Rhynchosporium secalis (leaf blotch)

Both winter and spring cultivars have high levels of specific resistance and at least 11 major resistance genes have been identified (Griffiths, 1988). Where major gene resistance has been used it has not proved durable. Non-specific resistance can provide adequate protection.

Seed treatments are not required. Foliar sprays containing mixtures of MBC, morpholine, strobilurin, conazole and dithiocarbamate fungicides give effective control (Margot et al., 1998; Anon., 1999). There is evidence of MBC (Philips and Locke, 1994) and DMI tolerance (Hollomon, 1997). Routine application of fungicide for the control of leaf blotch is not justified but the presence of the disease on any of the top three leaves, particularly at or soon after flag leaf emergence (GS 39) may be justified (Anon., 1985). Herbicides can significantly reduce spore numbers from volunteers after 4 weeks. Glyphosate is slightly more effective than paraquat (Stedman, 1982).

Adequate crop rotation, removal of debris and volunteers by soil inversion, optimum nitrogen usage and timing and soil inversion can reduce inoculum levels (Jordan and Stinchvombe, 1986), although in intensive barley growing areas these processes will have limited effect. The bacterium Pseudomonas chloraphis is reported to offer effective control (Gehardson et al., 1998).

Scald, caused by R. secalis is a common disease of barley throughout the world, and is a regular or periodic problem in many parts of the world. The economic importance of scald up to the early 1970s was reviewed by Shipton et al. (1974). Scald is a particular problem in winter barley (Polley et al., 1993) but can also be severe in spring barley (Salamati and Transmo, 1997). Scald is also found on rye and on triticale where it causes problems in the USA and Poland (Zamarski and Schollenberger, 1995; Welty and Metzger, 1996). Scald is an important disease of barley in many parts of Europe. It is an important disease of winter and spring barley in Denmark (Ostergard and Pinnschmidt, 1999), and in Finland (Robinson et al., 1996). In Norway, where it is often cold and wet during the growing season of spring barley, scald is prevalent and causes significant losses (Salamati and Transmo, 1997). The disease is prevalent in winter barley in the UK each year (Polley et al., 1993), and it can be a particular problem in Scotland (O'Donnell and Williams, 1981) and southwest England (Melville and Lanham, 1972) when there is frequent rain, mist or dew (Habgood, 1974). In Germany, scald is most severe in northern counties with a maritime climate (Beer, 1988a) and in humid areas (Cselenyi, 1996). However in Belgium, where it is a periodic problem (Meeus et al., 1978), scald is particularly associated with mild and dry summers (Cavelier et al., 1989). Losses due to scald are heavy in Romania (Bobes and Sfetchu, 1978; Bobes and Florian, 1984) and in Poland (Schollenberg and Zamborski, 1997), and the disease is important in Turkey (Certinsoy, 1995).Severity of scald varies between regions in the Middle East and North Africa. It is severe in the cooler areas of the central region of Tunisia (Yahyaoui et al., 1995), prevalent in all areas of Ethiopia (Lakew et al., 1995), but restricted to western areas of Egypt, where it was first recorded in 1991-92 (Rizk and El-Sayed, 1995).Scald is common in North and South America. It can be severe in Ontario, Canada, being very severe on some cultivars (Xue and Hall, 1991), and in Alberta (Penner et al., 1998). In the USA, scald is particularly severe in California (Webster et al., 1980). In Argentina, severe scald epidemics are most likely in the Southern Argentine Pampas region (Rodriguez-Amieva et al., 1973; Carmona et al., 1997).Scald is the principal leaf disease affecting yield in southern regions of Australia (Wallwork, 1995). In New Zealand, as much as 90% of crops can be infected (Cromey et al., 1980), sometimes with significant effects on yields (Wright et al., 1984). In Japan, incidence of scald varies between prefectures (Arai, 1991).

