Fungal diseases in rice: blast and sheath burn

Rice is a staple food for more than 2,4 billion people in the world, playing an important economic, social and nutritional role. Brazil is among the world's largest rice producers, with annual production in the 2022/23 harvest exceeding 10 million tons. According to the National Supply Company (CONAB), in the 2022/23 harvest, the planted area was 1.479.600 hectares (ha), and the average productivity was 6.781 kg/ha. Most of the planted area occurs in the floodplain ecosystem, where the crop is irrigated, accounting for around 75% of national production.

Both in the irrigated system and in highland crops, plants are attacked by diseases that can infect different tissues, at all stages of development, reducing crop productivity and affecting grain quality. Among the pathogens that can attack rice plants, we mainly include fungi, bacteria, nematodes and viruses. Of these, fungi are considered the most important, due to causing greater losses, and also due to the difficulty of control, high incidence and aggressiveness, infecting all cultivars, to a greater or lesser degree of severity, depending directly on the degree of genetic resistance, adopted management and climatic conditions in the region. Blast is considered the most worrying disease, followed by sheath burning.

About blast

Rice blast is considered the main rice disease worldwide, caused by the fungus Pyricularia grisea, which, in rice plants, is found in the mitosporic or asexual phase, and can cause total loss of the crop, due to damage to the leaves and panicles. The pathogen is disseminated by conidia (Figure 1a), formed under leaf wetness, in the infected tissues of rice plants (Figure 1b), alternative weed hosts or in crop residues.

Normally, the conidia formed in the lesions are carried by the wind, from one area to another. This is an important factor for the infection of new plants within the same crop and for rice crops close to an infected area. How does the fungus P. grisea It is also proven to be spread by seeds, this can be an important route of dissemination over long distances, including municipalities, states or even between countries.

In agricultural environments, blast epidemics have several cycles and the rate of increase in the disease is directly influenced by management, resistance and climatic conditions. Since, in regions with a colder climate, as is the case in most planted areas in Rio Grande do Sul and Santa Catarina, blast epidemics can present between 6 and 8 disease cycles per cultivation period; while in tropical regions, such as Tocantins, it is possible to occur 10 to 15 disease cycles per crop. Blast can occur in plants shortly after emergence, at the seedling stage until the grain maturation phase, shortly before harvest. Symptoms on the leaves begin with the formation of small, brown, necrotic lesions that increase in size, becoming elliptical, with a brown margin and a gray or whitish center (Figure 2).

When the disease occurs severely, in susceptible cultivars, under favorable conditions (low light, high humidity, leaf wetness, etc.), the lesions coalesce quickly, causing the death of the leaves and, often, the entire plant in large areas of the crop. (Figure 3A, B, C). 

The damage caused by panicle blast is significant due to its effect on grain filling. When infection occurs shortly after flowering, the panicle becomes whitish and does not fill the grains, that is, all grain production and quality are compromised (Figure 4).

Sustainable control of the disease requires management, integrating several measures, such as genetic resistance, use of healthy seeds, cultural treatments, fungicide and biological control, which, separately, are not efficient, but when adopted together they increase their effectiveness. Among the most important measures to be adopted are: use of healthy seeds treated with systemic fungicides; planting resistant cultivars and in rotation; balanced nitrogen fertilization; control of invasive plants alternative hosts of P. grisea; avoid water stress; seed treatment with efficient strains of the fungus Trichoderma or bacteria Bacillus; control with systemic and contact fungicides, applied to the leaves and during the reproductive period of rice plants, the frequency of application of which will depend directly on the resistance of the cultivar and the climatic conditions observed in the growing region (rainfall, temperature, leaf wetness, light, etc. .). After harvesting, it is very important to incorporate infected crop residues and destroy alternative host plants of P. grisea, as the pathogen, under these conditions, under the soil, does not survive for long, due to the action of antagonistic microorganisms, such as Trichoderma spp., Bacillus spp.

