Cereal rust diseases attract strong emotions in growers. NIAB plant pathologist Dr Rosemary Bayles talks through what affects rust development in the third of Farmers Weekly’s arable academy modules
What are rusts?
Five rust species are commonly found on cereals in the UK; yellow and brown rust infect wheat and barley, while crown rust is found in oats. Yellow rust is recognised by its small, yellowish, oval to circular pustules which appear in stripes on the leaves, hence its alternative name of “stripe rust”. A single infection can result in a stripe the full length of the leaf.
On seedling leaves, however, stripes do not form and pustules are scattered at random. In severe infections, pustules may also be found inside the glumes of the ears and on the awns.Brown rust is distinguished by its brown pustules.
Normally circular in shape, they are somewhat larger than yellow rust pustules, and are scattered at random on the leaf. A pale green halo can often be seen around each pustule. In severe infections, pustules may appear on the leaf sheaths. Oat crown rust produces bright orange pustules.
Rusts feed on the living cells of their hosts, meaning they are dependent on living green plant tissue for their survival. They obtain nutrients from living cells of the host plant by producing specialised nutrient-absorbing structures called haustoria.
Rusts in the UK have greatly simplified life cycles, compared with many worldwide rust fungi. For example, wheat yellow rust, unlike many other rusts, has only one host plant species, and the survival and success of the fungi is entirely dependent on infection by airborne asexual spores (uredospores). Cereal rust fungi survive the post-harvest period on volunteer seedlings infected by uredospores. Infection then spreads to early-sown autumn crops and from these to later-sown crops. This ‘green bridge’ is critical for survival. Rusts over-winter either in the leaf or on the leaf surface. During the spring and early summer, uredospores are produced, which re-infect the plant and are responsible for the epidemic rapidly cycling.
Severe infections of yellow rust on susceptible wheat varieties may result in yield losses of up to 40%. Losses due to brown rust are generally less, up to 20%, since infection tends to develop later in the season during grain filling. Rust infection on the flag leaf leads to poor grain filling and loss in grain quality.
CEREAL RUST SPECIES IN UK
Wheat yellow rust
Puccinia striiformis f.sp. tritici
Wheat brown rust
Barley yellow rust
Puccinia striiformis f.sp. hordei
Oats crown rust
Factors affecting rusts
- Weather While environmental conditions for yellow and brown rust development are similar, the optimum temperature for brown rust is slightly higher, which explains why it tends to occur later in the season and is more of a problem in the south of the country than the north.
For yellow rust the optimum infection temperatures are 8-13C, although infection can take place well outside these ranges. The optimum temperature for the disease to develop is around 12-15C. At 15C symptoms can appear in as little as 11 days from infection. Yellow rust development is favoured by cool moist conditions, but not heavy rainfall, which tends to wash uredospores off leaves. During hot or dry periods sporulation ceases, but will often re-start when favourable conditions return.
Optimum infection temperatures for brown rust are 8-20C, with disease developing fastest around 20C. Latent periods vary from 8-14 days at 10-25C to several weeks at 5-10C. Brown rust development is favoured by warm days (20-25C) and mild nights with adequate moisture for night time dew development. Only prolonged periods of frost which kill the leaves of the plant are likely to kill rust during the winter.
Inoculum can travel long distances via airborne uredospores so rotation has relatively little effect on the risk of infection. However, wheat following wheat, or barley following barley, is likely to be at increased risk if volunteers from the preceding crop survive to form a green bridge.
- Variety choice
Variety has a major impact on the risk of rust developing in the crop. At one extreme, wheat varieties with resistance ratings of 8 or 9 for resistance to yellow rust or brown rust are available on the UK Recommended List, and are unlikely to become infected or suffer significant yield loss. But varieties with ratings of 3 or 4 are at high risk of infection. There is also a wide range of resistance to the rusts among barley varieties.
- Pathogenic variation
Infection is complicated by rusts ability to exist as a large number of different races. These are distinguished solely by their ability to overcome the resistance of different varieties. One variety may be susceptible to one race, while being resistant to others.
The rust pathogens are able to adapt to overcome the resistance of previously resistant varieties by producing new races. When this occurs, a variety with a high resistance rating may unexpectedly become heavily infected. Although this is sometimes referred to as “breakdown” of the variety’s resistance, it is the pathogen, not the variety, that has changed. Constant monitoring of rust populations by the UK Cereal Pathogen Virulence Survey (UKCPVS) based at NIAB and funded by DEFRA and HGCA, helps detect new races.
Because the rusts are entirely dependent on the green bridge for survival between harvest and the emergence of the next crop, the destruction of volunteers is a key step in reducing the carryover of inoculum. Early sowing tends to increase the risk of autumn infection.
- Variety Choice
The use of resistant varieties is a major weapon against rusts. Varieties with good resistance are available and should be the first choice in areas known to be at high risk. Variety diversification is a further consideration for avoiding yellow rust in wheat. Most farmers spread their risk by growing more than a single variety of wheat on the farm.
There is a wide choice of triazoles and strobilurins with activity against the rusts. Fungicides within these groups differ markedly from one another in their activity, so it is important to select one with a relatively high rust activity for susceptible varieties and high risk situations.
Morpholines and spiroketalamines also have some rust activity and can usefully be added to a standard programme based on triazoles and strobilurins to strengthen rust control.
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