Disease biology

Yellow rust

Summary

Yellow rust attacks wheat the hardest, but it can also be found in barley. Yield losses of up to 50% can occur in years with severe yellow rust epidemics. Yellow rust shows as orange spores on the leaves. At first the spores are scattered – later, they are found in long rows. Attacks can occur as early as in the autumn and the early spring. Attacks can occur sporadically in the field or in patches, often at first near hedges and in places when the crop is dense. Use of resistant cultivars can prevent attacks. In susceptible cultivars control should be carried out when attacks are observed.

Mycology

Yellow rust has both summer and winter spores (uredo- and teleutospores). The summer spore cases are up to 0.3-0.5 x 0.5-1.0 mm. Parallel stripes of uredo spore cases are visible due to infection with running hyphae in the leaf. The uredo spores are 20-30 mm in diameter, round and yellowish-orange. The winter spore cases appear below the surface primarily on the leaves. They are the size of the uredo spore cases, dark brown and form long rows. The teleutospores are 30-42 x 14-17 µm and with rounded tips as in brown rust. Systematics: Basidomycotina, Teliomycetes, Uredinales, Pucciniaceae.

Symptoms

All above-ground parts can be attacked. On plants yellow/orange spore clusters are seen scattered on the leaf, especially near the leaf tip. Gradually, the spore pustules settle in stripes along the blade veins. At first, the pustules are surrounded by a pellicle, which bursts, leading to a “rub-off effect” (spores) of the pustules. Relatively sharply defined chlorotic areas on the leaves can also be caused by the yellow rust fungus. Pustule formation may not occur, for instance due to resistance in the plant, unfavourable climatic conditions or chemical control. Black teleutospores are formed on the leaves at the end of the growing season. Their significance is still unknown. Attacks can occur sporadically on a few plants in the field or in highly infected patches. Yellow rust is often first found near hedges, in double sowings or in other places with dense crops. Can be mistaken for: Yellow rust can be mistaken for brown rust. Brown rust is only found as individual pustules and not in stripes. If the rust clusters cannot be scraped off with a nail, it can be because it is another fungal disease, for instance Septoria leaf blotch or glume blotch.

Time of attack

Attacks can occur as early as the autumn or the early spring. The yellow rust fungus is weakened during hard winters, and after such winters proper yellow rust epidemics are not expected until late in the growing period.

Biology

Yellow rust is an obligate parasite, which in mild winters overwinters as mycelium in winter wheat and volunteers. Yellow rust dies in hard winters because the infected leaves are destroyed by frost. Widespread attacks can occur as early as February in mild winters, in which the fungus survives. Disease spreading takes place over large distances by the wind; up to 1000-1500 km. Thus, infection can be brought to Denmark from the United Kingdom and Germany. New infection races with the ability to attack (virulence) hitherto resistant cultivars can thus be introduced. Under favourable climatic conditions a tremendous propagation occurs, up to 10 spore generations during one growing season. One of the reasons for this is that the mycelium grows systemically inside the plant in the longitudinal direction of the leaf through the vascular bundles (which results in leaf streaks with clusters of rust). 50% of the epidemic growth is a result of systemic growth of previous infection, not new infection. Climatic requirements: Yellow rust thrives best under cool and humid conditions. Epidemic attacks have, however, been seen under hot and relatively dry conditions, provided that a large amount of infectious matter has already developed. The optimum temperature for mycelium growth and spore production is 15°C. Spore germination is possible in the interval of 0-25°C. Liquid water on the leaves for at least 3 hours is necessary for the spores to germinate. The incubation period, that is the period from infection until symptoms are visible, depends on the temperature and is 12-14 days in the summer and 1-1.5 months during the autumn and in March/April.

Control measures

Prevention: Remove volunteers that can sustain infectious matter and use resistant cultivars. Several cultivars possess built-in race-specific resistance, but many cultivars also possess field resistance that is based on several genes. Cultivars that are solely based on race-specific resistance are especially exposed to having their resistance destroyed. Early sowing of winter wheat and a high level of nitrogen promote the fungus. 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). The occurence of the yellow rust races is monitored continuously through virulence monitoring. The virulence monitoring records changes in the genetic composition of the yellow rust population and ensures early information on yellow rust resistance for each cultivar. Information can be found on the Internet in SortInfo under: www.planteinfo.dk . Chemical control: Use approved sprays. The long-term and curative effect (the ability to control established attacks) of the sprays differ. If yellow rust has previously been found in susceptible cultivars, treatments should be carried out at suitable intervals. Choice of dosage, expected crop prices, yield level, etc. all have an influence on the profitability of a fungicide treatment.

Names and distribution

English: Yellow rust. Other names: Rust. Distribution. In parts of both Europe and the rest of the world yellow rust is one of the most important diseases in wheat and barley. Cold and humid coastal areas are the areas most favourable to the fungus. The dependence of the fungus on climatic conditions leads to irregular occurrence and to its concentration in the north and north-western parts of Europe, in Switzerland, Spain, Portugal and north-western USA.

Hosts and importance

Importance: In years with major attacks of yellow rust yield losses of up to approx. 50% can occur. Attacks on flag leaves and ears are particularly serious. Under Danish conditions the attacks in barley are rare and the losses not so heavy. Hosts: Internationally, attacks are best known in wheat and next in barley. In rye the fungus is less widespread. Studies up till now have shown that at least 230 wild grasses are hosts. The yellow rust fungus is specialised so that for instance yellow rust on barley only attacks barley. Yellow rust in wheat can sometimes also be found in rye and other grasses, but this is not considered to be of practical importance.

Names in different languages

Danish :	Gulrust
German :	Gelbrost
English :	Yellow rust
Latin :	Puccinia striiformis
Norwegian :	Gulrust
Swedish :	Gulrost

Yellow rust in stripes on wheat leaves

Yellow rust on a wheat leaf

Dark points in the leaf can occur when yellow rust has been stopped – see the arrow

Right: a controlled attack which is the cause of the rather dull colour

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