How to minimize the appearance and effects of wheat blast

In the Southern Region of Brazil, wheat cultivation has traditionally been part of the farms' production system. It plays an important role not only as an alternative for grain production in winter, but also as a high-quality straw-forming crop in the direct planting system.

In some regions of Brazil where the tropical climate predominates, producers are beginning to notice some benefits of wheat cultivation in the off-season of the summer crop. One of them is the possibility of harvesting in periods of almost no rainfall, providing a high quality product, whose average hectoliter weight is greater than 80 kg/hl, normally higher than that achieved in the southern region of Brazil.

Also notable among the benefits is the low relative humidity during most of the crop cycle, which also contributes to reducing the occurrence of diseases and pests, another serious problem in southern Brazil. Another important aspect refers to the productivity levels that are much higher than the traditional regions of the country and the possibility of harvesting in the off-season in the Southern Region of Brazil and Argentina, the main exporter of wheat to Brazil, achieving better price competitiveness in the national market.

In Minas Gerais, also considered a tropical region for wheat production, the advance of the crop in recent years can be clearly seen: data from the Brazilian Institute of Geography and Statistics (IBGE) show that the planted area exceeded 21,5 thousand hectares in 2012 to 55,0 thousand hectares in 2014. In this same period, production increased from 80,7 thousand tons in 2012 to 175,9 thousand tons in 2014. In addition to the widely recognized importance of the crop in the agricultural production system of rural properties, where it is Highly advantageous, whether for the formation of straw and soil cover or for lowering inoculum sources for some fungal diseases, wheat cultivation has been attracting the attention of Minas Gerais producers also due to the good prices paid by the industry in recent years.

Wheat blast (anamorph Pyricularia oryzae, teleomorph magnaporthe oryzae) was described in wheat ears in Brazil in 1985. Its characteristic symptoms are observed in all parts of the plant, but it is in the ears that it causes significant economic damage. In recent harvests, this disease has gained importance and attracted the attention of producers, technicians and researchers due to its destructive potential. In some cases, where higher temperatures predominated during the cycle, losses reached 100%. Wheat grains affected by blast are flat and take on a wrinkled shape, very different from grains without the presence of the disease in the ears. These characteristics of grains from cobs with blast promote low hectoliter weight (kg/100 L) and reduce productivity, in addition to favoring poor industrial quality.

Various strategies for controlling and/or living with this disease are being researched. Some can be implemented immediately, while others take time to apply.

Currently, few chemical products are registered with the Ministry of Agriculture, Livestock and Supply (Mapa) to control blast. Furthermore, studies carried out by Embrapa Trigo, in partnership with several other research institutions and other companies in the sector, indicate that the efficiency of chemical control is low. The causes of this low efficiency in controlling the disease are still the subject of studies. Product doses, application technologies, number of applications, timing of application, are some of the variables studied with the aim of improving efficiency in the chemical control of blast.

Genetic resistance is generally the most economical alternative for disease control. In the specific case of wheat blast, research carried out to date indicates few wheat cultivars that are significantly tolerant. The absolute majority of materials recommended and/or cultivated in the tropical regions of Minas Gerais, Goiás, Distrito Federal, Bahia, São Paulo, Paraná and Mato Grosso do Sul, have been shown to be highly susceptible to the pathogen that causes blast.

Over the last three years, several research projects on wheat blast have been carried out by Epamig, in Patos de Minas (Funded by Fapemig) in conjunction with Embrapa Trigo. The results show that the incidence of the disease is extremely high when sowing is carried out in February and March, falling significantly when sowing is carried out from May onwards. Among the preponderant factors that can explain this more or less aggressive behavior of the disease, the variations in temperatures between February and August stand out. According to this research, the incidence of blast on ears drops to values ​​below 10% when the average monthly minimum temperatures are below 14 ºC. In most Brazilian regions with a tropical climate, average minimum temperatures are observed only in the months of May, June and July, indicating that sowing must be planned to coincide with wheat heading (a stage of the crop susceptible to penetration of the fungus that causes blast on the cob) in these colder months.

Another strategy for dealing with blast in tropical regions is knowing the duration of the cultivar cycle in each cultivation area. A given cultivar, with an early or late cycle, can change the duration of its cycle according to the environmental conditions of the sowing location. Higher temperatures and water deficit in the soil are the main factors that can shorten the cycle of cultivars. On the other hand, regions with higher altitudes, normally colder, places with higher rainfall, irrigated crops, tend to increase the cultivar cycle. The number of days that the cycle can be altered depends on the cultivar and the intensity of environmental factors prevailing in the region. In this sense, it is important that the wheat farmer is technically oriented or has knowledge of the influence of these factors, and evaluates the possibility of sowing in such a way that earing occurs under the most unfavorable conditions possible for the penetration and development of the fungus in the ear, that is, milder temperatures.

Another important factor that is related to the incidence of blast is the presence of water in the cob. This water may come from high relative air humidity, rain or irrigation. Results of work carried out at Epamig, in Patos de Minas, indicate that the months with a lower incidence of blast coincide with the drop in relative air humidity. When managing irrigated wheat in tropical regions, it is recommended that the producer avoid irrigating during heading, in order to create a less favorable condition for the penetration of the fungus and the development of the disease.

While research is still working to find cultivars with high levels of tolerance and even resistance to blast, wheat farmers, mainly in Brazilian tropical regions, can live with the disease at low levels of incidence. The main unfavorable factors for the fungus that causes blast are: crop heading in colder months; sowing planning according to the cycle and local environmental conditions; avoid irrigation and coincidence of rainy periods in the spike.

 The article can be read in full in issue 190 of Cultivar Grandes Culturas.