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Yellow rust is the most important rust affecting wheat and can cause devastating yield losses. Bill Clark, from Broom's Barn Research Centre, rounds up what growers need to know.

Yellow rust is caused by the fungus Puccinia striiformis, which has distinct crop-specific forms. Puccinia striiformis f.sp.tritici can attack wheat and triticale, whereas Puccinia striiformis f.sp.hordei can only attack barley. There are also specific races of yellow rust that can only attack certain varieties.

Symptoms

The classic yellow rust symptoms are parallel stripes of yellow/orange-coloured lesions on the leaves of adult plants.

Epidemics usually start in the autumn with individual plants showing symptoms, but yellow rust can then easily be confused with brown rust, which is also often present.

Autumn yellow rust infections tend not to produce stripes and are often more orange/brown. These initial infections are hard to see and usually missed until early spring, by which time the disease has spread to produce small patches of infected plants.

On older leaves, pustules tend to occur in obvious stripes. Severe attacks quickly give rise to leaf yellowing and, later, necrosis. Because the pustules tear open the leaf epidermis, severely affected leaves lose a lot of moisture and dry out quickly in May/June if the weather is warm and dry.

Later on, yellow rust can infect the ears. Infected glumes look very pale as the fungus sporulates on the insides of the glumes and outer parts of the grain.

Later still, black secondary teliospores can be produced within existing lesions on leaves and ears. These, the sexual stage of the fungus, are thought to play no role in its life cycle.

Importance

Globally, yellow rust is the most important of the rusts affecting wheat. In some countries it has become a major problem because aggressive strains adapted to high temperatures have emerged.

Recent mild winters in many European countries have led to greater survival over winter and earlier epidemics.

In the UK, severe epidemics are usually associated with very susceptible varieties, mild winters and cool, moist summers.

Epidemics tend to be remembered as linked to particular varieties, such as Slejpner, Robigus and, most recently, Oakley, that "break down". In fact it is not the variety that changes – the rust mutates or adapts to overcome the varieties’ resistance.

We are fortunate in the UK that extensive breeding programmes have given us varieties with a wide range of resistance genes that protect us from many of the rust races around the world.

However, it is only a matter of time before currently effective resistance genes are overcome by new races. So the battle is never won – it is more a series of skirmishes with new races appearing constantly, overcoming our current defences.

Yellow rust is most common in coastal areas that may have cool summer weather with regular mists. Yield losses of 40-50% have often been recorded in susceptible varieties grown in high-risk areas. However, in the UK yield losses are, on average, about 2%.

Yellow rust pustules tear open the epidermis and dry leaves out.

Fungicide strategies

In high-risk areas with susceptible varieties, yellow rust can be very severe and may need to be treated either with a systemic triazole seed treatment or an autumn spray. This prevents the disease establishing during the autumn, reducing the risk of a very early spring epidemic.

In most UK areas, a fungicide would not be necessary until the disease appears in the spring. Most triazole fungicides applied for controlling septoria at T0 or T1 give good control of yellow rust.

The disease can be very severe if conditions remain cool into May/June and flag leaves and ears can be badly affected. Flag leaf sprays of triazoles and strobilurins give good control.

Ear sprays may be needed specifically to control late infections in high-risk situations.

Life cycle

The fungus needs living green plant material to survive – it cannot grow on dead or dying plant material. It can only survive winter as dormant mycelium inside a living plant or as active sporulating lesions on cereal volunteers or in early-sown crops.

Yellow rust mycelium can survive freezing, so once plants are infected, the fungus will usually withstand the coldest UK winters.

In spring, the fungus starts to grow and produces active lesions. Yellow rust needs temperatures of 10-15C and high relative humidity for spore production, germination and infection.

The more common races are inhibited by temperatures above 20C. Hot, dry periods will stop an epidemic in its tracks, preventing sporulation and disease spread.

However, high-temperature-?tolerant strains, now common in the USA, would not be slowed by UK summer temperatures.

Infection to new spore production can take as little as seven days during ideal conditions, so the disease cycle can be repeated many times in one season. The late-season teliospores germinate to produce yet another spore type, the basidiospore; but no alternate host has been found, so the teliospores seem to have no function in the UK.

Varieties

Many varieties have very good resistance to yellow rust. However, resistance ratings can change dramatically within a season.

Many varieties rated 9 on the Recommended List may rely on a single gene and so are very prone to sudden changes in the yellow rust population.

In 2008, yellow rust was found on some resistant varieties, notably Solstice, Timber and Hereford. This indicates that there are new races of the disease in the UK that can overcome the resistance of these and perhaps other varieties.

In 2009, further outbreaks, some severe, were seen in crops of Solstice and Oakley, putting varieties such as Qplus and Ketchum (which share parentage with Solstice) at risk.

Other resistant varieties Humber and Viscount have also shown symptoms. So the resistance ratings for many of these varieties will be looked at carefully this autumn