Disease biology

Barley rust

Summary

Barley rust only attacks winter and spring barley. In Denmark attacks by barley rust often occur too late to do any damage. Increasing growing of winter barley results in a higher risk of major barley rust attacks. Barley rust shows as scattered lemon to orange spore clusters which come off the leaves when you touch them. Barley rust can be mistaken for yellow rust.

Mycology

Barley rust does not alternate between hosts in Denmark and has to overwinter on barley. There are two forms of spores: summer and winter spores. The spore clusters (the pustules) are relatively small, 0.3-0.5 mm long and 0.1-0.2 mm wide. The summer spores (uredospores) sit on stalks in the yellow-orange spore cases. They are round to elliptical and densely covered with small prickles. The size of the spores is 20-30 x 17-22 µm. The winter spore cases are black and measure approx. 0.5 mm in diameter. The winter spore cases are covered by the leaf surface. The winter spores (teleutospores) are both 2- and 1-celled. They are unsymmetrical and measure 34-44 x 18-23 µm. The 2-celled uredospores measure 40-60 x 18-23 µm. The bottom cell is narrower than the top cell. The teleutospore function is not known. Systematics: Basidomycotina, Teliomycetes, Uredinales, Pucciniaceae.

Symptoms

The spore clusters are primarily seen on the upper side of the leaves and to a lesser extent on the underside. Attacks also occur on straws and leaf sheaths but rarely in ears. The colour is lemon to orange. The summer spores are found spread over leaves and stems. Later, the black winter spores (teleutospores) appear on the underside of leaves and stems. In case of severe attacks the leaves die and the plant ripens incompletely. Can be mistaken for: Barley rust can be mistaken for attacks by yellow rust, but yellow rust in barley is rare in Denmark.

Time of attack

Attacks can in winter barley occur as early as the autumn, but these attacks are not important. Severe attacks develop rather late in the growing season, usually from June-July onwards.

Biology

Barley rust is an obligate parasite, which means that the fungus can only survive on living plants. The summer spores overwinter on winter barley and volunteers. Spring and winter barley volunteers can therefore be sources of infection for spring barley in the spring just as the fungal spores can be spread by the wind over large distances. The development of barley rust is usually slight during the winter and is easily overlooked. The summer spores are found spread over leaves and stems. Later in the summer black winter spores (teleutospores) appear on the underside of leaves and stems. Their function is not known. The development of barley rust on the leaves is inhibited by severe mildew attacks just as the opposite can also be the case. Climatic requirements: Spore germination occurs at 100% air humidity; water as drops is not necessary. The temperature must be between 5 and 28°C with an optimum at 16-20°C. The period from infection until rust pustules are visible is 6 days at 25°C, 16 days at 10°C and 60 days at 5°C. Spore production peaks at 20°C.

Control measures

Prevention: Keep a distance between winter and spring barley fields. Use cultivars that are less susceptible to barley rust, and avoid too early sowing of winter barley. Sow spring barley on time as late sowing increases the risk of attack. Forecast service: Aarhus University, Faculty of Agricultural Sciences and the Danish Agricultural Advisory Service issue forecasts. Keep up to date on the Internet under: Varsling/Registreringsnet (= Warning/registration network developed). Control: Several fungicides have been approved for control of barley rust. Choice of dosage, expected crop price, yield level, etc. have an influence of the profitability of fungicide treatment.

Names and distribution

English: Barley rust. Other names: Brown leaf rust, barley brown rust. Distribution: The fungus is adapted to a temperate climate and is known in both North and South America, Europe to East Asia, Australia and New Zealand.

Hosts and importance

Importance: In cases of early attacks yield losses higher than 20% can occur. Barley rust has in recent years become more important due to the increasing area with winter barley combined with mild winters. Hosts: Both spring and winter barley are attacked. Several grass species, for instance among rye-grasses, can also be hosts, but barley rust on barley is considered to be specialised so that only barley is attacked.

Names in different languages

Danish :	Bygrust
German :	Zwergrost / Braunrost
English :	Barley rust
Latin :	Puccinia hordei
Norwegian :	Dvergrust
Swedish :	Kornrost / Dvärgrost

Early barley rust attack

Early and severe barley rust attack

Late barley rust attack

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