Scald (barley)

Scald is common disease of barley in temperate regions. It is caused by the fungus Rhynchosporium secalis and can cause significant yield losses in cooler, wet seasons.^[1][2]

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Symptoms

Scald is a foliar disease of barley affecting the leaves and sheaths of the plant, however, lesions may also occur on coleoptiles, glumes, floral bracts and awns. Initial sympotms are oval, water-soaked, grayish-green spots, 1.0-1.5 cm long. As the disease develops, the centers of the lesions dry and bleach, becoming light gray, tan, or white. with a dark brown margin. The lesions are not delimited by the leaf veins and often coalesce.

Disease cycle

The fungus can infect and survive in barley seed. It exists as mycelium in the pericarp and hull of infected seeds. Infection of the coleoptile occurs as it emerges from the embryo. Optimal infections occurs at soil temperatures of 16C. At soil temperatures of 22C or higher, very little infection occurs.

In spring cropping systems, the fungus overwinters on the crop debris and stubble of previous diseased barley crops. The fungus produces abundant conidia on wet lesions during cool, damp weather after the leaf tissue has become necrotic. Conidia, spread by wind and splashing rain, infect young leaves of spring- planted grain. Optimum temperatures for sporulation and infection range from 10-18C. Hot, dry weather reduces the rate of disease development.

Geographical distribution and economic importance

The disease is economically important barley disease in Europe, North America and Australia. It has been reported from South America, Africa, the Middle east, Japan and Korea. Yield losses as high as 35-40% have been reported, however, losses of 1-10% are more common. Yield loss is primarily due to reduced kernel weight, but both kernels per head and number of heads per plant may also been affected.

In the wetter areas of the United Kingdom, scald is the most damaging disease of barley, affecting both spring and winter crops.^[3]. In the Victoria area of Australia, scald is widespread in barley crops in most seasons, but its severity varies greatly from crop to crop and between seasons. In Victoria, scald causes annual average yield losses of 10-20%, with individual losses as high as 45% in susceptible varieties.^[4] In Canada, yield losses in Alberta have been calculated at 2.4 per cent, although losses in particular fields may exceed 25 per cent. Losses are due to a decrease in photosynthetic area on the flag and second leaves resulting in reduced seed weight.^[5]

Management

Management of the disease involves the use of clean and/or treated seeds, resistant cultivars, crop rotation, residue management, and foliar fungicides.

Resistant cultivars

Cultivars with scald resistance are available in all major barley growing areas. The level of resistance occurs along a spectrum and the degree of resistance that is required to effectively control the disease will depend on the region where it is grown, cropping practices that reduce initial disease inoculum, wetter conditions and fungal pathotypes.

Fungicides

Foliar fungicides can be used to effectively control disease development. Product selection, application rates and timing, depend upon numerous factors.

The availability of fungicides as a management tool depends on whether the product has been evaluated and registered for use in a specific country or region.

Foliar fungicides: azoxystrobin¹, bromuconazole¹, cyproconazole¹, epoxiconazole¹, fluquinconazole¹, flusilazole¹, propiconazole^{1, 2, 3}, prochloraz¹, pyraclostrobin²,tebuconazole¹

Seed treatment fungicides: triadimenol²

1. EPPO - Europe
2. Canada: Alberta
3. US: Oregon

Fungicide resistance

In the United Kingdom, there are issues concerning MBC fungicides and some of the triazole fungicides (e.g. flusilazole and epoxiconazole). To date there are no issues with strobilruin fungicides (QoI fungicides), but the situation is being monitored closely. There are currently no issues concerning chlorothalonil, cyprodinil or the morpholines.^[3]

Research articles

Abang, M.M.; M. Baum, S. Ceccarelli, S. Grando, C.C. Linde, A. Yahyaoui, J. Zhan, B.A. McDonald. (2006). "[[1] Differential selection on Rhynchosporium secalis during parasitic and saprophytic phases in the barley scald disease cycle]". Phytopathology 96: 1214-1222.

Bjørnstad, Å.; V. Patil, A. Tekauz, A.G. Marøy, H. Skinnes, A. Jensen, H. Magnus, J. MacKey (2002). "Resistance to scald (Rhynchosporium secalis) in barley (Hordeum vulgare) studied by near-isogenic lines: I. Markers and differential isolates.". Phytopathology 92: 710-720.

