Damping off: Root decay
Pathogen: Fusarium species, Pythium species and Rhizoctonia solani
Symptoms
Roots are invaded and eventually turn dark red or black as they decay. The fungus may cause both pre- and post-emergence damping-off. Seedlings are unthrifty and stunted, eventually turn yellow, wilt and die.
Disease Development
The fungi that cause damping-off are usually common in an onion production field. These fungi generally survive for long periods in soil and may persist in plant debris or on roots of weeds. Damping-off tends to be most severe under conditions of high soil moisture and compaction. Moderate temperatures, especially when onion crops are grown in succession, favor this disease. In greenhouses, damping-off can be more common when improperly pasteurized soil or previously used seedling trays are used for planting. Water splash can move infested soil from diseased to healthy plants and spread this disease.
Disease Management
Crop rotation with cereal crops and soil fumigation or solarization may help reduce damping-off in fields. Improving soil drainage by using raised beds, and regulating soil moisture by avoiding excessive irrigation help to reduce disease. Good sanitation in greenhouses, including using sterilized planting trays and proper soil pasteurization, reduces damping-off. Onions are most susceptible between the flag leaf and first true leaf stage, especially under low light intensity. Therefore, reduced watering can lessen disease losses during this stage. Some fungicide seed treatments or soil drenches can help prevent serious damping-off.
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