What are the control methods for Spirogyra in rice fields?
The flocculent masses of Spirogyra in rice fields press and entangle the seedlings, causing delayed development, dead seedlings, or single plants with no tillering; they lower the water temperature, and when the Spirogyra coverage reaches 30%, they can reduce the water temperature by 2-3°C; when Spirogyra occurs in large amounts in rice fields, the applied fertilizer cannot reach the rice roots, and it competes with the rice for water, nutrients and growing space. In rice fields with severe Spirogyra infestation, the rice plant height is reduced, the leaves turn yellow, and the yield drops by about 30%.
Characteristics of Spirogyra
Spirogyra belongs to the family Zygnemataceae, an annual aquatic green algae plant. It reproduces by regeneration from broken filaments or by conjugation, and spores germinate into new individuals that spread with the water. Spirogyra has extremely strong vitality and reproductive capacity, and it likes fertilizer, growing vigorously and reproducing faster especially under conditions of heavy phosphate fertilizer application. It grows best in still water with a soil pH of around 7.5 in saline-alkaline conditions. Generally, after the rice fields are irrigated in spring, it begins to appear in mid-May, and in early June, as the temperature rises and the effects of phosphate and organic fertilizers in the field take effect, it rapidly reproduces and causes damage.
Occurrence pattern
Fields with drought and little rain, scarce water resources, frequent use of recycled water for irrigation, or irrigation without drainage are prone to Spirogyra damage. Spirogyra spores and filaments in irrigation and drainage channels flow with the water into rice fields and reproduce there. Fields with a high incidence of Spirogyra the previous year retain large amounts of Spirogyra masses, which will recur in large numbers after irrigation the following year. Deteriorated water quality and high application rates of phosphate fertilizer, forming nutrient-rich water, can promote the occurrence of Spirogyra. Situations that lead to major outbreaks of Spirogyra include: (1) Low-lying, still-water rice fields with long-term standing water, poor drainage, and deteriorated water quality. (2) Rice fields where large amounts of phosphorus-containing fertilizer are applied, forming nutrient-rich water. (3) Rice fields with a high incidence of Spirogyra the previous year.
Control methods
(1) Scientific fertilization with a reasonable formula of nitrogen, phosphorus and potassium. (2) Reasonable irrigation and drainage, using the shallow, dry, wet irrigation method. (3) Manual removal: when Spirogyra forms small patches in the field, first remove it by hand; although the remaining Spirogyra filaments are broken, their ability to absorb nutrients is extremely strong and they quickly reproduce to form new masses, but the wounds on the broken filaments contact the chemical solution, which will quickly poison and kill the Spirogyra, so after removal, pesticide should be applied promptly for control. (4) Chemical control: use 65 grams of 26% Shui Mian Jing wettable powder per mu, applying the pesticide 15 days after rice transplanting when Spirogyra is in the "block stage"; after July, when Spirogyra is in the "blanket stage", use 100 grams per mu, mixed with fertilizer or fine soil and broadcast, never with dry soil, and after application keep a water layer of 4-6 cm in the field for 5-7 days. Shui Mian Jing must not be used on diseased or weak seedlings or in leaky fields.