Changes to brown rust resistance ratings, the introduction of new fungicides and increasingly unpredictable weather patterns are all influencing the effects of brown rust – a disease that often plays second fiddle to yellow rust.

Traditionally more common in coastal regions and in the South, brown rust tends to develop later in the season than the other foliar diseases, preferring hotter, drier conditions than yellow rust.

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Severe infections take out significant green leaf area and get onto the ear, causing average yield losses of up to 40% and affecting grain quality.

Until recently, brown rust was a lesser problem than other diseases in most seasons. That changed with the discovery of new races, some of which have been able to overcome varietal resistance and cause problems where they weren’t expected.

A good example is the popular milling wheat variety Crusoe, which saw its brown rust rating fall to a 3 because of its susceptibility to a race of brown rust that re-emerged in 2015. Another big name is the soft Group 3 wheat Firefly, which has seen its rating drop from an 8 to a 5.

For 2021, there has been a change in the way brown rust resistance ratings are calculated. Rather than being based on the average levels of disease seen in trials over a three-year period, they are now weighted so that the most recent year of data has the largest influence.

Agronomists point out that brown rust is a disease that tends to explode from mid-June onwards, making fungicide choices at the ear spray timing important where susceptible varieties are being grown.

If it gets a hold, brown rust can be difficult to overcome. Most fungicides work better as protectants on the disease, with new introductions such as Revystar offering very high levels of control.

Symptoms

During autumn and winter, brown rust symptoms are confined to older leaves. At this stage, it can be difficult to distinguish from yellow rust, as both diseases are characterised by small, orange-brown pustules scattered over the leaves.

By the summer, brown rust and yellow rust are much easier to tell apart. With brown rust, the pustules are scattered randomly, whereas yellow rust pustules are arranged in stripes. There is also a greater differentiation in colour.

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Late in the season, brown rust can become very severe and result in leaf death, with leaf sheaths and ears also becoming infected. Tiny black spore cases may be visible on diseased tissue, indicating a second developmental stage of the fungus.

While these teliospores seem to have no function in the disease cycle, they may contribute to the development of new races through sexual recombination.

How it spreads

Brown rust requires living green plant material to survive, overwintering as dormant mycelium or active sporulating lesions on volunteers or early-sown crops.

The disease is more efficient at entering the plant than yellow rust because the spores have sensing mechanisms that allow them to identify ideal leaf entry points.

Once within the plant tissue, the fungus can survive at very low temperatures, although cold winters do help to reduce its survival. In the spring – especially in milder, moist weather – the fungus starts to grow.

Temperatures of about 10-15C and a relative humidity of 100% are perfect for initial spore germination.

Brown rust then spreads from disease foci in fields, transferred by wind-blown uredospores that can travel long distances. Typically, it develops later in the summer than yellow rust, during warmer days with dewy nights and humid spells of weather.

However, when there are higher-than-normal spring temperatures, the disease can develop much earlier in the season. As a result, crops should be monitored regularly.

As the season progresses, ideal growth conditions are days with temperatures of 15-22C, followed by overnight dews. Surface moisture on leaves is essential for spore germination and the production of the brown pustules on the leaf that release the uredospores.

Dry, windy days help to spread the disease, which is also favoured by late nitrogen applications. By mid- to late summer it has an optimum life cycle of seven days.

It is often more difficult to control than yellow rust, because it produces pustules that have the ability to form deep inside the leaf, making it harder to target with fungicides. It is also active over a wider range of temperature (7-25C) than yellow rust. 

Managing brown rust

Early-sown crops are at greater risk of brown rust, especially if a mild winter follows, as they are more likely to be infected by the wind-blown spores.

Southern and eastern parts of the UK, which tend to have higher summer temperatures than other regions, also have a higher incidence of the disease, putting susceptible varieties at greater risk.

Control of volunteers, which provide the “green bridge” between harvest and the emergence of new crops, is advised. While resistant varieties are available, they need to be monitored regularly for the disease, as it is a very diverse pathogen and new races can occur.

Fungicides containing azoles, strobilurins and SDHIs are very effective, providing treatment intervals do not exceed three to four weeks.

The latest results from the AHDB’s fungicide response work show that single active ingredients give very good control of brown rust, with the exception of prothioconazole.

The best SDHI/azole mixes proved to be Elatus Era and Revystar, with Ascra also performing well.

Control programmes

It’s rare to have to take early action against brown rust, or to do anything specific for its control over and above a standard fungicide programme based on SDHIs and azoles, says Bill Clark, technical director of Niab Tag.

He points out that it is a disease that doesn’t tend to cause problems early in the season, so any fungicide requirements are generally considered later.

Using azoles, strobilurins and SDHIs at T1 and T2 will help control the disease, he adds.

“Any differences in the performance of individual products are difficult to pick up in a field situation, although trials suggest that epoxiconazole and metconazole are more effective than prothioconazole on brown rust. So choose them according to the disease pressures you are facing at the time.”

Brown rust is temperature-driven, says Adas cereal disease specialist Jonathan Blake, so it is easier to predict than some of the other diseases if temperatures rise and there is moisture present.

“It can be hard to stop it once it gets going, despite the amount of chemistry we have for it. The T1 and T2 applications will check it, and then specific action can be taken at T3 if required.”

An azole at T3 will work well, he advises, although the best active ingredients for brown rust control are the opposite to those required for a septoria top-up.