Yellow rust in cereals: Disease symptoms and management
Yellow rust (Puccinia striiformis) is a fungus from the rust fungi family (Pucciniales). It belongs to the Puccinia genus, which is the largest group of rust fungi with around 3,000 species.
Yellow rust is common all over the world. Unlike brown rust which prefers warmer temperatures, yellow rust generally prefers cold and wet weather. The fungus can infect many pasture grasses and all cereal varieties. Wheat, barley and triticale are common host plants. It is well adapted and does not require alternate hosts to ensure its survival or reproduction. As such, Puccinia striiformis is one of the most threatening and serious rust diseases affecting our cooler climes.
How does yellow rust survive and spread?
Yellow rust pathogens require living plant material to survive. They overwinter as mycelium or spores on winter cereals, volunteers and couch grasses. The fungal mesh can survive in temperatures as low as minus twenty degrees, however, the spores die in frost.
Urediniospores are distributed by wind and rain, and this is primarily how the disease spreads. Persistent free moisture (rain or dew) combined with temperatures between ten and sixteen degrees provide the optimum conditions for germination and spread of the disease. Although the fungus can become active at lower temperatures, rapid and endemic disease development is greatest under optimum conditions. New races that have emerged in recent years because of the cultivation of resistant varieties also favour warmer temperatures and ideal conditions range up to 20 degrees. Drought and heat waves immediately halt the spread of the disease.
How are plants infected?
The pathogen enters via the stomata and infects the plant. It forms mycelium inside the leaf and spreads along the leaf veins. Throughout its growth, mycelium produces new urediniospores that erupt from the leaf. Due to the multiple paths of infection through landed spores and spores formed within the plant, several infection cycles can occur over the course of plant development.
What does the disease look like?
Yellow rust in cereals is characterised by yellowish-orange pustules on leaves. Initially, these appear in isolated cases or in small patches that are irregularly distributed across the crop. In the later stages, they appear in stripes or strings of pearls and may be located across leaf blades and plant ears. The leaf stalk and sheath are usually left untouched by the fungus. The rust pustules contain powdery urediniospores that spread across the surrounding area.
Yellow rust consequences
When grains are severely infected, yellow rust can lead to significant losses. Wheat is particularly at risk, with yield losses of 50 per cent or more in the worst-case scenarios.
Infection during the early stages of plant growth can mean that the fungus affects every stage of plant development. Population density then decreases, and infected leaves die, reducing their photosynthetic capacity and meaning that fewer grains can form. If plant ears are also infected, this results in reduced grain weights - at times, only seedlings remain.
How can yellow rust be prevented?
The grain variety presents a decisive factor in the management of this disease. In areas where yellow rust frequently occurs, more resistant varieties can be selected, or for compact locations, using a range of resistances can be successful. New rust races can overcome resistant genes, therefore resistant varieties do not offer a guarantee. Additional measures are therefore always necessary.
Most importantly, pathogen-rich soil must be reduced. It is essential to remove the volunteer grain, preferably before the winter cereals emerge, in addition to carrying out intensive stubble cultivation immediately after the harvest.
N fertilisation plays an additional role during plant growth. Since nitrogen feeds the fungus and promotes germination, nitrogen fertilisation of fields should be limited.
What plant protection measures are available?
Regular checks for the disease should be carried out as soon as plants shoot. If signs of disease are detected, immediate action must be taken to limit the rapid spread of the disease. Effective plant protection products exist for this purpose. From BBCH 31 on, fungicides containing azole are suitable. Depending on the severity of the disease, a full or reduced application can be used. Combined products containing azoles, strobulins and carboxamides that act against a range of diseases are recommended once the last leaf (BBCH 37) has appeared. This is because the effect, along with other aspects, is greatest at this time. Follow-up applications may be necessary depending on the weather and how far the disease has spread.
Additional preventative measures
Great results with GRAINGUARD
If leaves are damaged by disease (as is often the case with yellow rust), photosynthesis may be limited. This is fatal as it is crucial in yield formation.
GRAINGUARD comes into its own in situations such as this. Thanks to the stimulating effect of humic substances, organic acids and plant extracts, plants develop a leaf area index that is, on average, increased by 19 percent. The positive effect of this is clear - an increase in the rate of photosynthesis, and in turn, an improved chance that grain formation can be sustained longer despite the effects of the pathogen.
Read more about GRAINGUARD.