Agriculture Reference
In-Depth Information
seeds float at the top and can be (carefully!) poured off with the water. Dry seeds immediately, using,
for example, coffee or tea filters.
SELECTION CHARACTERISTICS
• healthy, vigorous growth in first year
• true-to-type bulb shape
• uniform bulb development; no double or multi-bulbs
• high-quality, sturdy scales with no tears
• true-to-type scale color
• drought tolerance
• flavor
• slim neck of onion (= stores better)
• long storage length
• synchronous, complete ripening
• minimal susceptibility to downy mildew
DISEASES AND PESTS
Crop rotation is very important in onion cultivation: wait at least five years be-
fore growing onions again in the same bed. Never fertilize with fresh manure or unripe compost as this
delays ripening and encourages onion flies (
Delia antiqua
). These look like small houseflies and lay
their eggs on young onions. The larvae then eat these young onions, turning leaves yellow and killing
them. Later generations can then infest larger plants later in the year. Onion fly attacks inflict the most
damage in hot weather when the soil is dry. The second generation eats its way into bulbs from the soil
in mid to late summer; sometimes a third generation appears later. Prevention: insect netting at the
time of egg laying (around the time of the last spring frost), not fertilizing with fresh manure, not
growing onions after spinach. Polyculture with carrot and undersowing subterranean clover (
Trifolium
subterraneum
) are said to help keep infestations under control. Treatment: remove and burn infested
leaves (to kill maggots, thus interfering with their reproductive cycle). Infestation of the seed stalks in
the second year can cause major problems for seed growers. Onion fly maggots can attack seed stalks
from within until they die and fall off. Observe plants closely.
Downy mildew (
Peronospora destructor
) occurs especially in wet years and in plots with little
wind. Symptoms: onion foliage with elongated, greenish brown spots, sometimes with a violet-gray
carpet of spores. In serious cases, onion leaves can entirely die off, bulb growth is stunted and harves-
ted bulbs do not store long. A primary source of infection is infected plant debris in the soil. Preven-
tion: grow in windy areas, maintain loose soils, apply horsetail slurry, five-year crop rotation, thorough
removal of plant debris from garden beds. Treatment: remove infected plants and compost thoroughly
or burn. Onion neck rot (
Botrytis aclada
) is a disease that pops up in storage. The fungus can enter the
plant through dead leaves in the fall or through injuries sustained during the harvest. After a few weeks
in storage, the upper part of the onion appears glassy and looks like it has been cooked until soft. A
layer of gray fungus with black spots also forms on the onion. Onions ultimately shrivel and rot into a
smelly porridge. The fungus is seed- and soil-borne (spread by the wind when soil-borne). Prevention:
do not excessively fertilize with nitrogen, sow seed early, protect crop from rain as it ripens (tarp), wait
until leaves are completely dead before harvesting, harvest only in dry weather. Treatment: check
stored onions regularly and quickly remove infected ones.
