If you are one of these growers who seldom has to tackle any of the rust diseases, then it is important to state that these are serious problems. Growers may be somewhat used to dealing with crown rust on oats, as most of our varieties are very susceptible but many wheat growers are less familiar with yellow rust and this an even more serious disease.

Yellow rust infection is now evident around the country. Like all the rust diseases, the fungus grows in the leaf tissue and the multiplicity of spores produced erupt through the leaf surface to spread the disease locally. For this reason it is generally found, initially, in small circles but if you do not stop a small circle it will become a big circle very quickly.

An active disease lesion continues to spew out spores for distribution and total defoliation of infected patches can occur very quickly. However, while spores mainly move locally, they can be carried in the wind and indeed it is common that the initial appearance of new races that can attack previously resistant varieties will arrive into north Leinster on an easterly wind. Sometimes spores can be carried further west and even up into Donegal. Then, as in 2017, a northerly wind distributed the infection southwards.

Yellow rust has been very evident in winter wheat crops in north Leinster all winter and especially in springtime. The young disease on juvenile plants tends to produce individual brown lesions which produce millions of spores for distribution. As the leaf becomes stronger and older the disease symptoms change and new lesions appear as elongated lesions between veins and the spore colour is definitely yellow. The lesion shape gives it its other name, which is stripe rust.

An untreated outbreak can severely defoliate a susceptible variety within a few weeks. For this reason it can be a severe yield robber with up to 60% yield loss commonly recorded in this country. So it is certainly not a disease to ignore.

Treatment

The challenge with yellow rust is to identify it early. In theory, all modern fungicides are effective against yellow rust but efficacy will be influenced by the specific actives applied and timing of application relative to infection.

Like all other diseases, yellow rust has a lag period between initial infection and the appearance of lesions so one can spray a crop not seeing seen infection to find bad disease a number of days later because the disease had broken out before the fungicide actives could stop it.

There are differences in the ability of actives to halt yellow rust. There has been no evidence of reduced sensitivity of fungicides to rusts but individual actives do have different capabilities. Triazoles such as epoxiconazole (Opus/Strand) and tebuconazole (Folicur) have always been strong against the disease. They tend to be able to dry up an existing infection and provide further protection but, as with any other disease, they cannot make the lesion go away.

Traditionally, a raging infection is best addressed through the application of a morpholine. These tend to provide the fastest knockdown but they are not the most persistent. Epoxiconazole and tebuconazole have always given good control but this can only begin from the point of application. In general, strobilurin and SDHI actives function as good preventatives or protectants and so work against future infection more so than a current infection.

Activity

Research compiled by AHDB in the UK suggests difference in product capability on yellow versus brown rust. Of the products tested, epoxiconazole was more effective than prothioconazole and the same holds in mixtures with other families.

In general azoles – epoxiconazole, tebuconazole and cyproconazole – are more effective than strobs (of which Comet/Modem is best) and these are more effective than SDHIs (exception may be benzovindiflupyr and isopyrazam, which both appear very active on yellow rust).

Yellow rust control must be considered in the light of recent or current T2 applications. In this regard, mixes containing metconazole or prothioconazole will be weaker against yellow rust, especially where infection is visible and vibrant. So if you are using products like Librax or Elatus or Ascra you should include a morpholine (0.7 l/ha) to get a quick knockdown of infection. If you are using products based on epoxiconazole (Adexar) you may need to add additional epoxiconazole if you are using a reduced rate of the mix.

Experience in the UK has indicated that yellow rust is difficult to knock completely with a single treatment.

Healthy lesions will continue to spew out spores so a gentle rub of your finger or a white tissue will give an indication of the level of spores being produced.

If you only get a few spores and little discolouration then the disease has obviously been hit. But keep a continuous eye on your crops especially for the appearance of new lesions.