Lemongrass Oil Eliminates Onion Seed Fungus

Based on the organic sector's demand for chemical-free vegetable seeds, Embrapa researchers evaluated alternative treatments based on the application of essential oils from aromatic plants and discovered that lemongrass is the most suitable species for disinfecting onion seeds contaminated with a particular species of fungus. The natural product can be an alternative for organic producers or those who have restrictions on using fungicides.

"In addition to not compromising the germination potential of the onion seed, lemongrass oil inhibited the germination of fungus spores by 100%", quantifies agroindustrialist Maria Isabel Lozada, master's student at the University of Brasília (UnB) and scholarship holder from Embrapa Hortaliças, for whom the oil can be considered a promising alternative for controlling this microorganism that is harmful to the quality of onion seeds.

This is the fungus Colletotrichum gloeosporioides f. sp. cepae which, in hot and rainy seasons, causes a very destructive disease called anthracnose or seven-turn disease. In addition to causing lesions on onion leaves, this disease affects the bulbs, which can present malformation and, in the most extreme cases, rot. Even when the plant resists the microorganism, onion bulbs do not reach commercial market standards.

"This fungus can deteriorate the seed during storage, making its germination or the emergence of the plant unfeasible, which, if it develops, can manifest the disease and compromise production", explains researcher Warley Nascimento, from the Seed Production Technology area, who leads the research project.

Generally, to control this microorganism, a conventional treatment based on broad-spectrum fungicides is applied to onion seeds, before direct sowing in the field. However, in the search for alternative methods for the organic system, essential oils from five species were tested: basil, sage, thyme, citronella and lemongrass. "Alternative treatments with essential oils do not require the use of fungicides and provide a better quality of life for rural producers", ponders Nascimento when informing that this method is recommended for family farming, but can also be used in the conventional system to minimize dependence on chemical inputs.

Different concentrations of essential oils diluted in water were analyzed to identify not only the best oil, but the ideal dose for the fungus to be eliminated without compromising the germination of the onion seed.

The highest concentrations, from four thousand to eight thousand parts per million (ppm), were discarded because they made the perfect germination of onion seeds impossible. Therefore, the other analyzes continued only with dilutions of one thousand and two thousand ppm. "After adjusting the dose, we also studied the effects of essential oils in relation to vigor and resistance, as well as other characteristics relating to the physiological quality of the seed", he adds.

Considering that the standard for the commercialization of onion seeds in the country establishes a minimum germination percentage of 80%, all the oils analyzed were above this value. The seed soaked in lemongrass oil was the only one that showed a higher germination rate than that obtained by the untreated seed, reaching 97% germination.

The effect of essential oils

Seed treatment has two main objectives: to disinfest the seed and protect it from harmful microorganisms present in the soil. The role of essential oil meets the first objective, as it eliminates the microorganism that is attached to the surface of the seed. "Essential oils do not have a residual effect to the point of shielding the seed in the soil, but they have a lot of merit in guaranteeing a healthy seed free from contaminants", analyzes Embrapa researcher Ricardo Pereira, from the Phytopathology area.

Essential oils contain chemical compounds that have a toxic effect on the fungus that harms onion seeds. The types and amounts of these compounds vary in each oil, which is why they present different results when treating seeds. According to Maria Isabel, the disinfectant action of lemongrass oil is not necessarily due to the presence of a major substance, but rather to the combination of chemical compounds.

In the next phase of research, funded by the National Council for Scientific and Technological Development (CNPq), the team intends to analyze the effect of essential oils on seeds of ten other species of vegetables, including lettuce and carrots, both for treatment against fungi and for bacteria.

National seed production

Brazil produces almost all of the open-pollinated vegetable seeds it uses, however, a large part of the production of hybrid seeds is carried out abroad. In a market that generates around R$550 million per year, more than 70% of this value corresponds to the sale of hybrid seeds. "Due to the scarcity and cost of labor, seed companies opt for production abroad and, as a result, we lose currency and become more vulnerable to the entry of exotic pests into the country", analyzes Nascimento.

The production of vegetable seeds, whether conventional or organic, demands expertise and technological investment on the part of the farmer. For the organic sector, specifically, the adoption of new practices becomes essential after Normative Instruction No. 46/2011, from the Ministry of Agriculture, Livestock and Supply (Mapa), which prohibits the use of synthetic pesticides in the treatment and storage of organic seeds and seedlings.

According to the regulations, starting next year, each state must produce an annual list of species that can only be grown with organic seeds, depending on the market's availability to meet these demands. If seeds from organic systems are unavailable, the use of other types of seeds is authorized, but preference will be given to those not treated with pesticides. "Therefore, based on alternative treatments, we hope that the organic seed production chain will become more competitive to serve this expanding market niche in the country", explains Nascimento, who adds: "a quality seed is the starting point for healthy and productive farming".