Yellow Rust (puccinia Striiformis): an Emerging Serious Threat to Wheat Production Worldwide

Fungicide application

Fungicide application is a necessary approach to fight against stripe rust disease. Various synthetic substances were applied to control this disease. Commercial fungicide products were used worldwide. Currently, the following active ingredients are labeled for control of stripe rust in Morocco: propiconazole, azoxystrobin, propiconazole in combination with trifloxystrobin, strobilurin, and azoxystrobinin combination with propiconazole. These labeled fungicides with different active ingredients provide choices for growers to use and may reduce selection pressure in the fungal pathogen to develop resistance to chemicals.

The importance of using fungicides was demonstrated in field experiments near Pullman Washington during successive growing seasons from 2002 to 2012. They were conducted to improve chemical control of stripe rust for major commercially grown cultivar with various levels of resistance. Findings of this study showed that fungicide application reduced AUDPC (Area under Disease Progress Curve) by more than 80% in both susceptible winter wheat and susceptible spring wheat when compared to untreated controls.

The AUDPC reduction depends on the duration and severity of disease. Tilt (Propiconazole) was used throughout this study and became the standard fungicide to control stripe rust during this period. Triadimenfon (Bayleton) has been largely used to control wheat stripe rust in China. The timing of spraying fungicides is crucial for an effective control of stripe rust. Viljanen-Rollinson et al. (2006) revealed that using fungicides early in the growing season leads to a better disease control.

However, the use of fungicides adds high input costs to wheat production, which is a burden for many growers, especially in developing countries. It causes numerous negative health and environmental issues. Furthermore, repeated applications may result in the selection of fungicide resistant strains of the pathogen.

Cultural methods

Cultural methods provide another strategy to partially control wheat stripe rust. Using a series of cultural practices significantly enhances the existing sources of resistance. As a result, crop management in terms of a combination of crop choice, timing of seeding and removing volunteer cereals may provide effective control of stripe rust.

Stripe rust requires green material to survive from one season to next, it is known as “green bridge”. Removing volunteer plants (the Green Bridge) that will support stripe rust survival is an effective control measure for epidemics that result from endogenous inoculum. Planting a mixture of wheat varieties with different resistance backgrounds may significantly reduce disease pressure, and may also increase or stabilize wheat yield. Mechanisms by which cultivar mixtures suppress disease may include dilution of spore’s density because of the greater distance between susceptible plants, a physical barrier created by the resistant plants in the canopy that interrupt spore movement, and induced resistance.

Challenge of stripe rust control

Increased temperatures could affect the phenological growth stages of wheat. For example, temperatures more than 34°C could reduce the grain filling period of wheat and accelerate plant senescence. Increased leaf senescence indirectly affect pathogens development especially biotrophic fungi such as Puccinia species. High temperatures also directly impede diseases development. Results showed that pathogens may adapt themselves to warmer temperatures.

Climate change, in terms of rising temperatures, and the timing and increasing variability of rainfall, influences the spread and severity of rust diseases. In wheat, the expression of many genes for resistance to stripe rust is influenced by temperature and/or plant developmental stage. Findings of a weakening of stripe rust resistance and pathogen adaptation due to temperature increases were well documented in annual race surveys in the Eastern USA.

In contrast, some stripe rust resistance genes, such as Yr18, are known to be temperature mediated and become more effective at higher temperatures. Therefore in varieties with this resistance gene, there may be an enhancement of the effectiveness of resistance in a warming climate. On the other hand, Kaur et al. (2008) have predicted that the importance of wheat stripe rust disease may reduce in the future in Punjab state of India as a result of climate change.

New pathotypes of Pst can adapt to increased temperatures. In the eastern USA Pst races, collected after 2000, have different virulence profiles than races collected before this year. These new races pose an increased risk to wheat crops as the results of latent period and spore germination indicated that the new population was better adapted to high temperature.

High epidemic potential

Many yellow rust epidemics were reported in Central and West Asia and East and North Africa. The 2009-2010 epidemic severely affected many countries including Morocco, Turkey, Algeria, Syria, Lebanon, Iraq, and Uzbekistan. Syria and Lebanon were the worst hit of this epidemic; Syria lost nearly half of its wheat harvest. In 2014, the Central Research Institute for Field Crops in Ankara and the Regional Cereal Rust Research Center in Izmir confirmed the detection of a new Pst race in Turkey.

The newly detected strain was “Warrior” race previously identified in the United Kingdom in 2011. Some of Turkish commercial cultivars known to be resistant to the previously characterized races of Pst were recorded as fully susceptible to this new race. The warrior race was much more widespread in the following year after its first detection. It was already present in high frequencies in most European countries and North Africa and it was confirmed in Morocco in 2013 and in Algeria in 2014. This race was very dissimilar to pre-2011 European races. It showed relatively higher genetic diversity than other previous races.

Conclusions

Wheat stripe rust continues to be a major worldwide limiting factor of wheat production. Global losses were estimated to be at 5.5 million tons per year. The evolution of pathogen races becomes larger and faster; the emergence of new races with high epidemic potentials and which can adapt to warmer temperatures has expanded virulence profiles. The new highly aggressive strains have defeated key resistance genes such as Yr27, used in breeding of many wheat cultivars across Asia and Africa, which led to the epidemic in 2009-2010.

Climate change is aggravating the severity and frequency of today’s new wheat problems. Warmer winters induce earlier stripe rust infection and spread. Thus, the severity of the epidemics will increase throughout all wheat growing regions.

Growing resistant cultivars is the major component of integrated control of stripe rust. However, “breakdown” of resistance following the introduction of new genes for resistance is a major problem. Successful deployment of resistant crop varieties at larger scales and in different regions would, however, require: a better understanding of pathogen diversity; regional and international collaboration to effectively address the disease through data sharing; and a long term effort to control new and existing challenges to stripe rust through research and development of resistant varieties to emerging strains.