Disease biology

Brown rust

Summary

Brown rust attacks especially wheat and rye. Brown rust needs a lot of heat and is therefore usually seen late in the growing season. In Denmark brown rust in wheat is only of minor significance as the attacks are often slight. Ochre- or rust-coloured pustules appear on the leaves, either scattered or densely on the leaf blade. The pustules are mainly found on the upper side of the leaves. This also applies to rye, even though severe attacks can occur in some years. There are several approved fungicides.

Mycology

Brown rust has both summer and winter spores (uredo and teleutospores). The uredo spore cases are up to 1.5 mm in diameter and scattered over the upper side of the leaf. In the spore cases the orange or brown, round or oval uredospores can be seen; they are 15-30 µm in diameter. The winter spore cases appear under the leaf surface, primarily on leaf sheaths and leaves. They are the size of the uredo spore cases, black and shiny and not frayed. The teleutospores are 30-42 x 14-17 µm and rounded at the tip as in yellow rust. Systematics: Basidomycotina, Teliomycetes, Uredinales, Pucciniaceae.

Symptoms

Ochre- or rust-coloured pustules that will rub off appear on the leaves, either scattered or close together on the leaf blade. The pustules are mainly found on the top side of the leaves. Yellow areas form round the pustules. Later on, the attacks occur on straw and leaf sheaths. Late in the growing season black, so-called teleutospores are formed. The pustules on rye are a little darker than on wheat. Can be mistaken for: Brown rust in wheat can be mistaken for yellow rust, but the brown rust pustules are darker, often appear later and are not arranged in stripes like those of yellow rust.

Time of attack

Attacks often do not appear until after earing, stage 55.

Biology

The brown rust fungus is an obligate parasite, which means that it lives on living plant tissue only. Overwintering is as summer spores on wheat, rye and volunteers. In the early autumn the sources of infection are volunteers and overwintering crops from which infection passes from winter to spring crops. The spores are spread by the wind over large distances. Later in the summer black winter spores are formed that are of no importance to the overwintering of the fungus. Brown rust on rye alternates between bugloss and alkanet as hosts, but this alternation is not obligatory. Climatic requirements: The fungus thrives best at high temperatures and humidity. The largest amount of spores is formed at 25°C. The spores of the brown rust fungus are highly resistant to frost and can survive under snow for months. The spores need liquid water to germinate, which means that even at 99% relative humidity the capacity for germination has decreased considerably.

Control measures

Prevention: Remove volunteers. Little information is available about cultivar differences in susceptibility to brown rust. Winter crops should not be sown too early, just as mitrogen fertilisation should be moderate. Forecast service: Aarhus University, Faculty of Agricultural Sciences and the Danish Agricultural Advisory Service issue forecasts if there are early and severe brown rust attacks in wheat. Keep up to date on the Internet under: Varsling/Registreringsnet (= Warning/registration network developed). Brown rust on rye is not studied. Chemical control: There are several approved fungicides. Choice of dosage, expected crop price, yield level, etc. influence the profitability of fungicide treatment.

Names and distribution

English: Brown rust. Other names: Rust. Latin: Puccinia recondita f. sp. tritici (brown rust on wheat), Puccinia recondita f. sp. secale (brown rust on rye). Distribution: Brown rust is found in all wheat and rye growing areas and is very widespread in countries with hot summers.

Hosts and importance

Importance: Only minor importance is attached to brown rust in Denmark as the attacks only occur rarely and often appear late in the growing season and then as slight attacks. The same applies to rye, although severe attacks can occur in some years. Hosts: Wheat and rye. At least 37 other grass species are also said to be susceptible to attack. Brown rust appears in different forms (forma specialis) so that for example brown rust on wheat cannot attack rye and vice versa.

Names in different languages

Danish :	Brunrust
German :	Braunrost
English :	Brown rust
Latin :	Puccinia recondita
Norwegian :	Brunrust
Swedish :	Brunrost

Wheat leaf with rust-coloured brown rust pustules

Scattered brown rust pustules on the leaf surface. Yellow areas show around the pustules

Attack closely together on the leaf blade

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