Fountaine, J.M.; M.W. Shaw, B. Napier, E. Ward, B.A. Fraaije. (2007). "Application of Real-Time and Multiplex Polymerase Chain Reaction Assays to Study Leaf Blotch Epidemics in Barley". Phytopathology 97: 297-303.

Goodwin, S.B.; R.K. Webster, R.W. Allard (1994). "Evidence for Mutation and Migration as Sources of Genetic Variation in Populations of Rhynchosporium secalis". Phytopathology 84: 1047-1053.

Hahn, M. (1993). "Cultivar-specific elicitation of barley defense reactions by the phytotoxic peptide NIP1 from Rhynchosporium secalis". MPMI 6: 745-754.

Lee, H. K.; J.P. Tewari, T.K. Turkington (2000). "A PCR-based assay to detect Rhynchosporium secalis in barley seed". Plant Dis. 85: 220-225.

Lyngsjorgensen, H.J.; V. Smedegaard-Petersen (1995). "Pathogenic variation of Rhynchosporium secalis in Denmark and sources of resistance in barley". Plant Dis. 79: 297-301.

McDonald, B.A.; J. Zhan, J.J. Burdon (1999). "Genetic structure of Rhynchosporium secalis in Australia". Phytopathology 89: 639-645.

Salamati, S.; J. Zhan, J.J. Burdon, B.A. McDonald (2000). "The genetic structure of field populations of Rhynchosporium secalis from three continents suggests moderate gene flow and regular recombination". Phytopathology 90: 901-908.

Steiner-Lange, S.; A. Fischer, A. Boettcher, I. Rouhara, H. Liedgens, E. Schmelzer, W. Knogge (2003). "Differential defense reactions in leaf tissues of barley in response to infection by Rhynchosporium secalis and to treatment with a fungal avirulence gene product". MPMI 16: 893-902.

Schürch, S.; C.C. Linde, W. Knogge, L.F. Jackson, B.A. McDonald (2004). "Molecular population genetic analysis differentiates two virulence mechanisms of the fungal avirulence gene NIP1". MPMI 17: 1114-1125.

Tekauz, A. (1991). "Pathogenic variation in Rhynchosporium secalis on barley in Canada". CJPP 13: 298-304.

Xi, K.; P.A. Burnett, J.P. Tewari, M.H. Chen, T.K. Turkington, J.H. Helm (1999). "Histopathological study of barley cultivars resistant and susceptible to Rhynchosporium secalis". Phytopathology 90: 94-102.

Xi, K.; T.K. Turkington, J.H. Helm, C. Bos (2002). "Pathogenic variation of Rhynchosporium secalis in Alberta". Can. J. Plant Pathol. 24: 176–183.

Xi, K.; A.G. Xue, P.A. Burnett, J.H. Helm, T.K. Turkington (2000). "Quantitative resistance of barley cultivars to Rhynchosporium secalis". CJPP 22: 217-223.

Xi, K; T.K. Turkington, J.H. Helm, K.G. Briggs, J.P. Tewari, T. Ferguson, P.D. Kharbanda (2003). "Distribution of pathotypes of Rhynchosporium secalis and cultivar reaction on barley in Alberta". Plant Dis. 87: 391-396.

Xue, A.G.; P.A. Burnett, J. Helm, B.G. Rossnagel (1995). "Variation in seedling and adult-plant resistance to Rhynchosporium secalis in barley". CVJPP 17: 46-48.

Xue, A.G.; R. Hall (1991). "Components of parasitic fitness in Rhynchosporium secalis and quantitative resistance to scald in barley as determined with a dome inoculation chamber". CJPP 13: 19-25.

Zareie, Reza; D.L. Melanson, P.J. Murphy (2002). "Isolation of fungal cell wall degrading proteins from barley (Hordeum vulgare L.) leaves infected with Rhynchosporium secalis". MPMI 15: 1031-1039.

References

1. ^ Mathre, D.E. (1997). Compendium of barley diseases. American Phytopathological Society, 120 pp. 
2. ^ Martens, J.W.; W.L. Seaman, T.G. Atkinson (1984). Diseases of field crops in Canada. Canadian Phytopathological Society, 160 pp. 
3. ^ ^{a b} Europe: Scotland
4. ^ Australia
5. ^ Alberta yield losses