Know your rusts to help protect your crops

Many cereal growers have never had to deal with rust diseases by virtue of the crops they grow or their location in the country. There are general comments that you can make about risk but there are no fixed rules. Rust diseases can pop up anywhere and at any time across a range of crops.

Perhaps the best-known and most widely publicised is yellow rust because it is a major problem in Britain and, also, because of its potential to devastate wheat crops. But yellow rust is not confined to wheat – it can also do serious harm in winter barley. This is a serious disease with the potential to reduce grain yields by over 60%.

Brown rust attacks barley and wheat and it is also a serious disease. It can move quickly through a crop but it tends to be less devastating than yellow rust. Many modern varieties have reasonable genetic resistance and so the problem is less common than a few decades ago.

Crown rust is somewhat similar to brown rust but it only attacks oats. However, it is also a potentially serious disease of grasses. It produces singular lesions, oblong in shape. Crown rust is a formidable disease in the areas and regions most prone to attack. The problem tends to be worst along the eastern and southern seaboards but it is highly influenced by local climate and often occurs in some western counties.

Rust will also attack beet and beans, and can be relatively serious.

Brown specks in the leaf are sometimes confused with rust. A simple rule is that the lesion colour on rust will rub off on your finger because the colour is generated by the spores produced. And the spores erupt through the surface of the leaf. Rust diseases can develop and spread quickly to devastate a crop if weather conditions are suitable.

Control tools

Rusts are broadly similar in terms of their control and the fungicides that can be used. All diseases are well controlled using triazole fungicides but epoxiconazole (Opus), tebuconazole (Folicur) and cyproconazole (originally Alto) are particularly effective. However, the longevity of control will be influenced by the rate used and temperatures following application. The triazole actives are good on curativity but if you are tackling an active and well-established epidemic, the use of a morpholine like fenpropimorph (Corbel) will provide faster knockdown but generally shorter protection.

Actives tend to be broadly effective on all the rusts but this is not always the case. Products can differ in their curativity versus protective abilities. Actives and products can also differ between species on the same crop.

Examples from the HGCA fungicide evaluation show that cyproconazole is stronger on yellow rust than on brown rust. So is epoxiconazole, which is actually stronger against both. Prothioconazole (Proline), which is regarded as being weak on rusts, is actually quite strong against yellow rust (similar to cyproconazole and tebuconazole), while being weak on brown rust. Tebuconazole is the only triazole that is regarded as being equally strong against both diseases.

Of the current SDHI mixtures, all are very good against brown rust but Aviator and Venture are not as strong as Seguris and Adexar against yellow rust. Penthiopyrad (Vertisan) and fluxapyroxad (Imtrex) are both strong on their own against yellow rust but they are not rated against brown rust by the HGCA due to a lack of information.

Of the strobilurins, pyraclostrobin is the strongest against brown rust and it and picoxystrobin are most effective against yellow rust.

The well-known HGCA rate response curves show that, in general, products have greater activity against brown rust than yellow rust.

Proline is the obvious exception to this statement as it is substantially poorer than all the other products against brown rust. Indeed, the HGCA efficacy chart shows that all the other products tested (Adexar, Aviator, Gleam, Comet epoxiconazole, Imtrex, Seguris and Vertisan epoxiconazole) were equally good at half rate as at full rate for the control of brown rust.

Good control was not so easily achieved with yellow rust, where Imtrex and Comet were somewhat poorer performers and Proline gave very good control at full rate. But the general level of control from half rates on yellow rust was less than was the case with brown rust. Some products were almost equally effective from half and full rates, such as Adexar, Aviator and the Vertisan epoxiconazole mix. But others, such as Proline, Epoxiconazole and Seguris showed better control when the full, rather than half, rate was used.

Crown rust

Crown rust only attacks oats but it will also attack grasses and can often be seen on same.

It is basically a single spot lesion but when the infection is high, the lesions will be close together and basically eradicate the foliage. Individual lesions are longer than they are wide, giving them an oval shape.

