Wheat leaf rust management

Wheat leaf rust is one of the main villains that affect the crop in southern Brazil. Managing it involves measures adopted even before the installation of the crop, with the choice of cultivar, planning applications with protective fungicides and maintaining the disease at low levels with the help of systemic and/or curative products.

Wheat grown in the southern region of Brazil is exposed to different biotic (pests and diseases) and abiotic (drought, excess rain, high or very low temperatures) stresses. These stresses prevent high-yielding cultivars from reaching their maximum productive potential. In Brazil, wheat is grown in the autumn-winter period, extending until mid-spring. During this period, there are large variations in environmental conditions, especially in temperature and relative humidity, which favors the development of various diseases, including rust. All over the world, leaf rust, stem rust and yellow rust cause significant productivity losses in wheat.

Yellow rust, caused by Puccinia striiformis, is of little importance in Brazil, where only sporadic cases have been reported. This disease is more important in colder regions (Europe, Australia, Chile), being less frequent in subtropical climate conditions, such as Brazil. Stem rust (Puccinia graminis) is considered the most destructive disease of wheat, and can cause losses of 100% of the crop due to fragility and breakage of the stalks when severely affected. The use of genetic resistance, however, has guaranteed control of the disease since the mid-90s. Currently, the majority of Brazilian wheat cultivars are resistant to this disease.

Wheat leaf rust, caused by the fungus Puccinia triticina Eriks. It is the most common disease and the most widely distributed around the world. The environment is a key condition for the dispersion and germination of P. triticina spores, directly influencing the variation in the severity of epidemics. The population dynamics of P. triticina in the Southern Cone (Argentina, Uruguay, Chile and Southern Brazil) has determined a short resistance life of commercial cultivars, as the fungus survives in summer-autumn parasitizing volunteer wheat plants, which correspond to main source of inoculum in Brazil.

Leaf rust is characterized by the presence of dark yellow to brown pustules (Figure 1) along the leaf surface, and can occur at all stages of plant development. Temperatures ranging between 15 ºC and 20 ºC and high relative humidity are ideal conditions for the disease to progress. Losses in productivity occur as a result of the reduction in the photosynthetically active area and the transport of photoassimilates to the ears, resulting in a reduction in grain weight. In cases of severe epidemics, values ​​of up to 65% loss of productivity have been recorded. The disease is also associated with reduced grain quality, which directly influences the quality of baked goods.

The use of resistant cultivars and the use of fungicides are often used as the most efficient methods of controlling the disease. There are currently around 100 commercial products on the market registered with the Ministry of Agriculture, Livestock and Supply (Mapa) for leaf rust in wheat. The groups of strobilurins, triazoles, carboxamides and dithiocarbamates stand out. With the change in legislation a few years ago, it is now possible to obtain commercial products with mixtures of active ingredients and mechanisms of action, which allows for better effectiveness in controlling this disease, with emphasis on: strobilurin + triazole, strobilurin + carboxamide and triazole + carboxamide.

There is a tendency on the part of farmers to prefer the mixture of strobilurins + triazoles, as it presents greater efficiency in the field, as they have better fungitoxic capacity and a broad spectrum of action, associated with greater residual power, which guarantees its effect for longer on the farm. However, care must be taken, and its application is not recommended when the grains reach the milky stage. At this stage, the grain is practically formed, resulting in a reduction in efficiency and a greater predisposition to the accumulation of fungicides, which makes its use as a food raw material unfeasible. 

The improvement of cultivars with genetic resistance is an environmentally safe option and, therefore, should preferably be used to control leaf rust. Furthermore, it reduces the need for fungicide applications, resulting in reduced costs for producers. Genetic resistance can be of two types. The first is complete resistance, which confers immunity to the plants, with no symptoms of rust observed in the field, that is, there is no development of pustules. This resistance is also known as vertical, as it confers specific resistance to certain races of the pathogen. Therefore, the use of complete resistance in commercial cultivars determines that it is overcome within a few years after its release, due to the high selection pressure exerted by the more virulent P. triticina races. The second type of resistance is expressed mainly in the adult stage and is characterized by a reduction in the quantity of the disease, which is why it is called adult plant resistance (RPA). In other words, the plant exhibits a susceptibility pattern, but the disease develops slowly. Therefore, RPA acts on the components of the disease progress rate, such as the latency period, infection efficiency and pustule size. Although the presence of RPA in cultivars alone does not guarantee adequate levels of control, it makes it possible to increase the interval and efficiency of applications, reducing the cost of fungicides.

Wheat leaf rust is one of the main villains for the production of this crop in southern Brazil, as it is present annually, to a greater or lesser extent, in crops. Therefore, strategies for managing this disease must begin before the implementation of wheat, with the choice of cultivar, planning applications with protective fungicides and maintaining the disease at low levels with the help of systemic and/or curative products.

The Wheat

Hexaploid wheat (Triticum aestivum L.) consumed today, especially in the form of bread, is the result of the domestication of a species that emerged more than 10.000 years ago, in the region that today represents Southeast Asia. It was the pioneer species for the creation of agriculture, as from its appearance men stopped being nomads and started to settle in regions where cultivation was possible. Currently, wheat is a staple food for more than 50% of the world's population, especially in Europe, North America and North Asia. In addition to being the third most cultivated cereal in the world, it is widely spread across all continents. Alone, wheat is responsible for about 1/5 (20%) of all human calories consumed worldwide, being the main source of food in many developing countries. 

According to estimates from the US Department of Agriculture (USDA), global wheat production for the 2016/2017 harvest is expected to reach a new record, with production estimated at approximately 248,37 million tons. In Brazil, triticulture is historically important in the South region, with Rio Grande Sul being the only self-sufficient state in grain production/consumption. National production, however, does not meet Brazilian demand, which makes Brazil an importer of this grain, with Argentina responsible for approximately 75% of imports. It is expected that in 2016, Argentina will harvest around 14 million tons of wheat. The Brazilian estimate is to produce 6,3 million tons, with Rio Grande do Sul being responsible for approximately 30% of national production. Despite a 14,5% reduction in the planted area in the 2016 harvest, it is estimated that Brazil will produce more wheat than in the previous harvest, due to the increase in productivity from 2,26 ton/ha to 3,0 ton/ha. 

In Rio Grande do Sul, wheat is a crop of great economic importance. When climatic conditions are suitable and there are no severe problems with pests and diseases, financial returns are guaranteed. Currently, there are several wheat cultivars developed by Brazilian entities that have excellent baking quality, making the product a little more attractive than that produced a few years ago. 

Gerarda Beatriz Pinto da Silva, Francisco Saccol Gnocato, Geísa Finger, UFRGS

Article published in issue 211 of Cultivar Grandes Culturas.