ONION HEALTH MANAGEMENT & PRODUCTION be scattered throughout a field. IYSV is transmitted by onion thrips. The virus does not appear to be seed-borne in onion. Control of onion thrips should help reduce incidence of IYSV; some onion varieties appear to be less susceptible to thrips and/or the virus. Destroying onion debris, including volunteers, and culls; locating new plantings away from overwintering onions or previous onion crops with a history of the disease; and avoiding planting annual bulb crops in close proximity to biennial seed crops because the ‘green bridge’ effect of overlapping crops promotes rapid buildup of thrips and IYSV, all help reduce losses to IYSV in onion crops. Onion Yellow Dwarf (OYDV) This disease is caused by a virus that is transmitted by various aphid species or transmitted mechanically to onions and other crops such as garlic, leek and some narcissus species. The first symptoms appear on the youngest leaves, which turn pale and develop many yellow streaks along the veins. Leaves may crinkle and flop over. Symptoms are more pronounced on leaves that develop from an infected bulb or transplant, and the yellow streaks begin at the base of the first leaves and successive leaves as they emerge. Later, there may be more pronounced yellowing, and leaves crinkle, flatten, twist and fall to the ground. Flower stems are shortened, streaked with yellow, and twisted. Generally the plant appears dwarfed, and has a wilted appearance. Symptoms of virus infection should not be confused with those of normal-shaped leaves that have alternate yellow/green bands caused by genetic or vegetative mutations (chimeras). OYDV is not spread by seed, but infected bulbs (transplants, volunteers) always produce diseased plants and serve as sources of contamination for subsequent seasons, especially when aphid populations are large. Therefore, planting disease-free transplants and rotating out of onion production for at least three years are recommended. Other disease management recommendations include isolation from other susceptible crops or volunteer onions, and aphid control. Aster Yellows - Phytoplasma This disease is caused by the aster yellow phytoplasma that affects many weeds and crops including carrot, celery, lettuce and onion. The pathogen is transmitted by the aster leafhopper. Symptoms initially appear as a yellowing at the base of young leaves, which then spread toward the top. Leaves flatten, become marked with green and yellow streaks, but do not twist. Flower stems are abnormally elongated, and have malformed, sterile floral clusters. Disease management recommendations include crop rotation out of onions for at least three years; elimination of weed hosts in and around onion fields; insect vector control; and isolation from other susceptible crops or volunteer onions.
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