Egypt. J. Exp. Biol. (Bot.), 5: 1 – 20 (2009) © The Egyptian Society of Experimental Biology REVIEW ARTICLE Abdellah Akhkha Barley Powdery Mildew ( Blumeria graminis f.sp. hordei ) : Interaction, Resistance and Tolerance ABSTRACT : In the present review, the effect of 1. The importance of barley as a crop and powdery mildew ( Blumeria graminis f.sp. the economic significance of barley mildew hordei) on growth, physiology and metabolism (Blumeria graminis f.sp. hordei ) of barley crop ( Hordeum vulgare ) is discussed. Barley ( Hordeum vulgare ), a small-grain Furthermore, the interactions between the host cereal, belongs to the tribe Hordeae of the (barley) and the pathogen ( B. graminis ) are family Gramineae. It is a major world crop and reviewed in details. Different types of ranks as the most important cereal after rice, resistance including, complete and partial wheat and maize (Bengtsson, 1992). Barley is resistance were discussed. Plant tolerance of widely cultivated, being grown extensively in diseases was also presented in details as one Europe, around the Mediterranean rim, and in of the alternatives to protect crops from Ethiopia, Russia, China, India and North damage caused by the pathogen or the America (Harlan, 1995). In Britain, barley has disease. However, this phenomenon would not been the crop with the largest land acreage for involve pathogen limitation and the pathogen a considerable period of time and still would not affect the crop in a way other represents today, together with wheat, one of intolerant crops would do. The use of the major crops. tolerance in integrated disease management is It has been suggested that cultivated discussed. barley originated from the wild barley, Hordeum spontaneum C. Koch, which has its centre of origin in the Fertile Crescent of the Middle East (Zohary, 1969), with scattered stands over a much wider area from Tunisia to Afghanistan and with doubtful occurrence in Morocco and Abyssinia. The fungus Blumeria graminis (DC.) KEY WORDS: Powdery mildew, Erysiphe, Speer f.sp. hordei Marchal (Syn. Erysiphe Blumeria, Wild barley, cultivated barley, Hordeum graminis DC. f.sp. hordei Marchal) causes vulgare, Hordeum spontaneum , Resistance and powdery mildew, the most important disease of Tolerance. barley throughout the world where the crop is grown (Bennet and Scott, 1971). The importance of powdery mildew on barley was recognised at the beginning of this century when the disease was observed to cause economic losses (Wolfe and Schwarzbach, 1978). Since then barley mildew has remained a constant problem in many parts of the world, including Europe. For example, annual losses of about 9% are reported in England and Wales (King, 1972 & 1977), 25% in USA (Schaller, 1951) and 30% in North Africa (Yahyaoui et al ., 1997). Even CORRESPONDANCE: greater yield reductions have been found in Abdellah Akhkha experimental studies and losses in grain yield * Division of Biochemistry and Molecular Biology, in excess of 50% have been reported (Rea and Institute of Biomedical and Life Sciences, The Scott, 1973). Bower Building, University of Glasgow, Glasgow G12 8QQ, Scotland, UK. 2. Interactions between barley and B. + Department of Biology, Faculty of Science, graminis f.sp. hordei Taibah University, Madinah, Kingdom of Saudi The responses of barley to infection by Arabia B. graminis have been found to be extremely E-mail: [email protected] varied. Some cultivars may be highly ARTICLE CODE: 01.02.09 ISSN: 12895-2007 http://www.egyptse b . o r g 2 Egypt. J. Exp. Biol. (Bot.), 5: 1 – 20 (2009) susceptible and support high levels of fungal because resistance provides the cheapest and development resulting in the death of the host, the most effective means of controlling while others may be immune. In between the pathogens, particularly powdery mildews of two extremes there is a continuous range of cereals. However, the cultivation of resistant cultivars which support all levels of fungal cultivars on a large scale imposes a strong development (Jones and Clifford, 1983a). selection pressure on the pathogen population for virulent races that can overcome the 2.1. Host resistance resistance. In general, major gene resistance Resistance of a host to a parasite is remains effective for only a few years before a defined by Agrios (1997) to be the ability of the virulent race of the mildew pathogen arise host to prevent, completely or in some degree, which can overcome the resistance. This was the growth and development of that parasite. first observed in the case of the major Different types of resistance in barley to resistance gene Ml-g, which was introduced infection by B. graminis f.sp. hordei have been into European barley varieties in 1930s (Wolfe noted from complete resistance to varying and Schwarzbach, 1978). For example in levels of partial resistance. Germany, when the area under cultivation with 2.1.1. Complete resistance cultivars with Ml-g gene was still small during In many instances, complete or near the 1930s and 1940s, it remained effective. complete