Yellow (Stripe) Rust

Pathogen

Puccinia striiformis

Hosts

The disease affects wheat, barley and triticale.

There are distinct forms of the fungus which are specific to crops.

  • P. striiformis f.sp.tritici can attack wheat
  • P. striiformis f.sp.hordei can only attack barley

Within the forms of P. striiformis there are races which can only attack particular varieties.

Symptoms

The characteristic symptom is of parallel rows of yellowish orange coloured pustules on the leaves of adult plants. Epidemics of yellow rust often start as individual plants, usually in the autumn. Symptoms develop slowly over winter and are often missed until the early spring when small patches or foci of infected plants can be seen in fields  Early on the yellow to orange coloured pustules are very difficult to distinguish from brown rust  However, yellow rust lesions tend to spread as a band across young leaves, often with a yellow band on the leaf moving ahead of the sporulating lesion  On older leaves pustules tend to occur in obvious stripes  Severe attacks quickly give rise to chlorosis and later necrosis of leaves. Infected leaves can rapidly desiccate in May/June if weather conditions are warm and dry. In severe attacks yellow rust infection of the ears can occur, resulting in the formation of masses of spores between the grain and the glumes. At the end of the season, secondary black spores (teliospores) are sometimes produced amongst the stripes of pustules.

Life Cycle

Yellow_rust-life-cycle.gif

P. striiformis requires living green plant material in order to survive. The fungus survives the winter as dormant mycelium or active sporulating lesions on volunteers or early autumn-sown crops. Set-aside provides an excellent source of yellow rust overwintering inoculum. Yellow rust within plant tissue can survive very low temperatures so once infected the fungus will usually survive the coldest of UK winters. In the spring, particularly in cool moist weather, the fungus starts to grow and produces active sporulating lesions. Temperatures of 10-15OC and a relative humidity of 100% are optimal for spore germination, penetration and production of new, wind-dispersed spores. The fungus is inhibited by temperatures over 20OC although strains tolerant of high temperatures do exist. The complete cycle from infection to the production of new spores can take as little as 7 days during ideal conditions. The disease cycle may therefore be repeated many times in one season. During late summer, the dark teliospores may be produced. These can germinate to produce yet another spore type, the basidiospore, but no alternate host has been found. Although the teliospores seem to have no function in the disease cycle they may contribute to the development of new races through sexual recombination.

Importance

The disease is very sporadic in the UK occurring more often in the east of the country and in coastal areas which may have cool summer weather accompanied by regular mists. Severe epidemics are usually associated with very susceptible varieties, mild winters and cool moist summers. The development of new races of P. striiformis can result in varietal resistance being overcome within a short period of time. Yield losses of 40-50% have often been recorded in susceptible varieties.

Cultural control

In the UK, disease resistance has been a very successful means of control. Severe epidemics of yellow rust were a frequent occurrence on winter wheat in the mid 1980's and late 1990's because of a combination of very susceptible popular varieties and weather conducive to yellow rust epidemics. New races of yellow rust can appear and develop very rapidly if conditions are suitable and varietal resistance ratings can plummet as a result. When new races of yellow rust appear the disease may attack varieties which were previously regarded as highly resistant.

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