Disease Name: Septoria tritici blotch

Pathogen:

• Mycosphaerella graminicola infects the leaf of both hexaploid and tetraploid wheat, although it can also infect spikes.
• Septoria tritici blotch lesions initially appear as small yellow flecks mainly on lower leaves closest to the soil. Flecks expand into irregular red-brown lesions 1-5 mm by 4-15mm. Later yellow to reddish-brown or grayish-white to brown spot or blotches may develop on all above groung parts of the plants.
• S. tritici lesions are at first restricted by the veins and tend to develop longitudinally along the leaves, often appearing parallel-sided with rather blunt ends.
• The elongated blotches range up to 5 x 15 mm and eventually coaleasce.
• Mycosphaerella graminicola infects lower leaves and gradually progresses up to the flag leaf.
• Conspicuous pycnidia (spore-producing structures) of S. tritici are dark black and circular and one-third smaller than those of Stagonospora nodorum (syn. Septoria nodorum) another pathogen causing similar foliar lesions and the disease known as glume blotch.
• Leaves infected at the base of the plants may be killed.
• Epidemics can result in yield loss of 25 to 50%, with the resulting seed being shriveled and unfit for milling.

Confirmation:

• Septoria tritici blotch lesions are more or less angular at an early stage and pycnidia are visible to the naked eye, as they are black and are superficially embedded in the leaf tissue.
• During wet weather S. tritici conidia are oozed from the pycnidia in milky white to buff cirrhi (gelatinous spore masses), whilst the cirrhi of S. nodorum are pinkish.

Why and where it occurs:

• Both Septoria diseases occur worldwide wherever wheat is grown in areas with high rainfall but the distribution of both species may differ geographically and based on varieties and climate.
• Infection by Mycosphaerella graminicola requires cool temperatures (15-25 °C) with wet and windy weather conditions. In contrast, infections of S. nodorum occur over a wider temperature range ideally between 20 to 27 °C with wet and windy weather conditions.
• Under ideal conditions infection can occur at any stage of the plant development.
• At least 6 hrs of wetness is required for infections to occur with Mycosphaerella graminicola.
• The presence of wheat stubble, volunteer wheat, susceptible grasses and diseased seeds sustains survival of the disease from year to year. These become sources of the inoculum for infection of the new wheat crop.
• Spores from the pycnidia and sexual spores (ascospores), initiate the disease on wheat in the fall.
• The sexual stage is not found easily everywhere but ascospores are a major source of inoculum, and sexual recombination in nature occurs many times per growing season.
• M. graminicola isolates of different mating types are found but different sexual organs also exist. Spermagonia produce the male spermatia, and ascogonia produce the female trichogynes.
• Pycnidia are produced in abundance on necrotic leaves.
• Septoria tritici conidia are readily dispersed within the wheat canopy by wind and rain splash.

Causal agent or factors:

• Mycosphaerella graminicola and S. nodorum infections may occur individually either early or later in the season respectively, but frequently occur together in the same field and on the same plants.
• When one or both fungi infect the leaves they are referred to as the septoria leaf blotch complex or septoria complex.
• Genetic studies on Septoria spp. have shown that there is a broad range of virulence genes in the fungi and some of which can be associated to specific geographical regions.
• Resistance to STB may be isolate-specific or quantitative. Isolate-specific resistance is near-complete, oligogenic and follows a gene-for-gene relationship. Whereas quantitative or partial resistance is incomplete, polygenic and isolate nonspecific.
• Eight resistance genes identified in bread wheat cultivars in recent years, but the targeted incorporation of these genes is still underway.
• In Europe, the use of strobilurin fungicides has been a widespread failure leading to resistance of Mycosphaerella graminicola isolates.

Host range:

Major hosts include: Triticum aestivum (bread wheat), Triticum turgidum (durum wheat), triticale, Hordeum vulgare (barley), Secale cereale (rye) and wild grasses.

Life cycle:

Mechanism of damage

• The pycnidiospores of Mycosphaerella graminicola germinate on the wet leaf surface and enter the leaf tissue through natural openings (stomata) or directly through the epidermal tissue. Moisture is required for all stages of infection: germination, penetration, mycelial development and formation of pycnidia. The fungus produces toxic compounds that kill plant cells around the infection points.

