The first evidence of downy mildew is a fine, furry, grayish white to purple growth on the surface of older leaves. Leaf tissue under the growth becomes pale green, then yellow, and finally collapses. Large, yellowish, circular clumps of infected plants may be the first symptom noticed in the garden. The yellowing patterns often enlarge in the direction of prevailing winds.
Life cycle
Downy mildew can develop very quickly from an initial infection into an epidemic spread by airborne spores if humidity and temperature conditions are favorable (1.5 - 7 hours of leaf wetness and 43 – 80°F). Spores can travel long distances in moist air, but are quickly killed by dry conditions. Initial sources of disease can be infected bulbs, sets, seeds, and plant debris.
Solutions
Use disease-free bulbs, sets, and seed. Use a 3-year rotation away from onions, garlic, and related crops where the disease has occurred. Destroy volunteer and diseased plants. Well-drained soil, freely circulating air, and allowing plants to dry between irrigations help prevent the disease.