Severe outbreaks are usually associated with wet or moist weather, although in at least one region, it has been associated with relatively dry weather (Cavelier et al., 1989). Various authors suggest that changing agricultural practices, such as earlier sowing of crops, close rotations, and increased areas of barley sown in wetter areas, have contributed to increases in the severity of the disease in England (Shipton et al., 1974). In the UK, one fungicide spray will usually control scald in spring barley, but two may be needed in wet seasons and in winter barley (Casanova et al., 1977). Winter temperatures can be important in determining the amount of inoculum available in spring, since mild wet winters can lead to repeated sporulation in infected debris which can exhaust primary inoculum before spring (Davis and Fitt, 1992). In Poland, scald occurs every year, but severity depends on weather conditions and genotype susceptibility (Schollenberg and Zamborski, 1997). Rainfall during stem elongation is reported to be important (Beer, 1988b), and epidemics are associated with wet or moist conditions in the UK (Priestley and Bayles, 1979) and Germany (Sachs, 1995). Scald occurs in Turkey almost every year (Cetinsoy, 1995). Changes in cultivars, practices which leave crop debris on the soil surface, reduced crop rotation, increasing barley areas and the use of susceptible cultivars, have been implicated by several authors in the increased incidence of scald in parts of Canada (Shipton et al., 1974).

Estimates of crop losses due to scald vary. Losses of between 21 and 67% due to scald have been reported in Ethiopia (Semeane, 1995), 14% or more in Romania (Bobes and Sfetcu, 1978), over 20% in Germany in years of severe epidemics (Beer, 1988a), and over 10% in Finland (Robinson et al., 1996). Losses in spring barley in the UK have been estimated at 35-40%, but are thought to more commonly be less than 10% (Jordan et al., 1982), although another report estimates losses at 0.2 to 1.7% in the UK in the 1970s (King, 1977). Yield losses in crops due to scald are reported to average 10% but reach as high as 30% in Alberta, Canada (Penner et al., 1998) and are as high as 35% in California, USA (Webster et al., 1980). Yield reductions of 27-40% occurred in inoculated plots in Oregon, USA (Zencirci and Hayes, 1990). Losses averaged across all spring barley crops were calculated at approximately 1% in New Zealand in the 1970s (Arnst and Fenwick, 1973), although severe losses occurred in some crops (Sheridan and Grbavac, 1977).Yield increases of 21% were reported in trials in Tunisia by controlling scald (Nasraoui and Mansour, 1999). In Denmark (Nielsen and Jorgensen, 1993), Germany (Beer and Bielka, 1988) and Australia (Khan, 1986), yield increases of 9, 10, and 15-48%, respectively, were reported following fungicide application in trials where scald was the dominant disease. In a New Zealand field trial, yield increases of 35% were recorded in a highly susceptible cultivar following fungicide applications compared with 11% in a partially resistant cultivar (Cromey et al., 2000).In controlled environment conditions, grain yield reductions of up to 30% were recorded from inoculated plants, mostly through a reduction in the number of heads per plant (Mayfield and Clare, 1991). Another report puts the potential yield losses at between 30 and 40% (Griffiths, 1988). Similarly, the number of spikes per unit area was most affected in a field trial in Oregon, USA (Zencirci and Hayes, 1990). However, in trials in Finland (Karjalainen, 1990) and Russia (Ishkova, 1987) the greatest effect was on grain weight, the number of grains being only slightly reduced. In Australia, thousand-grain weight was most adversely affected by scald, showing reductions of 4-19% (Khan and Crosbie, 1988). The percentage of plump grains was reduced by 3-30%, depending on cultivar. The reduction in grain weight can obstruct the marketing of harvested grain as malting quality grain (Penner et al., 1998). In an Australian study, the severity of infection by R. secalis was not affected by sowing rate (Khan, 1988). Losses can remain minimal (around 1%) if fungicide treatment is applied (Mercer and McGimpsey, 1984; Gair et al., 1987).The approximate yield loss can be calculated by halving the infection on leaf 2 at the early milk development stage (GS 75) (James et al., 1968). The relationship between scald severity at various growth stages and yield has been determined (Mayfield et al., 1978).