Barley scald is caused by the fungus Rhynchosporium commune (formerly known as R. secalis ) and is common in Victorian barley crops. Its severity varies between crops and seasons, being more prevalent in high rainfall areas and wet seasons. Scald is also favoured by an early break to the season. Grain yield losses due to scald are estimated to be between 10-30 percent in susceptible varieties, while individual losses as high as 45 percent have been recorded.
The disease causes lesions of the leaf blades and sheaths. At first, scald appears as water-soaked, grey- green lesions, which change to a straw colour with a brown margin that are ovate to irregular in shape, (Figure 2.26). In severe infections, the disease may virtually cause defoliation by coalescing of the lesions, (Figure 2.27).
Figure 2.26 Scald of barley. Early water-soaked, grey- green symptoms compared to later straw colour lesions with a distinctive brown margin
Figure 2.27 Severe scald of barley: Note the scald-like lesions can coalesce and cause complete leaf loss
Rhynchosporium commune (Scald) primarily survives over summer on stubble of infected plants. During wet stages of the growing seasons, spores are produced on the stubble and are dispersed by rain splash into the new season’s crop, where they start the primary infection, (Figure 2.28). Scald is usually first observed in isolated patches in the crop when plants are tillering. Further spread is caused by splash dispersal of spores which is more rapid in the warmer months. By the end of the growing season scald can be found throughout the crop with distinct hotspots. The disease is most severe in seasons of above average rainfall, particularly during the winter/spring.
Scald can also be seed-borne, can infect barley grass and survive on volunteers. These sources are not as important as infected stubble but can be an inoculum source for barley crops, especially during seasons with favourable climatic conditions.
Figure 2.28 Disease cycle of barley scald. Illustration by Kylie Fowler
Carry-over of scald inoculum can be reduced by grazing, burning or cultivation of stubble, volunteers and barley grass, however, these practices do not eliminate the disease altogether as scald will survive on small amounts of remaining residue. Rotations that avoid consecutive barley crops and ideally a two year or more break between barley crops is recommended to allow residue to sufficiently breakdown. Scald is also more severe in early sown crops, so avoiding early sowing of susceptible varieties, especially in high rainfall areas will reduce the loss caused by scald.
Cultivation of resistant varieties gives the best control of scald, with the risk of grain yield and quality loss being greatly reduced by avoiding growing susceptible and very susceptible rated varieties. Unfortunately R. commune is highly variable pathogenically and able to overcome resistances rapidly, meaning that variety ratings may also change frequently. It is important to check the current Victorian Cereal Disease Guide (AG 1160) for the resistance status of varieties.
There are a range of fungicides available that will provide suppression of scald. Fertiliser and seed applied fungicides provide effective suppression during the seedling stages of crop development, while foliar fungicides are most effective when applied between the beginning of stem elongation (GS31) and flag leaf emergence (GS39). Two applications of fungicide are generally required to minimise grain yield and quality loss where disease pressure is sustained during the season. Application that coincide with the early stages of scald development are more effective than later applications as scald can rapidly cause damage when conditions are favourable. Crop monitoring if very important during seasons of risk of scald development.
More detailed information can be obtained from:
Victorian Cereal Diseases Guide (AG 1160) www.vic.gov.au/cerealdiseaseguide
Victorian Winter Crop Summary www.vic.gov.au/victorianwintercropsummary
For rust identification, send rusted plant samples in a paper envelope (do not use plastic wrapping) to: Australian Cereal Rust Survey. Plant Breeding Institute. Private Bag 4011, Narellan NSW 2567
BARLEY GRASS STRIPE RUST
NET BLOTCH OF BARLEY