Introduction

InGenetic resistance has been used extensively to control powdery mildew (Blumeria graminis f sp hordei) of spring barley during the twentieth century. Many major genes have been introduced into barley cultivars by plant breeders but most have only controlled powdery mildew effectively for a short time. Mutations in the diverse mildew population rapidly result in new strains of the fungus capable of overcoming the resistance genes in new cultivars. The mlo gene has been widely used to confer mildew resistance in spring barley cultivars since the 1970s (Jorgensen, 1992). Following the addition of cv. Atem to the Recommended List of Cereal Varieties for the UK in 1980, there has been a succession of spring barley cultivars carrying the mlo resistance gene grown in the UK. In 2000, over 40% of spring barley grown in England and Wales carried mlo (NIAB, 2000) and ten of the fifteen cultivars on the Recommended List for 2001 are believed to have the mlo resistance (Slater & Clarkson, 2001). Despite exposure to the UK mildew population for twenty years, mlo resistance is still effective for controlling mildew. Occasional mildew pustules are observed on some mlo cultivars but all maintain a high NIAB resistance rating of 9 (NIAB, 2001). Although there have been sporadic reports of mlo crops infected with mildew, further examination has usually shown the infection to be temporary and probably the result of environmental factors such as alleviation of water stress (Newton & Young, 1996).

During 1998, reports suggested that some spring barley cultivars with the mlo resistance to powdery mildew were carrying increased levels of infection (Slater & Clarkson, 1999). In the Recommended List Trial at NIAB, cv. Riviera showed considerable infection at GS 45 in one replicate, but less mildew in the other two replicates. Mildew colonies were also observed on cvs Chariot, Dandy and Derkado. However, the disease did not develop further and was not detectable at later growth stages. In differential tests, the level of infection on cv. Apex, the mlo differential, is usually low, with only occasional colonies of infection type 4 (Moseman et al.,1965). However, in standard differential tests carried out in the spring of 1998 using isolates collected from airborne spora in January, more colonies than expected were observed. A selection of isolates which gave higher than expected levels of infection on cvs Apex and Riviera (designated Apex + isolates) was compared with isolates from the same source, which showed no infection on cvs Apex and Riviera (designated Apex 0 isolates). Isolates showing higher than expected infection on mlo cultivars collected from plots in the Recommended List Trial were also included in the experiments (designated mlo isolates).

Some isolates also gave increased levels of infection on cvs Apex and Riviera in standard differential tests in 1999 and 2000. A selection of these isolates was compared with the 1998 isolates.

Materials and Methods

In the initial test, 12 cultivars believed to carry the mlo resistance, together with the susceptible cultivar cv. Golden Promise, were inoculated with selected isolates collected from the air spora and NIAB trial plots in 1998 (Table 1). Leaf segments detached from seedlings grown in a spore-proof glasshouse were placed on the surface of benzimidazole agar (0.5% agar, 100ppm benzimidazole) in rectangular clear polystyrene boxes and inoculated using a settling tower. Following incubation for 10 days at 15 - 18°C, numbers of colonies per leaf segment and leaf area infected on a 0 to10 scale were assessed. In addition, the number of infected leaf segments of the mlo cultivars was recorded.

Further tests were carried out in 1999 and 2000 with fewer cultivars but including additional isolates collected in 1999 and 2000 (Table 1). The level of infection in these tests was expressed by assessing the leaf area infected on a 0 to 10 scale.

The isolates used in the experiment and their source are shown in Table 2. Most were obtained using static nurseries of cv. Golden Promise to sample the air spora in Cambridge, with the remainder collected as infected leaf samples from NIAB trials throughout the UK.

Results

The results are shown in Table 3 and Table 4.

In all seven tests, nearly all the isolates designated Apex + gave higher levels of infection on the mlo cultivars than the isolates designated Apex 0. The isolates collected from mlo cultivars gave the highest infection levels in test 1, but intermediate levels in tests 2 - 4. In all cases, the number of colonies per segment was much lower than on cv. Golden Promise but most of the colonies were of infection type 4. It appears, therefore, that isolates collected in 1998, 1999 and 2000 produced differential reactions on cultivars that carry the mlo resistance to mildew.

The mlo cultivars used in these experiments varied slightly in the levels of infection carried but generally ranked consistently within the Apex +, Apex 0 and mlo groups of isolates. The differences between cultivars were small, except for cv. Chalice. This cultivar had higher levels of infection than the other cultivars with both Apex + and Apex 0 isolates, suggesting that perhaps it has less background resistance or does not carry the mlo resistance factor. However, like the other mlo cultivars, the number of colonies was usually much lower than on cv. Golden Promise.

The number of leaf segments that were infected with mildew in tests 1 and 2 are shown in Table 4. Results for the Apex + and Apex 0 groups were similar in both tests, with the Apex + isolates consistently giving more infection than the Apex 0 isolates. However, the isolates derived from mlo cultivars appeared to give differing results in the tests. This is believed to be due to the inclusion of three additional isolates in this group, originally classed as mlo-infective, which did not give higher than usual levels of infection in these tests. The three isolates, which originated in Northern Ireland, had given increased levels of infection in earlier tests in Northern Ireland (PC Mercer, pers.comm.). However, in test 2 and subsequent tests these isolates failed to infect any mlo cultivars, appearing to be similar to the majority of isolates found in previous years.

