Puccinia graminis f. sp. tritici (Pgt) is a rust fungus that occurs globally in all locations where wheat is grown but is most prevalent in areas with warmer and moist climates. When Pgt infects a plant, conducting tissue within the stem is severely damaged, resulting in lower nutrient absorption and less nutritious grains. In extreme cases, the crops can even be unviable.
The fungus progresses through many different structures during infection. The most characteristic structures are the reddish-colored urediniospores that form infectious structures on the wheat plants and are spread by wind. They form on the stoma of the black teliospores that form towards the end of the growing cycle and enable the fungus to survive during the winter season. Aecia are reproductive structures that form on barberry and release aeciospores that are windborne and infect wheat. Due to this, one common precaution taken by farmers is the removal of barberry near their crops.
In order to infect crops, Pgt spores contain avirulence factors that are released into the plant cells. Differences in types of avirulence factors distinguish different races of Pgt, and each race often has multiple types of avirulence factors. Resistance proteins in plants recognize the effectors secreted by Pgt and trigger an hypersensitive immune response in the plant, causing localized cell death and preventing the spread infection. Different resistance proteins are naturally occurring in plants affected by Pgt and are encoded for by resistance genes. The immune response protects the plant by preventing the infection from spreading further. Often, this recognition occurs at a very low level of infection, so levels of cell death are low and do not significantly affect the plant. However, effectors can mutate, in some cases causing them to no longer be recognized by the plant. If the effector is not recognized, no immune response occurs and the pathogen can continue to infect the plant. In this case, the strain is considered virulent to the resistance gene the mutated effector corresponds to.
Pgt spores are able to be distinguished from other fungal spores by the presence of ribitol. When the spores germinate, they form infectious structures that contain hyphae, or branching filaments. Ribitol is a metabolic product in the hyphae and its production is unique to the hyphae of Pgt (i.e. there are no records of Pgt being produced in any other types of airborne fungus). Therefore, the presence of ribitol from airborne spore samples will in turn indicate the presence of Pgt.
Pgt, while most notable for its impact on wheat, is a destructive force for many other cereal grains, including barley, rye, and triticale. These grains are a major staple in many diets worldwide, and shortages of these could cause a disruption in food availability and in many major industries. For most of the 20th century, Pgt was controlled around the world by the Sr31 resistance gene.
In 1999, the emergence of Ug99 or TTKSK caused the threat of Pgt to arise again. The new race of the fungus stopped producing AvrSr31, prohibiting an immune response to halt the progression of the disease. Since then, Ug99 has spread to areas East Africa and the Middle East, and new virulent strains have also developed. Two of the most damaging races include TKTTF in East Africa, Middle East, and Northwest Europe in addition to TTTTF in Italy. Pgt has also spread to the United States, but the races present are not as virulent and are better controlled by fungicides and chemicals. Climate change and global warming have exacerbated the effect of Pgt by increasing pathogen survival. The fungus is unable to survive winter climates characteristic of the locations it infects; however, milder winters are able to extend the prevalence of the disease.
By targeting many of staple foods of the world population, Pgt heavily contributes to food insecurity and causes disastrous monetary losses. Wheat rust fungus accounts for 30% of food insecurity in Ethiopia alone. In 2014, over 600,000 hectares of wheat were destroyed by wheat rust fungus in Ethiopia. This measured around one third of the wheat crop that year and a monetary value of over $375 million. In a recent report by the United Nations’ Food and Agriculture Organization, they estimate that even with a ten percent loss in crops due to TTKSK, the monetary value of these losses could exceed $7.5 billion.
The conventional method of detection involves farmers examining their fields in a W pattern, checking for physical symptoms. The most prevalent symptom is the formation of pustules, and it takes approximately one to two weeks for physical symptoms to be expressed by the plants. Therefore, by the time the farmer is alerted of the presence of Pgt, the disease has progressed rapidly and is in its final stages and ready to infect neighboring crops. In addition, after the formation of pustules, farmers only have a couple days to spray their crops. Given that farmers in remote areas must travel to obtain suitable fungicides, early detection also gives them enough time to gather their materials.
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