Rice thrives in warm and humid conditions, but such environments also often lead to problems because pathogens also favor warmth and humidity. From germination through growth to flowering and seed setting, rice is susceptible to various diseases throughout its life. The most severe disease is blast disease: if the seedlings are infected, they die; if the leaves are infected, they develop lesions and the leaves die; if the stem nodes are infected, they rot, preventing nutrient transport, and if the panicles are infected after heading, they will die; if the grains are infected, spots develop on the husks, and the grains fail to fill properly. Additionally, various other unusual diseases often occur, such as bacterial blight, which causes rice leaves to whiten; sesame leaf spot disease, which creates spots on the leaves and prevents grains from developing properly; and various diseases that cause abnormal growth or dwarfing in seedlings, and diseases affecting the tips of the rice stems, causing them to dry and twist. There is also rice sheath blight, which is quite severe.
These pathogens reproduce and spread generation after generation in the fields, eventually reaching the seeds, which may carry fungal mycelia or spores either inside or on their surfaces. Planting these diseased seeds can introduce the disease into the fields, facilitating the spread of pathogens. To prevent seeds from carrying pathogens into the fields, it is necessary to disinfect the rice seeds, killing any pathogens on them.
Pathogens’ mycelia and spores are very fine and stubborn, not easily eliminated, thus requiring toxic chemicals for treatment.
For instance, seeds can be disinfected by soaking them in Thiram or Cercobin solution. First, the chemical powder is mixed into a paste, poured into a large tank, and diluted with water to a concentration of one-thousandth or two-thousandths (approximately 100 kilograms of solution can soak 100 kilograms of rice seeds). The selected rice seeds are soaked in this chemical solution, stored in a cool place to prevent the solution from deteriorating. The duration of soaking depends on the weather and the concentration of the solution. If the temperature is below 20°C, a one-thousandth concentration of Thiram needs 48 hours of soaking, while a two-thousandths concentration requires only 24 hours.
Formalin can also be used for soaking seeds. A 2% formalin solution is made by mixing one part formalin with 50 parts water. Rice seeds are first soaked in clean water for 24 hours, drained, and then soaked in the formalin solution for 3 hours. Alternatively, seeds can be soaked in a one-thousandth concentration of a mercury-based solution for 6-12 hours.
A one percent or five-thousandths concentration of lime water can also be used for soaking seeds. Soaking seeds in lime water suffocates the pathogens due to lack of oxygen. The duration of soaking varies with temperature; for example, 15 days at 10°C and only 4 days at 25°C. The method of fumigating seeds with chemical solutions involves wetting the seeds with the solution, piling them up to fumigate, which does not require as long as soaking to kill the pathogens. For instance, fumigating with formalin solution only needs 3 hours.
Dusting seeds with chemical powder involves applying the toxic chemical on the outside of the seeds, then piling them in a container to prevent air circulation. The chemical vapors will steam and kill the pathogens. For example, after dusting with Thiram or Cercobin powder and fumigating for more than 3 days, the pathogens are killed.
These disinfection methods can kill not only the blast fungus but also sesame leaf spot and dwarf disease pathogens. After soaking or fumigating seeds with chemicals, they must be rinsed three or four times with clean water to remove any residual chemicals, as these could affect germination.
The hot water soaking method can kill pathogens hiding inside the seeds, like the rice stem tip nematode, which hides inside the rice husk. First, rice seeds are soaked in cold water for 24 hours to allow any nematodes inside to become active. Then, the seeds are placed in water at 45°C – 47°C for 5 minutes, followed by 52°C – 54°C for 10 minutes to kill the stem tip nematodes. However, the temperature and duration of hot water soaking must be carefully controlled to avoid damaging the seeds. After hot water treatment, the seeds are immediately cooled with cold water, spread out to dry in the shade; otherwise, it can also affect germination.