand-lens by the production of pycnidia.
• Most of the time incubation of lesions in a humid chamber and a microscope observation are needed.
• Tan spot lesions can be confirmed by recognizing the conical, snake’s head shaped basal cell of the single cylindrical conidia (12-21 μm x 45-200 μm) which have 1-9 septa (commonly 5-7) and the black, erect and simple conidiophores that are 100-300 μm x 7-8 μm in size, single or in small groups.
• On straw, after overwintering, the teleomorph can be observed as pin-head sized, raised, fruiting bodies (pseudothecia) containing the ascospores. This is a good indicator for the presence of the disease in the field but itcan only been observed on straw from the former season.

Why and where it occurs:

• Tan spot occurs worldwide In some areas, it is the major leaf spot disease affecting spring wheat and durum wheat.
• The fungus tolerates cool temperatures and is found in highland, mountainous regions.
• The tan spot fungus survives winter on wheat plant residues. Thus, the disease is oftern found in association with zero tillage in areas were stubbles are remaining on the soil surface until the next wheat season. Ascospores develop through the winter and are discharged as the primary inoculum during the wheat growing season.

Causal agent or factors:

• P. tritici-repentis is a non specific pathogen surviving saprophytically on plant debris.
• The symptoms of necrosis and chlorosis caused by P. tritici-repentis is due to specific interactions between the plant host and individual isolates.
• Genetic studies with P. tritici-repentis have shown that for every resistant in the host plant there is a corresponding virulence gene in the fungus that will allow the fungus to overcome the host plant’s defense.
• There are at least 8 races in tan spot fungi based on the production of host-specific toxin(s) but the practical importance of this on disease severity in the field has to be further studied since resistance to the fungus and the toxin cant differ.

Host range:
Major hosts include: Triticum aestivum (bread wheat), Triticum turgidum (durum wheat), Secale cereale (rye), and wild grasses.

Life cycle:

Mechanism of damage:

• The pathogen is seedborne and this is very often overlooked or ignored.
• Wild grasses can be a reservoir of inoculum.
• The pathogen survives between wheat cropping seasons on wheat stubble. Zin this case during the wheat growing season ascospores are discharged from perithecia and carried by wind to nearby wheat plants.
• At least 6 hours of leaf wetness is required for infection with temperatures of 15-28 °C and periods of dew.
• Two or three weeks after initial infection on leaves, necrotic tissue develops and produced conidia which are dispersed by wind to caused further secondary infection.
• Ideal conditions for secondary infection are: leaf wetness, high relative humidity and temperatures above10°C.
• Extended periods of dry weather more than 5 days inhibit disease progression.

When damage is important:

• Percentage yield losses are dependent on: weather and susceptibility of the wheat cultivars. Continuous wet weather during the wheat growing seasons, results in high concentration of conidia in the air causing severe tan spot epidemics affecting even moderately resistant varieties.
• The diseases reduce the photosynthetic area of leaves resulting in reduced grain filling and lower yields particularly when the flag leaves are severely infected during heading.

Economic importance:

Yield losses caused by tan spot may reach as high as 30 - 40% but generally range from 3 - 15%.

Management principles:

• Crop rotations that avoid planting of wheat into cereal stubble may reduce disease incidence, rotation with non-cereal crops is beneficial in reducing the risk of early infection on young wheat plants.
• Plowing-in of wheat residue into the soil reduces the amount of surface straw that can produce air-borne spores during the growing season. The process also aids in decomposition of infected leaf matter and physically prevents air-borne spores from infecting young leaves. However, the practice of plowing in will not affect disease caused by spores blown in from other fields later on in the growing season.
• Varieties with moderate to intermediate resistance are available.
• If resistant genotypes are not available, application of a foliar fungicide to protect the flag leaf can reduce yield loss due to leaf spot diseases.
• Crop monitoring during the growing season is important in assessing the level of disease.

References:

Bergstrom, G.C. and A.M.C. Schilder. 1998. Seed pathology of tan spot. In E. Duveiller, H. J. Dubin, J. Reeves and A. McNab (eds.), Helminthosporium Blights of Wheat: Spot Blotch and Tan Spot. Mexico, D.F.: CIMMYT. Pp. 364-8.

Bockus, W.W. 1998. Control strategies for stubble-borne pathogens of wheat. Canadian Journal of Plant Pathology 20:371-5.

