Puccinia hordei (barley brown rust)

Control of leaf rust is usually necessary to prevent crop losses or total failure of susceptible cultivars grown under conditions that are favourable to the pathogen.

Leaf rust can be controlled through use of foliar fungicides, particularly those with systemic action such as triadimefon.Triadimefon applied as a foliar spray gives good control of leaf rust (Teng and Close, 1977), as does the related compound triadimenol when applied as a seed dressing (Lim and Gaunt, 1981). Seed dressing of triadimenol has been reported as remaining effective in the plant for 51 days, but late epidemics of leaf rust require an additional foliar spray for control (Moore et al., 1979). Both compounds are prophylactic, therapeutic and transported acropetally but triadimefon owes some of its effect to vapour activity. This is highly advantageous in a crop situation but can lead to problems of interplot interference in glasshouse and field experiments. Other compounds with similar properties which are active against P. hordei include diclobutrazol (Skidmore, 1980) and propiconazole (Urech et al., 1980). Of the four fungicides currently approved for control of leaf rust of barley by the United Kingdom Ministry of Agriculture, Fisheries and Food (Anon., 1982), propiconazole, triadimefon, and prochloraz are all grouped together as having the same mode of action through an inhibition of ergosterol biosynthesis. The fourth chemical, fenpropimorph, also inhibits sterol biosynthesis but apparently at a point different from that of the other group of compounds. Such site specificity has led to the evolution of fungicide-insensitive strains of some plant pathogens but there is no evidence for the emergence of such strains of P. hordei.

For most situations, the use of resistant cultivars is the best and most useful control measure. Several major genes for resistance, designated Pa, Pa2, Pa3 etc. (presently also designated Rph, Rph2, Rph3, etc.) are known. They are assumed to operate on a gene-for-gene basis with corresponding virulence genes in the pathogen. However, virulence to these major resistance genes is widespread throughout many of the important barley-producing areas of the world.Clifford (1985) distinguished two main types of resistance to P. hordei in Hordeum spp.Type I ResistanceExpression: 'Hypersensitive' host response. Host cell death giving immune response, or, more commonly, visible chlorotic or necrotic flecks. Sporulation may occur, but uredia are usually associated with host tissue necrosis or chlorosis (See Pictures).Genetic control: Major or oligogenes. Usually single completely or partially dominant factors, which may be temperature sensitive in expression. Many of these major genes have been mapped (Chelkowski et al., 2003).Durability: Commonly completely overcome by matching virulence in the pathogen resulting in host susceptibility in the field. Virulence surveys have shown that nearly all Rph genes have been overcome by the pathogen. Virulence to Rph1, Rph2, Rph4, Rph6, Rph9 and Rph12 are so common that it is hard to find a P. hordei isolate carrying the avirulence to one of these genes (Niks et al., 2000). The only gene that has long been characterized and is still effective in Europe is Rph7 (Niks et al., 2000). However, this gene is being deployed in the USA and has lost its effectiveness by mutation in the American P. hordei population (Steffenson et al., 1993). In Europe, Rph7 is probably still not deployed in modern cultivars, which could explain why the pathogen has not yet shifted there towards virulence to this gene. The genes most commonly deployed in European spring barley cultivars seem to be Rph3, Rph9 and Rph12 (Niks et al., 2000) and Rph2 may also be common (Dreiseitl and Steffenson, 2000).Descriptive terms: Hypersensitive; major gene; race-specific; differential, vertical, low reaction.Type II ResistanceExpression: Quantitative reduction in compatibility with little or no host cell necrosis. Reduced numbers of uredia that are slower to develop and are reduced in size and sporulation capacity. This resistance is also called 'partial' (See Pictures) (Parlevliet and van Ommeren 1975; Niks et al., 2000).Genetic control: Quantitative, recessive major gene and/or limited numbers of modifying minor genes. In various mapping populations minor genes have been mapped that contribute to partial resistnace to P. hordei (Jahoor et al., 2004).Durability: Pathogen variation has been demonstrated experimentally but the resistance is also effective after widespread agricultural use to date.Descriptive terms: Non-hypersensitive; partial; slow-rusting; polygenic; minor gene; horizontal.The population of Hordeum spontaneum indigenous to Israel constitutes an abundant and very diverse gene pool for leaf rust resistance (Anikster et al., 1976). Both low reaction and slow rusting resistance are available in this gene pool (Manisterski et al., 1986) and serve as important sources of resistance in breeding programmes in various countries (Moseman, 1986). Hybrids that show high resistance to leaf rust and stripe rust in the Andean region were recently obtained by crosses between Ecuadorian commercial barley cultivars susceptible to both rusts and selections of Hordeum vulgare subsp. spontaneum from Israel (Brodny and Rivadeneria, 1997). Meanwhile the level of partial resistance in modern European spring barley cultivars has increased since the 1970s (Parlevliet et al., 1980; Niks et al., 2000). There are many different loci on the barley genome that may carry an allele that contributes to the level of partial resistance. Accumulation of such quantitative genes may increase the level.