Silver or white blotches or streaks
Two different conditions can cause very similar white or silver blotches on onion leaves: Thrips (Thrips tabaci), a tiny, almost microscopic yellow/brown insect, and Botrytis Leaf Blight (Botrytisspp.), a fungal disease.
Thrips: Adult thrips are difficult to identify due to their small (1/12 inch long) size. Damage is caused by feeding, producing white blotches that become yellow or brown with time. Bulbs may be stunted or distorted. Multiple generations of thrips are often produced in a single year, especially in hot dry weather.
Botrytis Leaf Blight: This fungal disease causes small oval white or yellow spots surrounded by a silver halo. Outbreaks are most severe in wet conditions, when plants are damaged by thrips, or when plants are exposed to air pollution. Disease spreads rapidly and entire fields can be killed within just days.
Since the fungus survives in soils and crop debris, only plant in well drained soil in areas with good air circulation and rotate crops annually. No resistant varieties are available. Rapid disease progression makes timely fungicide applications difficult. Field sanitation and frequent crop rotation are essential.
Thrips: Contact your local Extension agent for information about currently approved pesticides. Insecticide resistance is an ongoing problem with this pest and whenever possible cultural controls should be employed. This includes the removal of crop debris, and the use of resistant varieties--red onions are the most susceptible and sweet Spanish varieties are the most resistant. Yellow or white sticky traps can be placed in fields as a monitoring tool.
Botrytis Leaf Blight: Since the fungus survives in soils and crop debris, only plant in well drained soil in areas with good air circulation and rotate crops annually. No resistant varieties are available. Rapid disease progression makes timely fungicide applications difficult. Field sanitation and frequent crop rotation are essential.
Wet brown lesions
Purple blotch (Alternaria porri), a fungal disease.
Like many other fungal diseases, purple blotch outbreaks are worse in wet weather. Plant in well-drained soil, rotate crops annually and remove crop debris. Contact your local Extension agent for information on approved fungicides.
Dusty or oily black or brown lesions
Onion smut (Urocystis cepulae), a fungal disease that only affects seedlings, especially in cool weather conditions.
Resistant varieties are not available. Rotate crops annually and remove crop debris. Seed treated with fungicides provide good protection against this disease.
Root or Bulb Problems
Tunnels in bulbs
Onion maggot (Hylemya antiqua), the larva of a small fly. Adult files are 1/4 inch long and resemble small house flies. The immature form of this insect also feeds on the roots of leeks, shallots, garlic, and chives. The onion maggot is most problematic where continuous production is practiced. Larva overwinter in piles of cull onions and in crop residue. Adults emerge in May and begin laying eggs, leading to several generations during a single growing season. Feeding damage can cause secondary rot diseases and outbreaks are worse in cool weather.
After damage is detected it is too late to treat with insecticides. Rotate crops annually to avoid creating large populations of this insect. Provide at least 1/2 mile distance between new seedlings and previous crops and cull piles if possible. Also destroy cull onions and crop debris. Planting onions as soon as possible in the spring also helps establish young onions before adult insects emerge.
Soft, rotten roots covered in white, moldy growth and accompanied by soft, watery decay of the bulb
Fusarium Basal Rot (Fusariumspp.), a fungal disease.
This disease is usually worse in warm weather especially in mid to late summer. Treatment options include annual crop rotation and the planting of resistant varieties like Daytona, Cortland, Endure, Fortress, Frontier, Fusarion24, Mars, Millennium, Quantum, Spectrum, and many others.
Note: several other plant diseases can cause soft, rotten bulbs while in storage, consult your local Extension agent for information about problems with stored vegetables.