Presence of root-knot nematode Meloidogyne graminicola in crops irrigated rice in Rio Grande do Sul and Santa Catarina lights up warning sign about the potential harm of this silent and hidden threat. Monoculture and The use of susceptible cultivars are among the aspects that contribute to the rapid population growth of this parasite in the soil.
World rice production (Oryza sativa L.) annually reaches 700 millions of tons. In Brazil, Rio Grande do Sul stands out as the largest producing state, with an area of 1,10 million hectares dedicated to the cultivation of irrigated rice, which guarantees more than 70% of the national production of this cereal. Although the numbers are significant, the demand for increased productivity of rice crops is increasingly constant, mainly due to the social importance of culture in relation to human nutrition.
Although this demand is pressing, several phytosanitary problems present capacity to limit the productive potential of irrigated rice cultivation. Among these factors, plant parasitic nematodes can stand out in rice farming scenario. These microorganisms are specialized in infecting and harm the growth of the plant root system. One of the main The groups involved in this process are root-knot nematodes, considered the most economic importance in world agriculture. This group can parasitize number more than two thousand species of plants, with more than 100 already catalogued. species belonging to the genus.
Despite the great diversity that makes up this group and the various species that can attack rice crops irrigated, Meloidogyne graminicola and the more present and harmful in different parts of the globe. Estimates indicate that the impact caused by this nematode on irrigated rice crops in Asian countries can reach 80%.
Symptoms related to the infection caused per Meloidogyne sp. in rice vary from according to the degree of resistance of the plants, the population density of the nematode in the soil and with the management practices used in crops. At the However, symptoms usually begin with the formation of galls on the tips of the roots (commonly known as umbrella handles), which harm significantly the development of the root system, reflecting on the aerial and plant physiology.
Nematode infection in roots of plants directly interferes with the flow of water absorption and transport and nutrients, which may result in symptoms in the aerial part, such as rickets, chlorosis, loss of vigor, delayed maturation and reduction in tillering, which harms plant growth and can cause a substantial reduction in crop yield.
Faced with this situation, in recent years the Phytus Institute has carried out surveys in irrigated rice crops in the state of Rio Grande do Sul in partnership with Basf and FMC in Santa Catarina, in order to study the distribution of this nematode in producing regions. For this, 112 samples were collected containing soil and roots in several commercial crops in 11 municipalities producers in the state of Rio Grande do Sul (Rosário do Sul, Uruguaiana, Formigueiro, Mata, São Sepé, Itaqui, São Vicente do Sul, Alegrete, São Pedro do Sul, São Borja and Jaguari), and five municipalities in the state of Santa Catarina (Turvo, Ermo, Timbé do Sul, Meleiro and Rio do Oeste). In these crops a sample composed of rice plants and soil from hair was collected least five points in each area, carried out in a zigzag pattern. After Therefore, the samples were taken to the nematology laboratory of the Instituto Phytus, for identification and quantification of nematodes.
In laboratory, the collected samples were subjected to extraction and the population estimated by counting, using an optical microscope. The number was determined of nematodes in 200 cm³ of soil and in 10 grams of root. The gender identification through the assembly of temporary slides, followed by observation, based on morphometric characters from taxonomic keys.
According to the results, practically all municipalities sampled in both states showed the presence of the nematode of galls in different population densities. In samples from Rio Grande in the South there was a variation between 10 and 4.370 juveniles in 200 cm³ of soil, and in the roots from 10 to 14.800 juveniles in 10 g of root. In samples collected in the state of Santa Catarina the variation was from 170 to 1.590 juveniles in 200 cm³ of soil and from 1.050 to 18.420 juveniles in 10 g of root.
The average of the values obtained in the respective municipalities demonstrates a population variation of 890 juveniles in the soil and 1.810 in the roots, referring to samples from the state of Rio Grande do Sul. In samples from Santa Catarina, the average value was 540 juveniles in the soil and 5.590 in the roots (Graph 1).
Against of the previous results obtained in this study, there is a high presence of this nematode in rice crops. These results raise some interesting questions about this issue. One of the likely causes for the high occurrence of this nematode is the monoculture of rice with cultivars susceptible over the years, due to the difficulty of rotating crops with non-host plants, which contributes significantly to the rapid population growth of this nematode in the soil. Another possible cause is related to the lack of knowledge and lack of diagnosis of the problem due to technicians and producers, where the damage caused by the nematode ends up not being linked, damage being confused with other events, such as deficiencies nutritional, attack by other soil pathogens (Orizophagos oryza- Rootworm), phytotoxicity caused by herbicides or even indirect damage due to excess iron in the soil.
Despite all the symptoms mentioned are known, the rice plant has “plasticity” mechanisms, which enable it to adapt to specific conditions of cultivation, where initial morphological characteristics undergo changes to enable the plant to adapt to overcome a given situation stress, being a natural defense mechanism. In certain situations, this possible adaptation can cause symptoms caused by nematode attack to be masked, not generating reflections on the aerial part, which can go unnoticed easily by producers and technicians.
Another possible explanation for the symptoms in the aerial view of plants are not very expressive lies in the application of nitrogen fertilization, as the effect of the nutrient affects the growth of plant tissue, stimulating the vegetative cycle and, consequently, forcing the plant to emit new roots. This tends to hide or reduce the damage caused by the nematode. galls. The fact that these microorganisms do not be visible to the naked eye and the main initial symptoms of problems occur in the root system of plants, an “underground” environment difficult to see, also make it difficult to observe symptoms in the field.
All these problems of diagnosis and identification have provided favorable conditions for the growth of root-knot nematodes in rice producing areas. Considering the biotic potential of this group of nematode and high reproduction capacity when environmental conditions are favorable conditions, there is great concern about the possible growth of populations in irrigated rice crops.
It is very important to correctly diagnose symptoms in property, so that identification can be carried out and security practices adopted. management to contain the population increase of this microorganism. The ability to movement of this microorganism in the soil is relatively low, therefore, both In the conventional cultivation system as in the pre-germinated system, water and Agricultural implements play a crucial role in the spread of nematodes, being able to promote their movement to areas that are still exempt, increasing the extent of the problem.
New studies should be carried out this harvest, with the objective of obtaining a general overview of the distribution of this nematode in rice crops in the states of Rio Grande do Sul and Santa Catarina. In addition Therefore, it is necessary to measure the impacts caused on the performance of the culture, as well as alternatives management that make it possible to assist producers in reducing damage caused by this nematode, which is considered a pest silent and hidden, but which deserves full attention due to the possibility of lead to a drop in production and depreciation of crops.
Article published in issue 225 of Cultivar Grandes Culturas, February, 2018.
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