Integrated management against rice blast

The most important disease of rice crops, rice blast, can lead, in severe cases and without adequate management, to productivity losses of around 100%. Prevention and the adoption of integrated measures are essential to minimize damage. In crops with a history of the pathogen, sown late and/or with susceptible cultivars, one to two applications of fungicides may be necessary.

Rice blast is caused by the fungus magnaporthe oryzae (perfect form) or Pyricularia oryzae (imperfect form). It is the most important disease in rice cultivation in the world, with its occurrence confirmed in more than 85 countries. Under favorable environmental conditions, it can present high levels of severity and rapid dissemination. In severe cases and without adequate management, productivity losses can reach 100%. The fungus P. oryzae It can develop under different conditions, benefiting from cloudy days, relative air humidity greater than 93%, temperatures in the range of 15ºC to 35ºC, with an optimum of 25ºC to 28ºC and, mainly, leaf wetness.

Symptoms of the disease can be observed, mainly, from the beginning of tillering and, when the crop is in the reproductive stage, in its flowering panicles. When blast infects the leaves, it is possible to see small elliptical spots with a gray center and brown edges. These lesions grow towards the veins and their size varies according to the cultivar's degree of resistance and phenological stage. In the center of the patch of P. oryzae tissue necrosis occurs and, under conditions of high humidity, intense sporulation occurs. Under these conditions, the spread of the disease is facilitated, allowing it to spread quickly to adjacent rice plants through wind, rain splash and dew. It is important to highlight that P. oryzae presents survival structures such as mycelium and conidia, which in cultural remains, rice seeds and weedy plants allow its propagation and future reinfestations.

When symptoms are identified in the stem or more specifically, in the insertion of the panicle, the disease is called neck blast. Injuries caused by the pathogen can cause the base of the panicle to break, characterizing the symptom known as “broken neck”. In this case, the passage of photoassimilates is interrupted, preventing the grain filling process, which consequently reduces the weight and causes partial or total shock of the grains in the panicles, causing severe reductions in productivity.

In Rio Grande do Sul, the incidence and severity of blast vary according to the year and region, with the central depression and the north coast being the places with the highest incidence. They are characterized by high precipitation, high relative air humidity and, therefore, a longer period of leaf wetness, benefiting the development of the disease. The phenological development stage of the crop is also a factor to be considered. Plants in R₂ aR₄ (rubberization and flowering) are more susceptible to P. oryzae, Intense monitoring of the crop is recommended during this period.

Management strategies for blast must begin even before planting, with the choice of cultivars that are resistant to the disease and/or more suitable for the region and cultivation system. In Rio Grande do Sul, it is recommended to sow rice from September to mid-December. However, due to unforeseen events, such as rain occurring during the beginning of the sowing window, some producers have sown late. These delays can benefit the development of the disease, which intensifies when planting is carried out from the end of November, due to weather conditions more favorable to the pathogen. This occurs because these plants will reach the reproductive stage under conditions of less sunlight, lower temperatures and longer leaf wetness, increasing the presence of the pathogen.

Assessment of the area's history is also a relevant factor for choosing the cultivar, in places where the disease frequently occurs. Whenever possible, resistant genotypes should be chosen. Preparing areas in advance is also an alternative that minimizes the chances of developing P. oryzae, as it makes planting possible at the desired time. This practice is extremely important in years of high rainfall, as it allows the area to be ready for sowing regardless of the weather conditions.

The use of healthy, certified and, preferably treated seeds, must be taken into consideration when planning the crop. Contaminated seeds serve as a source of initial inoculum, because when young seedlings are infected, the entire plant dies, dispersing spores (conidiospores) to adjacent plants. Balanced fertilization, avoiding excessive amounts of nitrogen, combined with maintaining the water depth and crop rotation, are also important practices to mitigate the damage caused by the disease in irrigated rice crops.  

Failure to use the previously described strategies to control blast may result in areas with a high incidence, requiring the use of chemical products. In crops with a history of the disease, sown late and/or with susceptible cultivars and under favorable conditions, one to two applications of fungicides are recommended. Currently, there are several commercial products on the market to control the disease, with emphasis on Tricyclazole, belonging to the chemical group of benzothiazoles, which is a systemic and specific product for the disease. P. oryzae. When blast infects the leaves, the first application is recommended during the complete tillering phase and a second application when a greater incidence of the fungus occurs or 20 days after the first. As for neck blast, the first application must be carried out at the end of rubberization (up to 5% panicle emission) and the second, approximately 15 days later, or with 75% of the panicles exposed.

Blast control, in irrigated or rainfed crops, is not an easy task for the producer. However, the best recommended strategy is integrated disease management, which corresponds to the joint use of cultural management, elimination of crop residues, use of resistant cultivars and chemical control, when necessary. In this sense, the reduction in the occurrence of blast and the need for intense use of fungicides allows the full development of the crop, as well as the maintenance of rice's productive potential and high crop yields.

The rice 

The rice (Oryza sativa L.) is the second most cultivated cereal in the world, surpassed only by corn. Forecasts made by the US Ministry of Agriculture (USDA) for the 2016/2017 harvest suggest an increase in world production by 2,37%, reaching around 483 million tons (Mt) of rice. Meanwhile, Brazilian production should increase from 10,6 Mt in the last harvest to around 12 Mt. This increase is a reflection of the climatic conditions of this harvest, which made it possible to sow within the preferred season in a greater number of crops.

 Other factors such as increased investment in technology and the use of cultivars with high yield potential reflect an increase in national average productivity, which could increase from 5,3t/ha in the 2015/2016 harvest to 5,7t/ha in 2016/2017, representing an increase of 9,4%. The average productivity obtained in Rio Grande do Sul, which plants more than one million hectares with this crop, was 6,9 t/ha in 2015/2016. This state represents 68% of national production and together with Santa Catarina they are responsible for around 80% of national rice production.

The commercial exploitation of rice in Brazil occurs mainly in two cultivation systems: irrigated or lowland rice, used mainly in the South region, and rainfed or highland rice, used in the rest of the country. Southern rice crops present peculiar exploitation conditions, due to the predominant soil classes being Planosols and Gleisols, which have poor drainage, favoring their cultivation under flooded conditions. Irrigated rice has a higher productive potential compared to rainfed rice. In addition to the supply of water, given the high demand of the crop, the presence of water in the cultivation areas is an important tool in the management of weeds, especially red rice.

The impact on rice productivity caused by competition with weeds reached a high level, culminating in the development of cultivars with Clearfield technology (CL or RI), which allow the use of herbicides from the imidazolinone group, providing control of weeds without causing damage to culture. However, the development of this technology, in some cases, was not associated with genetic resistance to rice blast, which is the main fungal disease in irrigated rice. Among the main high-yielding cultivars sown in Rio Grande do Sul, Guri INTA CL, Puitá INTA CL, IRGA  424 and IRGA  424 RI stand out. However, only the two developed by the Instituto Riograndense do Arroz (IRGA) present considerable levels of resistance to blast.

Luiza Elena Ferrari, Gerarda Beatriz Pinto da Silva, Tiago Edu Kaspary, UFRGS 

Article published in issue 214 of Cultivar Grandes Culturas.