resistance has been found to be However, when areas under cultivation in the controlled by one or at most two or three late 1940s started to increase rapidly, this genes with major effect. This type of resistance gene was defeated (Wolfe and resistance is often called race-specific Schwarzbach, 1978, Wolfe, 1984). The resistance or major gene resistance and is instability of major gene resistance has caused often expressed as a hypersensitive reaction plant breeders to look for ways to use it, which (Jones, 1987). might make it more durable. The specificity of most types of major Three methods have been used to gene resistance suggests that there is some improve the durability of major gene relationship between specific avirulence genes resistance, pyramiding resistance genes, in the different physiologic races of the multiline varieties and variety mixtures. pathogen and the different resistance genes in Pyramiding resistance genes consists of the host cultivars incorporating them. breeding as many of the genes as possible into a cultivar before releasing it into commercial Many B. graminis f.sp. hordei avirulence production. This means that the parasite must alleles and barley resistance genes were found overcome all the resistance genes before it to follow a gene-for-gene relationship, a becomes virulent. relationship which was first demonstrated by Flor in 1956 between flax and flax rust Multiline varieties are formed as (Moseman, 1957and1959). combinations of isogenic lines, identical in all agronomic characters but differing in the race- The application of Flor’s gene-for-gene specific resistance gene they contain (Jones hypothesis has facilitated the identification of and Clifford, 1983b, Manners, 1993). In order specific resistance genes in barley and their to break down the resistance of the multiline, corresponding pathogenicity genes in B. the pathogen must acquire enough different graminis f.sp. hordei. In this way, a large virulence alleles to overcome all the resistance number of resistance genes in cultivated genes present. barley and wild species of barley have been identified and mapped (Wolfe, 1972, Giese, Variety mixtures consist of several 1981, Giese et al ., 1981, Søgård and varieties, which are similar to each other in Jørgensen, 1987). agronomic characters, but which have different resistance genes. Mixed varieties have been Resistance to at least some variants of shown to have reduced levels of infection and B. graminis f.sp. hordei is determined by consequently reduced yield loss of the alleles located in at least seven loci. At least individual components of the mixture, when five of these loci appear to be located on the compared to the mean yield from pure stands long arm of chromosome 5, whereas the other (Wolfe, 1985). However, mixed varieties, like two loci, Ml-g and Ml-o, are located on multiline varieties and pyramiding, could lead chromosome 4 (Wolfe, 1972). The resistance to the development of new and more virulent alleles are designated after the cultivar or line races (super-races) that are virulent on all of in which they were first identified; eg. Ml-a (cv. the varieties in the mixture (Groth, 1976). This Algerian), Ml-at (cv. Atlas), Ml-g (cv. Goldfoil), possibility was supported experimentally by the Ml-h (cv. Hanna), Ml-k (cv. Kwan), Ml-p (cv. work of Huang et al . (1994) who suggested Psaknon) and Ml-o (McIntosh, 1978). that super-races would dominate a mildew Plant breeders produce new resistant population when the same cultivar mixtures or cultivars by incorporating single major genes multilines were used continuously over long derived from Hordeum spontaneum . These periods and large areas. The search for more resistant cultivars are extensively used, durable forms of resistance should continue. particularly in Europe and North America, 2.1.2. Partial resistance ISSN: 12895-2007 http://www.egypts e b . o r g Akhkha A., Barley Powdery Mildew ( Blumeria Graminis F.Sp. Hordei): Interaction, Resistance … 3 Many barley varieties have been found to have stable tolerance which was expressed in be partially resistant to B. graminis pathotypes. each season, while others expressed unstable Such varieties support the growth of the tolerance which was expressed in one season fungus but the growth is limited. Partial but not in another. resistance tends to be more durable than race Simon (1966) examined 24 oat cultivars specific resistance (Roberts and Caldwell, for their reactions to crown rust ( P. coronata ) 1970). It is in fact not specific and affects and concluded that cv. Cherokee and several several of the pathogen infection processes, other cultivars with susceptible reactions were pathogenicity and sporulation. It is believed to significantly more tolerant of given levels of be controlled by a number of genes each with infection, as measured by kernel weight ratio, small effect (Parlevliet, 1981) and it is than cvs Clinton and Benton. sometimes referred to as polygenic resistance However, in none of these cases, were (Asher and Thomas, 1987).