The infection levels on the mlo cultivars in test 7 given by the isolates collected in 1998, 1999 and 2000 are shown in Table 5. Although the number of colonies observed on the mlo cultivars remained considerably lower than that on cv. Golden Promise, Apex + isolates gave higher levels of infection on the mlo cultivars than the Apex 0 isolates. The Apex + isolates from 1999 and 2000 gave similar levels of infection to the 1998 isolates, suggesting that no further adaptation to these cultivars had occurred since 1998.

Discussion

In previous years, low levels of infection have sometimes been observed on mlo cultivars in the field but virulence has never been confirmed in laboratory tests in the UK. Differential tests sometimes exhibit infection on Apex, the mlo differential, but the reactions have not proved repeatable. Usually, most of the isolates tested in a given batch have exhibited similar levels of infection, perhaps producing several colonies in one set of tests but no infection in subsequent tests. This suggests that environmental factors are the cause of the sporadic infections seen in tests and in the field to date.

However, isolates giving increased levels of infection on mlo-carrying cultivars have been recorded outside the UK. The laboratory culture HL-3, obtained by selection on a mlo host, is over 50 times more infective on mlo cultivars than the wild type population (Schwarzbach, 1979). A similar race has been identified in Japan (Lyngkjaer et al., 1995). In 1997, Schwarzbach detected “partially virulent” isolates which gave infection on mlo cultivars up to 10 times higher than the bulk of the population, but none were as infective as HL-3 (Schwarzbach, 1998). Isolates collected in Northern Ireland in 2000 also gave virulent reactions on mlo differential cvs Apex and Atem in tests, although no associated field breakdown was apparent (Mercer & Ruddock, 2001).

Since isolates giving differential reactions on mlo cultivars have been detected, the non-specific nature of the resistance is questioned. Although originally described as a non-specific resistance effective against all powdery mildew races, it is now evident that pathotypes have evolved with differing responses to the mlo resistance. The barley cultivars themselves also give differing responses to attack by powdery mildew, although the contribution made by additional specific genes and background resistance in these cultivars is not yet clear. However, to date, no specific cultivar/isolate interactions have been observed. Isolates giving increased infection on mlo cultivars do so on all the cultivars tested, with the mlo cultivars ranking similarly to all isolates.

So far, adaptation in the barley mildew population to mlo cultivars in the UK is very low and does not represent the presence of true virulence in the field. However, since the reactions of the partially virulent isolates in these experiments are consistent with the classification made from the original differential test, it seems possible that there has been some adaptation to the mlo resistance gene, which is present in the majority of current UK recommended spring barley cultivars. In view of the reliance on the mlo gene for mildew resistance in spring barley in the UK (mlo cultivars comprised 41% of the spring barley area grown in England and Wales and over 90% in Northern Ireland in 2000), it is vital that regular screening of the barley mildew population continues in order to detect any changes in reaction to these cultivars.

Acknowledgements

This work was carried out as part of the United Kingdom Cereal Pathogen Virulence Survey, funded by UK Home-Grown Cereals Authority (HGCA) and the UK Government Ministry of Agriculture Fisheries and Food (MAFF).

References

Jorgensen JH, 1992. Discovery, characterization and exploitation of Mlo powdery mildew resistance in barley. Euphytica 63, 141-152

Lyngkjaer MF, Jensen HP, Ostergard H, 1995. A Japanese powdery mildew isolate with exceptionally large infection efficiency on Mlo-resistant barley. Plant Pathology 45, 786-790.

Mercer PC, Ruddock A, 2001. Mildew of barley in Northern Ireland. UK Cereal Pathogen Virulence Survey 2000 Annual Report, pp 60-62.

Moseman JG, Macer RCF, Greely LW, 1965. Genetic studies with cultures of Erysiphe graminis f. sp. hordei virulent on Hordeum spontaneum. Transactions of the British Mycological Society 48, 479-489.

NIAB, 2000. Seed Production No. 26 (November 2000).

NIAB, 2001. UK Recommended Lists of Cereals: 2001 cereals variety handbook.

Newton AC, Young IM, 1996. Temporary partial breakdown of Mlo-resistance in spring barley by the sudden relief of soil water stress. Plant Pathology 45, 973-7.

Schwarzbach E, 1979. Response to selection for virulence against the mlo based mildew resistance in barley, not fitting the gene-for-gene hypothesis. Barley Genetics Newsletter 9, 85-88.

Schwarzbach E, 1998. The mlo based resistance of barley to mildew and the response of mildew populations to the use of cultivars with the mlo gene. Czech Journal of Genetics & Plant Breeding 34, 3-10.

Slater SE, Clarkson JDS, 1999. Mildew of barley. UK Cereal Pathogen Virulence Survey 1998 Annual Report, pp 43-51.

Slater SE, Clarkson JDS, 2001. Mildew of barley. UK Cereal Pathogen Virulence Survey 2000 Annual Report, pp 50-59.