The spores tend to be between orange and dark brown in colour.

Crown rust responds to temperature and so it grows and infests faster in warmer conditions. It can occur in autumn if conditions enable infection and its rate of progress will then depend on crop growth and weather in the autumn and winter.

Crown rust does not tend to show itself in patches in the crop. Infection tends to quickly spread through a crop, especially when it occurs initially in late spring.

Yellow rust

This is the best known of the rust diseases because of its historical impact and the rapidity with which a resistant variety can break down and become susceptible in the field. This resistance was important before the fungicide era and it remains important today. Robigus and Oakley are recent examples of initially resistant varieties breaking down completely against this disease.

Yellow rust is also called stripe rust due to the most common symptom that it produces on adult leaves. However, neither colour nor shape should be used to verify yellow rust identification. This is because the disease can take on two different forms on the foliage, depending on the stage of growth of the crop.

The yellow or stripe rust form tends to occur as the leaves get stronger and the disease is confined to the soft tissue between the veins. But on a juvenile plant individual pustules are much smaller, orange brown in colour and it can blitz the entire leaf, especially from the centre to the tip.

This juvenile stage can be confused with brown rust due to its colour. But it is much more prolific than brown rust and produces five to 10 pustules on the same footprint as one brown rust lesion. Its prolific spore production on a juvenile plant can turn the soil beneath orange in colour.

Yellow rust tends to appear initially in foci in the field. So yellow patches in the crop are the initial giveaway. These patches occur because the infection on one individual plant moves out and up the neighbouring plants. And it is the yellow lesions on the upper leaves that make the patch.

Most of the damage caused by this disease, which can infect wheat and barley, is caused by the yellow or stripe form. This can multiply with frightening rapidity to quickly destroy all green foliage. These lesions will be yellow in colour, tend to be about 2mm wide and can be any length.

The individual pustules that produce the spores and make the lesions are longer than they are wide, a bit like crown rust. But they produce huge spore numbers which infect nearby leaves and expand the disease. An infected crop can quickly go from a few small circles to a fully yellowed crop. This will mean significant yield reduction.

Yellow rust infection will often occur initially over the winter. Disease development is temperature-dependent and it will be at its worst when infection happens in late spring.

Brown rust

Brown rust also attacks wheat and barley and background infection is generally influenced by variety resistance. Most varieties of both crops show good resistance but, this spring, many winter barley varieties, thought to have good resistance, have picked up the disease.

For this reason, growers need to be aware of this threat and how to identify it. Brown rust is commonly confused with brown spotting in the leaf, which can be caused by a range of different things. If it is rust, the active rust pustules will rub off on your finger. If the spores (or colour) do not come off on your finger (or a white tissue) then it is not rust (or at least not active rust).

Brown rust pustules on barley are often circular in shape and around 1mm in size. The pustule is surrounded by a pale or yellow halo, which is also about a millimetre wide, and this makes each lesion about 3mm in size.

The presence of a halo around the pustule is important. If a spot does not have a halo, it is not active brown rust. Brown rust appears to have a much greater effect on the crop than the area covered by its lesions. It is almost like the toxins it produces suppress plant growth.

The flip side of this can be seen when a leaf is senescing. In this situation, the fungus acts to keep the leaf tissue alive so that it can survive itself. This is often seen by the presence of older rust lesions surrounded by a green rather than a pale halo on senescing leaves.

It is important to be able to identify the rust diseases. Each has a relatively distinct lesion but colour is not the key to identification. Rust can be verified with the help of a lens.

All of the rust diseases have one common characteristic – spores are produced in a specific spot in the leaf which eventually burst through the surface like a volcano spewing spores all over the place.

Each individual lesion is a bit like a pimple on your skin. The pus develops beneath the skin to form a white tip and can then burst through. So, with a lens you can clearly see that the epidermis, the surface of the leaf, has been punctured from the inside out and you can see the fungal spores coming up from the leaf. All rusts have this appearance characteristic but the individual types have mainly shape characteristics.