Brown spot, grain spot and stalk rot in rice

The rice (Oryza sativa L.) is considered one of the most produced and consumed cereals in the world, being the main food for more than half of the Brazilian population. According to IBGE (2023), in Brazil, there is a forecast of growth in paddy rice production for 2024. The estimate points to a production of 10,5 million tons, which could represent growth of 2,5% with an increase of 4,5% in the area to be harvested. The largest planted area in the country occurs in the floodplain ecosystem, where the crop is irrigated, accounting for around 75% of national production.

In the lowland ecosystem, where cultivation is irrigated and also in highlands, under rain or irrigation regimes, rice plants are infected by phytopathogens that affect crop productivity. Among them, fungi are the most worrying due to the difficulty of control and the damage caused to plants. Despite the technologies used in plant management, it is known that diseases cause a large part of the losses in grain production and quality. Among the main diseases, we can mention grain spots and brown leaf spot.

Brown spot and grain spot

Brown spot is caused by the fungus Bipolaris oryzae, in its asexual or mitosporic form (Figure 1) and Cochliobolus oryzae, in its sexual or teleomorphic form.

May become important under high temperature and humidity. Under these conditions, the disease forms dark, reddish-brown, oval-shaped lesions with a white to grayish center (Figure 2A), which can significantly reduce rice productivity by destroying most of the photosynthetic area of ​​the leaves of all plants in the crop. (Figure 2B). 

In some crops grown on more sandy or less fertile soils, the incidence of brown spot may be more severe, potentially compromising grain production and quality. This disease has assumed great economic importance throughout the country, affecting mainly crops sown in October and adult plants close to maturity. The disease can cause losses of 12% to 30% by affecting the weight of the grains.

When planting seeds infected with B. oryzae, there is a significant reduction in germination and emergence of seedlings. In those seeds that manage to germinate, the seedlings have lesions on the coleoptile that are brown in color and circular or oval in shape. Later, the pathogen can also infect the sheaths, which have lesions similar to leaves. In the reproductive phase, when it appears after the panicles are emitted, the infection causes the spikelets to become sterile. Infections at the beginning of grain formation, the glumes present dark brown spots that often coalesce, covering the entire grain, affecting the quality of the grains, through the stains caused on the grains. Infected seeds and infected plant debris, in addition to alternative weed hosts, constitute the main sources of primary inoculum. In seeds, the fungus is located within the internal tissues of the seed, causing discoloration and wrinkling, depreciating industrial quality after processing.

Grain staining can be caused by several other pathogens, in addition to B. oryzae, and is currently considered one of the most important rice diseases due to the spots directly affecting the appearance and quality of the grains, externally (Figure 3) and internally, and reducing the weight and industrial yield of the processed grains.

Among the etiological agents of grain spot, we can highlight: Bipolaris oryzae, Phoma sorghina, Rhynchosporium oryzae, Pyricularia oryzae, Sarocladium oryzae, Alternaria padwickii, Fusarium spp., Nigrospora spp., Curvularia lunata. Among the bacteria that may be involved, as grain staining agents, we have: erwinia spp. And Pseudomonas spp.

To control brown spot and grain spots, it is important that the producer acquires seeds of good health quality and good vigor, as most of the microorganisms involved in these diseases are also transmitted via seeds. It is recommended to treat seeds with systemic fungicides to eliminate possible foci of infection in seed tissues and also to protect seedlings for the first 20 to 30 days after emergence. Biological seed treatment with efficient strains of the fungus Trichoderma or bacteria Bacillus is recommended due to the beneficial effect of these agents on pathogens and also because their effect on promoting plant growth and solubilization of phosphate present in the soil has already been proven.

It is important that genetic improvement programs also prioritize the development of resistant cultivars, with a lower level of severity for these diseases. For planting in regions with a hot and humid climate, it is recommended to use systemic and protective fungicides, registered for rice cultivation, in alternation and in mixtures during the panicle emission period and grain formation, seeking to eliminate pathogens associated with these. illnesses. It is also recommended to control alternative hosts of these pathogens, such as red rice, rice grass, and other invasive grasses that host some of these microorganisms in the absence of rice plants. The destruction of crop residues, after harvest, through incorporation into the soil, is recommended, aiming to reduce the inoculum in the area. Crop rotation with plants from other families is also an important measure for managing these diseases.

In addition to these diseases, the presence of Sclerotium rolfsii, a fungus that is the causal agent of stem rot, in the Lagoa da confusion region, in Tocantins.

Stalk rot

This disease is caused by the soil-dwelling fungus S. rolfsii Catt. It is important in other rice areas around the world and may also be responsible for some losses. In Brazil, it was recorded in rice cultivation infecting plants in cultivation in Lagoa da Confusao, Tocantins (Osório et al, 2021). The authors arrived at the diagnosis of the disease after isolating S. rolfsii in culture medium, pathogen DNA extraction, PCR amplification, sequencing and subsequent phylogenetic analysis.

Due to the mode of propagation and persistence of S. rolfsii in the soil, in addition to its diverse range of hosts, which are around 500 botanical species, its control becomes difficult, due to the formation of sclerotia, pathogen resistance structures that can remain in the soil for years without the presence of rice. . Symptoms of the disease in plants appear as watery brown lesions and necrotic tissue present in the stem regions. Subsequently, the stem darkens and rots with the appearance of white mycelium around the stem, which results in the destruction of the cortex and main root, causing the death of the tillers (Figure 4).

As this is a new disease in Tocantins and not very widespread in rice cultivation, control measures are not yet recommended, however this pathogen also attacks other plant species, where the use of resistant cultivars, healthy seeds, seed treatment is recommended. with fungicides and planting in well-drained soils. Biological control with Trichoderma e Bacillus It can also be an efficient measure because these antagonists have antibiosis and mycoparasitism action on the sclerotia present in the soil.

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 295 of Cultivar Grandes Culturas Magazine