An introduction to disease resistance in cereals and oilseed rape

Varieties that are more resistant allow greater flexibility in fungicide programmes – including the omission of sprays and the use of reduced doses, especially where disease pressure is low.

Varietal disease resistance is the foundation of control. 

Broadly speaking, there are two forms of resistance – based on minor and major genes.

Minor genes

Race-non-specific/multigene resistance

• Individual minor genes provide a low level of resistance but can be combined to give moderate-to-high levels of resistance
• Usually, this type of resistance is durable and not subject to sudden failures
• This is the main type of resistance available against septoria tritici in wheat

Major genes

Race-specific/single gene resistance

• A major gene can provide a high level of resistance
• Usually*, this type of resistance is more susceptible to changes in pathogen populations
• In particular, new races/types of the fungus may result in a relatively sudden loss of varietal resistance
• When such a change occurs, varietal susceptibility depends on the underlying minor-gene resistance
• This is the main type of resistance available against yellow rust

*However, the strong mlo resistance to powdery mildew in spring barley and the moderate Pch1 resistance to eyespot in wheat have been durable for many years.

AHDB Recommended Lists (RL) trials assess disease symptom expression in wheat, barley, oat and oilseed rape varieties across the UK.

Varietal disease resistance ratings are published on a 1 to 9 scale for several key diseases, where 1 indicates low resistance and 9 indicates high resistance.

Modern cereal varieties with good levels of resistance often have little or no yield penalty.

Some varieties provide exceptional disease-control packages.

For example, in wheat, the untreated yield of some resistant varieties is just 5% lower, compared with fungicide-treated yields.

Such varietal resistance can be exploited to help reduce the intensity of fungicide programmes.

Yield responses to fungicides, between the most- and the least-resistant varieties, can vary as much as 3 t/ha.

A variety with a treated yield benefit of 0.5 t/ha will only justify having the value of 0.5 t/ha of grain spent on it.

Note: As pathogen populations evolve, previously defeated genes may become effective again, so varietal disease ratings can go up, as well as down.

Some varieties are susceptible to yellow rust when plants are young, but go on to develop some level of resistance after early stem extension during the adult-plant stage.

The verticillium ratings in oilseed rape are defined as: moderately resistant (MR), susceptible (S) or intermediate (I).

Some disease resistance or tolerance characteristics are presented as a ‘breeder’s claim’, which are not verified in RL tests.

Accepted claims are associated with clear genetic markers that allow breeders to verify if a variety has inherited the trait.

For information on these claims, contact the breeder or UK contact.

• Barley yellow dwarf virus (BYDV) resistance
• Pch1 eyespot resistance

Orange wheat blossom midge resistance is also presented as a breeder’s claim.

• Barley yellow dwarf virus (BYDV) tolerance or resistance
• Barley yellow mosaic virus strain 1 (BaYMV1) and strain 2 (BaYMV2) resistance
• Barley mild mosaic virus (BaMMV) resistance
• mlo resistance to powdery mildew in spring barley
• Wheat dwarf virus (WDV) tolerance or resistance in winter barley

Oilseed rape disease claims:

• Turnip yellows virus (TuYV) resistance
• Clubroot resistance

Tolerance to specific imidazolinone herbicides (a Clearfield® variety) and pod shatter resistance are also presented as a breeder’s claim.

Varieties may be described as resistant (R) or tolerant (To) to a plant virus.

A resistant variety will show few or no symptoms, with no significant yield loss and no, or very limited, viral replication.

Although a tolerant variety may show substantial symptoms, it does not suffer significant yield loss as a result, but viral replication will occur.

These images show septoria tritici symptoms on winter wheat varieties with a range of disease ratings rounded to the nearest whole number.

The winter wheat was drilled on 11 October 2022 at an RL trial site in Devon. 

The pictures, which were taken on 13 June 2023, showed clear variety and fungicide interactions.

Septoria tritici disease pressure was relatively high in the 2022–23 growing season.

Varietal mixtures increase genetic diversity within a field. This can slow or reduce the spread of some diseases, especially when varieties with different disease resistance profiles are grown.

Levels of disease reduction and yield increases depend on many factors, including disease and nutrient levels and the crop and varieties chosen.

Our variety selection tool makes it easier to assess the merit of each variety, by taking into account the relative importance of each disease.

The tool also provides a visual way to compare fungicide treatment benefits across varieties.