Archive: Ryegrass disease risk

Published 29 August 14

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Managing the disease risk

In the warm moist conditions of late summer, early autumn disease risk in ryegrass plants can increase considerably, reducing grass dry matter (DM) yields and quality, and increasing sward rejection. Dr Debbie McConnell, DairyCo’s research and development manager, looks at identifying and minimising the risk from the most common diseases in ryegrass in England and Wales.

It is not something we often look out for, however, disease in ryegrass plants does not only affect yield but also alters sward quality and composition. Though some fungicides are effective against diseases, these are often limited in use. Instead, it is better to know what diseases are prevalent in your area and consider selecting resistant varieties to manage the disease risk. On average, a disease can reduce grass DM yields by around 3% in the first three years. Although this may seem small, combined with reductions in quality and increased sward rejection, this can often result in under-utilised, difficult to manage grass swards.

Leaf diseases often affect the chlorophyll content of the plant, reducing the photosynthetic area available and stunting plant growth. This also increases the amount of dead material in the ley which in turn affects sward quality. Mildew and Rhynchosporium in Italian ryegrasses, for example, have been known to cause 1-2 unit reductions in sward D-value, while crown rust has be found to increase rejection in grazing swards.

The severity of disease is dependent on overall climate in different areas of the country. Some diseases are more prevalent in the generally wetter and warmer west and south west. Figure 1, below, gives a good indication of the risk of some more common diseases.

Figure 1: Regional disease risk for major diseases found in ryegrass across England and Wales

Ryegrass disease map


Identifying diseases

At this time of year Crown Rust and Drechslera are perhaps the most likely diseases to see in swards. Crown Rust (Figure 2) usually occurs in the late summer and autumn, particularly in warm and dew conditions. It can be identified by yellow-orange pustules across the surface of the plant leaf. Similarly, Drechslera is often apparent at the shoulders of the season and is encouraged by cool, wet, humid conditions. Black spots/marks on the plant are symptomatic of Drechslera (Figure 3).

Picture 2 Ryegrass disease

Figure 2: Crown rust in grassland


Picture 3 ryegrass disease


Figure 3: Drechslera can be identified by black markings

Mildew and Ryegrass Mosaic Virus (RMV), two diseases which are more commonly seen in spring and summer, are more likely to be found in dry conditions and do not reach high risk levels in wet areas. Mildew (Figure 4) is denoted by white powdery pustules on the upper leaves and parts of the stem. Mildews can also affect clover.

RMV can be identified by streaky paler areas on the upper surface of plant leaves (Figure 5). This is particularly common in Italian ryegrass varieties.

Picture 4 ryegrass disease

Figure 4: Mildew on ryegrass. (Image courtesy of

Picture 5 Ryegrass disease


Figure 5: Ryegrass Mosaic Virus is identified by striped pale patches on leaves. (Image courtesy of

Rynchosporium (Figure 6) is a springtime, wet weather disease and is usually confined to the west and south west of England and Wales. It can be identified by brown patches on the leaf surface. There are also a number of less prevalent diseases which are more sporadically seen across years. These include brown rust, stem rust and barley yellow dwarf virus.

Picture 6 Ryegrass disease

Figure 6: Rhynchosporium is a wet weather disease evident by brown patches on leaves. (Image courtesy of UVM Extension)

Minimising disease risk

Like any living organism, stressful conditions weaken the immune system of the plant making it more susceptible to disease. As a result, drought, nutrient deficiency, lack of sunshine, heavy dews, etc. can cause greater infection. In addition, young shoots in mature swards are susceptible to disease, as the lack of photosynthesis deeper in the canopy and self induced nutrient deficiencies, caused by the mobilisation of nutrient to the parts of the plant with access to sunlight will increase disease risk.

Hence, maintaining good soil and plant fertility is one method by which we can reduce plant susceptibility to disease and there may be some benefit to removing heavy covers or cutting slightly earlier to reduce the harbouring of pathogen spores.

Introducing disease resistance, when selecting ryegrass varieties at reseeding time, is arguably the most effective disease control method. On the Recommended Grass and Clover Lists (RGCL) each ryegrass variety is graded on a 1 (poor) to 9 (good) scale for resistance to Crown Rust, Drechslera and Mildew. In areas of high disease risk, selecting varieties with a moderate (6 – 7) or high (8 – 9) disease resistance rating is essential.

On the new RGCL online tool (available from varieties can be filtered and sorted dependent on their resistance to crown rust (Figure 7). 

Ryegrass disease figure


Figure 7: Use the new RGCL tool to identify disease resistance in varieties.