When damage is important:

• Percentage yield losses are dependent on susceptibility of the wheat cultivar, weather conditions (wet and windy with moderate temperatures), and amount inoculum on diseased stubble, volunteer wheat or use of diseased seeds.
• Dry weather tends to slow or completely stop disease development.

Economic importance:

The disease is economically very important in many wheat-producing areas of the world particularly where high rainfall occurs. Losses can be very high and may reach 50% if fungicides are not used in susceptible genotypes.

Management principles:

• Practices such as crop rotation can reduce the incidence but fungicide may need to be used if infections are severe and varieties susceptible.
• Crop monitoring during the growing season is important to assess the level of disease damage i.e. if the second leaf shows 0-5% and/or the third leaf shows 10-20% infection, then the application of a foliar fungicide is recommended.
• The practice of plowing-in of stubble into the soil after the wheat harvest can speed up the decomposition of infected stubble, thereby limiting the amount of Septoria spores released from the residue.
• Avoid the planting of susceptible varieties.
• Although seed transmission of M. graminicolla is not often considered as a major source of inoculum unlike for S. nodorum, seed treatment may reduce disease severity.
• In some areas, M. graminicolla survival on wild grasses is not negligible.

References:

Arraiano, L.S., P.A. Brading, F. Dedryver and J.K.M. Brown. 2006. Resistance of wheat to septoria tritici blotch (Mycosphaerella graminicola) and associations with plant ideotype and the 1BL-1RS translocation. Plant Pathology 55:54–61.

Arraiano, L.S. and J.K.M. Brown. 2006. Identification of isolate-specific and partial resistance to septoria tritici blotch in 238 European wheat cultivars and breeding lines. Plant Pathology 55:726–38.

Chartrain, L., P.A. Brading, J.C. Makepeace and J.K.M. Brown. 2004. Sources of resistance to septoria leaf blotch and implications for wheat breeding. Plant Pathology 53:454–60.

Chartrain, L., P.A. Brading, J.P. Widdowson and J.K.M. Brown. 2004. Partial resistance to septoria tritici blotch (Mycosphaerella graminicola) in wheat cultivars Arina and Riband. Phytopathology 94:497–504.

Eyal, Z., A.L. Scharen, J.M. Prescott and M. van Ginkel. 1987. The Septoria diseases of wheat: Concepts and methods of disease management. Mexico, D.F.: CIMMYT.

Gilchrist, L., B. Gomez, R. Gonzalez, S. Fuentes, A. Mujeeb-Kazi, W. Pfeiffer, S. Rajaram, R. Rodriguez, B. Skovmand, M. van Ginkel and C. Velazquez. 1999. Septoria tritici resistance sources and breeding progress at CIMMYT, 1970–99. In M. van Ginkel, A. McNab and J. Krupinsky (eds.), Septoria and Stagonospora Diseases of Cereals: a Compilation of Global Research. Mexico, D.F.: CIMMYT. Pp. 134–9.

Gilchrist, L. and H.J. Dubin. 2002. Septoria diseases of wheat. 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.

Goodwin, S.B., B.A. McDonald and G.H.J. Kema. 2003. The Mycosphaerella sequencing initiative. In G.H.J. Kema, M. van Ginkel and M. Harrabi (eds.), Global Insights into the Septoria and Stagonospora Diseases of Cereals: Proceedings of the Sixth International Symposium on Septoria and Stagonospora Diseases of Cereals. Tunis, Tunisia. Pp. 149–51.

Henze, M., M. Beyer, H. Klink and J.-A. Verreet. 2007. Characterizing meteorological scenarios favorable for Septoria tritici infections in wheat and estimation of latent periods. Plant Dis. 91:1445-9.

Kema, G., K. Verstappen, C.P. Els and C. Waalwijk. 2000. Avirulence in the Wheat Septoria tritici Leaf Blotch Fungus Mycosphaerella graminicola Is Controlled by a Single Locus. MPMI 13(12):1375–9.

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