Burning of the hems

Sheath blight is one of the most important diseases of rice in Brazil, caused by the necrotrophic fungus Rhizoctonia solani Kühn. The fungus belongs to the Basiodiomycetes class, forms branched hyphae at an angle of approximately 90º, with constriction at the base of the branch (Figure 5) septa close to the insertion of the lateral hyphae.

The disease occurs in floodplain areas and also in highlands, with a greater or lesser degree of severity, depending on the resistance of the cultivars and management. The damage it causes to plants affects grain productivity and quality and symptoms generally occur on the sheaths, stems and leaves, with waterlogged lesions and brown edges (Figure 6). 

the pathogen R.solani In addition to rice, it attacks a wide range of host plants from different families, making it difficult to control through crop rotation. The fungus is considered a soil inhabitant and can survive for years in the absence of rice, in cultural remains, cultivated plants, various weeds, or in the form of sclerotia, which are resistance structures disseminated during cultural practices, such as plowing. and soil harrowing and also by the movement of irrigation water. R.solani it can also be spread by seeds, in the inner layers, or the sclerotia can be mixed between the seeds. In this case, after emergence, the seedling may fall over or, later, the development of lesions will occur as the density of the plants increases, between tillering and the beginning of flowering. The greatest losses caused by the disease are seen under high severity of infection in the stalks and sheaths, resulting in lodging of the plants and sterility of the spikelets, directly affecting the productivity and quality of the grains. The conditions conducive to sheath burning are high relative humidity, temperature between 25 to 35 ºC, low light, nitrogen fertilization above 90kg N/ha, soils with a history of intense cultivation, with a high density of sclerotia, etc.

Despite the current importance of the disease, there is little work carried out in Brazil seeking to identify new sources of resistance, control with fungicides in rice and biological control. In the State of Tocantins, in the floodplains where intense rice cultivation occurs, in rotation with other crops, such as soybeans, watermelon, corn, beans, the burning of the sheaths has increased with each growing season, due to the constant multiplication of sclerotia. R.solani which serves as an infective inoculum for rice and other cultivated plants.

The only way to control this disease is by adopting several measures, in an integrated management program, as once present in the soil, there is no way to eradicate the pathogen, due to its adaptability in surviving through a resistance structure (sclerotium), or infecting other plant families, including weeds present at the site. It is recommended to use seeds of good origin, with health and vigor. Also use the recommended sowing density for each cultivar, according to its tillering capacity. And do not apply more than 90 kg of nitrogen per hectare. Base fertilizer with a source of silicon helps reduce the severity of the disease, due to strengthening the cell wall of the stem and leaf tissues.

Rotating rice with corn or sorghum is preferable to soybeans, as the latter is a highly susceptible host to R.solani and which helps in the rapid multiplication of sclerotia in the soil. The direct planting system helps manage the disease due to little soil movement, reducing the spread of sclerotia. Regarding the use of sources of resistance, it is important that genetic improvement programs seek new sources of resistance and make available cultivars demonstrating the degree of severity to sheath burning, so that producers can choose less susceptible varieties.

The use of biological control is a powerful tool against R.solani, mainly with the fungus Trichoderma or bacteria of the genera Streptomyces e Bacillus. These antagonists also live in the soil and can survive in the same environment as the pathogen. In this case, it is important to carry out seed treatment, according to the indicated dosage. You can also make sequential foliar applications, according to the manufacturer's recommendations. In Brazil, there is practically no fungicide registered to control sheath blight in rice. However, in countries such as the United States, fungicides that are efficient in managing sheath blight are reported, such as Azoxystrobrin and Propiconazole. In Brazil, these fungicides are registered to control other diseases in rice crops.

By Gil Rodrigues dos Santos, Dalmarcia de Souza Carlos Mourão, Maykon Rodrigo Gomes de Barros, Lorena Ribeiro Lima, João Victor de Almeida Oliveira, Ritielle Siqueira Batista, Joele Andressa Zanfra e Paulo Ricardo de Sena Fernandes, Federal University of Tocantins

Article published in issue 294 of Cultivar Grandes Culturas Magazine