Barley fungus: Mating could make net blotch more dangerous

Marja Jalli, Research Scientist at MTT Agrifood Research Finland, describes a method for assessing the virulence of net blotch in greenhouse conditions in her doctoral dissertation. The dissertation reveals that the virulence of the Finnish net blotch pathogen population has changed very little in the last 15 years.

"Novel Finnish quantitatively resistant barley cultivars are relatively kind to the disease. If the barley showed higher specific resistance, the pathogen would also have to grow stronger," Ms Jalli explains.

Net blotch can cause major crop losses

Net blotch produces netlike symptoms on barley leaves. The disease destroys chlorophyll and reduces the plant's leaf area. At worst, a severe net blotch infection can cause crop losses of up to 30 percent. In its current form, however, the disease can be managed.

In Finland, the ascomycete that causes net blotch disease (Pyrenophora teres Drechs.) mostly reproduces asexually by means of spores. The pathogen is also capable of sexual reproduction, which is known to happen to some extent.

Sexual reproduction changes virulence

The doctoral dissertation was based on populations of approximately 240 net blotch isolates collected from barley grown in different parts of Finland between 1994 and 2007. Ms Jalli studied the effects of sexual reproduction on net blotch virulence in laboratory conditions. The results were surprising.

"By crossing two net blotch isolates that were both unable to overcome the resistance of barley, we managed to produce progeny isolates that were able to cause symptoms even in barley plants that had previously been fully resistant to the disease," Ms Jalli explains.

In the future, the sexual reproduction of the pathogen is expected to also increase in field conditions, as climate change and the increasing popularity of low-tillage cultivation and specialisation in barley monoculture are making conditions more favourable to the disease.

Breeding should not focus on just one gene

The virulence of net blotch is not the product of a single gene alone but a much more complex hereditary process. Ms Jalli believes that resistance breeding should therefore not focus on just one factor.

"Efforts should be made to also introduce new resistance factors to barley cultivars during breeding. The genomes of barley landraces and wild barley, for example, are hoped to prove useful in this respect," she explains.

The barley cultivars discussed in the doctoral dissertation were also found to contain elements that were highly resistant to net blotch and that have not been previously used in resistance breeding.

Benchmarks for international research

Net blotch has previously mostly pestered barley cultivars grown in northern climates. In recent years, however, interest in the pathogen has begun to also increase in other latitudes.

Ms Jalli's doctoral dissertation specifies nine different barley cultivars that were selected in cooperation with international research scientists and that can be used as benchmarks for reliably measuring the virulence of net blotch across the globe.