The infection can occur at any growth stage and is first visible on mature leaves of first-year plants. Initial symptoms appear as irregular, yellow stripes that gradually form a mosaic pattern. As the symptoms progress, these leaves become wrinkled, softened, curled downward and can finally wilt. When the infection is heavy, it can lead to a complete yellowing of the leaves and plants have a stunted appearance. Bulbs may not develop and, if they do so, they are reduced in size and may shoot prematurely, for example during storage. Onion plants used for seed production can show deformation of flower stems, reduction of flowers and seeds as well as a loss in seed quality. Germination rates are significantly affected.
The symptoms are caused by a virus called Onion Yellow Dwarf Virus (OYDV). It can survive in plant debris in the field for a long period. Most commonly the virus is transmitted by infected plant parts such as bulbs, sets and volunteer plants in the field. It has a limited host range that is confined to plant species of the allium family (onions, garlic, shallots and few ornamental alliums). The disease can also be transmitted in a non-persistent manner via several aphid species (for example Myzus persicae). They carry the virus in hteir mouthparts and inject it into healthy plant while sucking their sap. Very often, the virus occurs in combination with other viruses in the same plant. The infection can result in reduced yield to a greater or lesser extent depending on that. For example, yield losses are higher when plants are also infected with leek yellow stripe virus.
No biological treatment seems to be available at the moment to treat this disease. Treatments against aphids include solutions with neem oil at 2% and neem seed kernel extract (NSKE) at 5%.
Always consider an integrated approach with preventive measures together with biological treatments. In the case of viruses a chemical treatment is not possible. Treatments against aphids include emamectin benzoate, indoxacarb or NSKE.