Numerous diseases can affect the quantity and quality of onions. While there are many practices that can help reduce onion diseases, they are not infallible, and several diseases require different treatments. There are, however, certain methods that can be employed to ward off most types of ailments. Drip irrigation has been shown to reduce the occurrence of diseases because the water is directly given to plant roots instead of wetting the plant or crop field. Without standing water, many diseases have a harder time infecting onion bulbs.
Many diseases thrive in similar conditions. Excessively wet soil, moderate temperatures, and dense crop spacing can all increase the risk of infection from bacteria, fungi, and viruses by insects. Farming practices such as Tillage and Cultivation, Crop Rotation, Fertilizer Application Methods, Best Management Practices for Insect Control, and Best Management Practices for Weed Control contribute to disease prevention and control.
Of the many onion diseases, the most significant are botrytis leaf blight, neck rot, downy mildew, purple blotch, damping off, white rot, pink root, plate rot, basal rot, and iris yellow spot virus. Each of these diseases occurs under specific circumstances and has to be managed in different ways. Botrytis leaf blight is a fungal disease that kills foliage. Neck rot begins when the plants are in the field, but develops in the bulbs while the onions are in storage, and can damage an entire crop. Downy mildew attacks the leaves and bulbs of onions, greatly reducing crop yield. Purple blotch also kills foliage. Damping off is another fungal disease that can kill onion seedlings if they are suffering from dampness or over-watering. White rot is a fungal disease that can spread extremely quickly and wipe out entire fields of onions. White rot is persistent in fields where it has occurred in the past and cannot be controlled through crop rotation. Pink root attacks the roots of seedlings and makes it very difficult for the bulb to grow. Plate rot (basal rot) also attacks roots and decreases bulb growth. plate rot and can attracts maggots (Onion Disorders).
Iris yellow spot virus (ISYV) is perhaps the most important pathogen affecting onions in the Pacific Northwest and is spread by onion thrips. Once plants are infected, there is no cure or treatment; however, crop rotation, weed control, disposal of culls, physical separation of fields, and thrip management are all preventative measures. (Iris Yellow Spot Virus in Onions). Water stress can aggravate IYSV symptoms, so sustainable irrigation practices are important for disease management. Onion plate rot and pink root are managed through crop rotation and soil fumigation. White rot is managed by avoiding planting onion or any other Allium crop in soil where white rot has occurred in the past. For detailed information on several more diseases, visit the Onion Disease Guide.