Lifting of the onions should be done as close to harvest as possible. Lifting the onions before the foliage has fallen over exposes a more sensitive bulb to sun burning or color discoloration of the bulb. Providing some sun protection by covering the onions with the foliage of adjacent plants reduces the sun scorch damage significantly. Uncovered onions suffer up to 50 percent more sun scorch damage. Any mechanical lifting should focus on maximizing this overlapping of foliage. Temperatures above 90˚F tend to increase the damage to the bulbs.
After the bulbs are lifted and the tops are clipped, the major disease that affects the bulbs is Botrytis neck rot. This fungus causes decay in the bulbs by delaying the natural drying process that occurs after the bulbs are lifted from the soil. Proper drying of the bulbs after lifting for 10 to 14 days will decrease the vulnerability of the bulbs to this disease. Leaving the foliage on the bulbs for this period is critical to minimizing loss in storage. This does not mean that the bulbs have to be kept out in the elements during this drying period. If conditions are warm and dry, then curing in the field is preferred. When rain or sun burning of the bulbs is possible during field drying, it is better to bring the bulbs in and dry them in storage by providing air movement.
Transmission of Botrytis from diseased bulbs to healthy bulbs occurs during storage. Rough handling of the bulbs causes injury to their outer protective wrapper scales and exposes the fleshy scales. Improper clipping of the foliage can cause damage to the bulbs, which also drastically increases the spread of neck rot.
Certain varieties are less susceptible to Botrytis neck rot. Harder bulbs are much less likely to be affected by this disease; they tend to have less water content in the cell structure and therefore have a less favorable environmental condition for the spread of the disease.
In summary, to extend the shelf life of your crop, you should:
Cover your onions after lifting
Cure them for 10-14 days before clipping the tops
Be careful not to drop or damage the onions in storage