Abstract

The breakdown of race specific resistance to powdery mildew in Norwegian spring wheat has been much more rapid in the northern Mjøsa region than in the southern Oslo region. An explanation is sought in the population genetics of powdery mildew in the respective regions, as affected by the mildew situation in winter wheat, the meteorological conditions and the migration of spores between districts and from areas outside Norway.

Introduction

Rapid breakdown of race specific resistance to fungal diseases is a well known reality in cereal mildew as well as in other plant diseases. In Norwegian wheat cultivation an interesting picture has appeared together with the introduction of new wheat varieties with different race specific genes against mildew: A strong differentiation between districts exist. The breakdown is much more rapid in the northern than in the southern part of the cultivation area of South- Eastern Norway.

Materials and Methods

A hypothesis for the differentiation between districts is sought in the population genetics of powdery mildew in the respective regions:

(A). The region around Oslo normally has little snow. Due to lack of cover the winter wheat leaves and hence the mildew would often die during the winter because of frost damage. Because the prevailing wind direction in May/June (vegetative growth period) is southern, new inoculum could normally migrate by wind from Sweden, Denmark etc. To some extent wind from the north could provide inoculum from region (B). Accordingly the pathogen would basically be asexual in this region.

(B). In contrast, the northern part around the lake Mjøsa has more snow cover, which enables the mildew population to survive from year to year. A hypothesis is proposed (c.f. McDonald and Linde, 2002) that the mildew population around Mjøsa is characterized by:

- a considerable population size. Although the winter wheat fields with snow cover
could be small and scattered in years with limited snow cover, a gene flow would occur between the areas.

- sexual recombination as the pathogen is assumed to have a continuous life cycle.

- migration of mildew from southern districts (Sweden, Denmark, UK, Germany etc.).

- a continuous conservation of individuals of the population over time which makes it behave like a gene-pool. It would accumulate new virulence genes from immigrants and mutations and produce recombinations without losing old virulences.

- winter wheat varieties which until now has possessed a low degree of race specific resistance, enables the genetic variation to be conserved without selection for virulence. Continuous breeding material (over years) from Graminor Plant Breeding with a lot of different genotypes including susceptible material located in the centre of the district (at Hamar) is also ensuring that virulence genes are not lost.

Results and Discussion

Results (Table 1 and Figure 1a, 1b, 1c, 1d) from commercial growing of the spring wheat varieties 'Bastian', 'Polkka', 'Brakar' and 'Avle' which have different race specific genes for resistance to mildew (Lombnæs, 1995) are the same: New virulence first appeared in the northern cultivation area around Mjøsa, then in the southern area around Oslo. Acreages of the varieties are shown in Table 2 as well as in the Figure 1a, 1b, 1c, 1d. The varieties have been distributed relatively equally with respect to northern versus southern districts of the cultivation area.
To follow the development of virulence, we start with the variety 'Avle' that has been released recently. It gives an example of what happens the first years after a variety is taken into cultivation. Then varieties are presented in successive order with respect to time of release to show what happens after a variety has been cultivated for a longer time.

Figure 1 shows that the virulence against the variety 'Avle' first occurred in the Mjøsa/Hamar region, already in the first year of cultivation, 1997, when it covered only 1 % of the spring wheat growing area. The absence of mildew on 'Avle' in 1998 is probably due to non-random distribution of the virulent inoculum in the Hamar region. In 1999, however, when the cultivation area had reached 59%, the severity of mildew on 'Avle' had reached 12%. The following year 'Avle' was quite susceptible, not only in the Mjøsa/Hamar region, but also around Oslo (Aas), obviously because of transfer by wind. A similar breakdown of resistance in the Mjøsa region has probably taken place before 1992 in 'Brakar', 'Polkka' and 'Bastian' (Figure 1a, 1b, 1c and 1d). In 'Brakar' the resistance was overcome at least four years before the variety was taken into cultivation. Although the history of cultivation is different for the three varieties, they have the following features in common:

(a) Virulence in the Mjøsa/Hamar region in the whole period (1992- 2001).
(b) Some virulence in the Oslo/Aas region from 1992 to 1993.
(c) No virulence in the Oslo/Aas region from 1994 to 1995.
(d) A high virulence frequency in Oslo/Aas region from 1996 until 2001.

It is realistic to assume that the virulence in the Mjøsa area has been rich enough (c.f. hypothesis above) to provide virulences to all varieties, already during the testing of the varieties, in a low frequency. Later, as the acreage has increased, the virulence frequency has also increased, by means of asexual reproduction. It is also obvious that the inoculum has been transferred by wind to the Oslo region and introduced the virulence there.

According to meteorological data from the period 1992- 2001 (Department of Agricultural Engineering, Agricultural University of Norway) the absence of virulence in the Oslo region in two seasons could be explained by:

- 1993/94: Periods in the winter had low temperature and not sufficient snow cover for survival of mildew, and the prevailing wind in May was not southern.

- 1994/95: Periods of continuous rain in May/June, and the prevailing wind in May was not southern.

Also the reaction on differential varieties (cf. Figure 2, Figure 3, data from 1997 and 1999) show the difference in virulence spectrum between the mildew populations of the Mjøsa and the Oslo area. The race composition around Oslo is more like the composition in Northern Europe, c.f. data from UK, Germany and Poland in 1997 (Clarkson, 2000) which is in correspondence with transfer of inoculum by southern winds. The unique composition of the inoculum around Mjøsa is consistent with the hypothesis above.

Acknowledgements

The field testing for resistance was supported by Graminor Plant Breeding, Hamar, Norway. The author wish to thank professor Aasmund Bjørnstad at Department of Horticulture and Crop Sciences for valuable criticism of the manuscript.

References

Clarkson JDS, 2000. Virulence survey report for wheat powdery mildew in Europe,1996 -1998. Cereal Rusts and Powdery Mildews Bulletin [www.crpmb.org/]
2000/1204clarkson.

Lombnæs P, 1995. Resistensforedling mot mjøldogg i hvete (Resistance breeding to powdery mildew in wheat). Aas, Norway: Agricultural University, M.Sci. Thesis.

McDonald BA, Linde C, 2002. The population genetics of plant pathogens and breeding strategies for durable resistance. Euphytica 124, 163- 180.