Ciuffetti, L.M. and R.P. Tuori. 1999. Advances in the characterization of the Pyrenophora tritici-repentis–wheat interaction. Phytopathology 89:444-9.

De Wolf, E.D., R.J. Effertz, S. Ali and L.J. Francl. 1998. Vistas of tan spot research. Canadian Journal of Plant Pathology 20:349-444.

Duveiller, E., H.J. Dubin, J. Reeves and A. McNab (eds.). 1998. Helminthosporium Blights of Wheat: Spot Blotch and Tan Spot. Mexico, D.F.: CIMMYT.

Duveiller, E. and H.J. Dubin. 2002. Helminthosporium leaf blights: spot blotch and tan spot. In B.C. Curtis, S. Rajaram and H. Gómez Macpherson (eds.), Bread improvement and production. FAO Plant Production and Protection Series. Rome: Food and Agriculture Organisation of the United Nations.

Duveiller, E., Y.R. Kandel, R.C. Sharma and S.M. Shrestha. 2005. Epidemiology of Foliar Blights (Spot Blotch and Tan Spot) of Wheat in the Plains Bordering the Himalayas. Phytophotology 95: 248-56.

Duveiller, E., R.C. Sharma, B. Çukadar and M. van Ginkel. 2007. Genetic analysis of field resistance to tan spot in spring wheat. Field Crops Research 101:62-7.

Francl, L. 1998. Components of the tan spot disease cycle. In E. Duveiller, H. J. Dubin, J. Reeves and A. McNab (eds.), Helminthosporium Blights of Wheat: Spot Blotch and Tan Spot. Mexico, D.F.: CIMMYT. Pp. 28-36.

McMullen, M. and L. Franci. 1993. Tan spot of wheat. PP-766. North Dakota State University Extension Service. Available online at http://www.ag.ndsu.edu/pubs/plantsci/smgrains/pp766w.htm.

Martinez, J.P., S.A. Ottum, S. Ali, L.J. Francl and L.M. Ciuffetti. 2001. Characterization of the ToxB gene from Pyrenophora tritici-repentis. Molecular Plant-Microbe Interactions 14:675-7.

Prescott, J.M., P.A. Burnett, E.E. Saari, J. Ransom, J. Bowman, W. de Milliano, R.P. Singh and G. Bekele. 1986. Wheat diseases and pests. A guide for field identification. Mexico, D.F.: CIMMYT.

Rasmussen, J.B., T.L. Friesen and S. Ali. 2003. Proceedings of the Fourth International Wheat Tan Spot and Spot blotch Workshop., North Dakota State University, Fargo, ND, USA.

Singh, P.K., Gonzalez-Hernandez, M. Mergoum, S. Ali, T.B. Adhikari, S.F. Kianian, E.M. Elias and G.R. Hughes. 2007. Identification and molecular mapping of a gene conferring resistance to P. tritici-repentis Race 3 in tetraploid wheat. Phytopathology 96:885-9.

Singh, P.K., M. Mergoum, S. Ali, T.B. Adhikari, E.M. Elias, J.A. Anderson, K.D. Glover and W.A. Berzonsky. 2006. Evaluation of elite wheat germplasm for resistance to tan spot. Plant Disease 90:1320-5.

Strelkov, S.E. and L. Lamari. 2003. Host-pathogen interaction in tan spot [Pyrenophora tritici-repentis] of wheat. Can. J. Plant Pathology 25:339-49.

Tuori, R.P., T.J. Wolpert and L.M. Ciuffetti. 1995. Purification and immunological characterization of toxic components from cultures of Pyrenophora tritici-repentis. Mol. Plant-Microbe Interact. 8:41-8.

Wegulo, S.N., R.N. Klein and R.M. Harveson. 2006. Tan spot disease of wheat. NebGuide G429. Lincoln: University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources. Available online at http://www.ianrpubs.unl.edu/epublic/live/g429/build/g429.pdf.

Wiese, M.V. 1987. Tan spot (yellow leaf spot). In M.V. Wiese (ed.), Compendium of wheat diseases. St. Paul, MN: The American Phytopathological Society (APS Press). Pp. 42-3.

Contributors: H. K. Buhariwalla, E. Duveiller, and P. Kosina