PESTS & DISEASE
Allium or onion leafminers can quickly infest plants, causing leaves to become curled and deformed. Adult leafminers, which are small black or gray flies with a characteristic orange spot on top of their body, lay eggs at the base of the plant. Once hatched, the small larvae pierce the leaves and create channels through the leek, which can allow bacteria and pathogens to enter the plants.
Onion thrips are small insects varying in color from yellow to black. The adults lay eggs on leeks and other onion plants. Once hatched, the larvae feed on plant leaves and then move to the soil to pupate. Onion thrip infestation can result in severe crop damage including stunted growth, twisted leaves, smaller bulbs, reduced yield, and plant death.
Onion maggots are a major threat to leeks, shallots, onions, garlic, and chives. The pests burrow underground and feed on plant bulbs, digging tunnels and destroying tissue. It is believed the maggots may also infect plants with a variety of organisms, which can lead to the development of deadly diseases.
Spread by infested plants, white rot is a serious disease that can live in soil for many years. When leeks are infected, the leaves turn yellow and wither and plants do not grow to full size. In some cases, a fuzzy coating forms on the bulb and sheaf, making the leeks unmarketable.
Downy mildew typically forms on leek plants with overly wet leaves and cooling temperatures. The disease leads to the formation of long, light-colored blotches and velvety purple spores on leaves. It also causes the sheaf to soften and may lead to premature sprouting of leaves.
Basal rot is a common disease of leeks, onions, and garlic, particularly in transplants. The soilborne fungus causes leaves to turn yellow and wither. Roots of infected plants will turn dark brown or pink, and a white fungus may appear on the bulb/sheaf.
Other diseases of concern include botrytis leaf spot and neck rot, pink root, purple blotch, rust, smudge, and white tip.
References: Michigan State University Extension, Ontario Ministry of Agriculture, Food and Rural Affairs, PennState College of Agricultural Sciences, University of California, University of Massachusetts Amherst, University of Wisconsin Extension.
CULTIVATION, STORAGE & PACKAGING
Leeks are a labor-intensive crop because they are typically transplanted, harvested, cleaned, and packed by hand.
A cool-weather crop, leeks thrive in temperatures below 75°F in full sun and well-drained slightly acidic soil. If temperatures fall precipitously, plants may bolt. It can take 70 to 150 days for leeks to grow to full maturity, depending on the variety. Because of their lengthy growing season, leeks are often transplanted. Direct seeding, however, is also an option.
Although it varies by region, leeks are generally planted in late summer or early fall for winter harvesting. Transplanted leeks should be grown in a greenhouse or indoors for 6 to 8 weeks before outdoor planting. Leeks are planted 4 inches apart with 1 to 3 feet between rows. To encourage the growth of a longer edible sheaf, transplants should be placed in fertile channels up to 12 inches deep. Plants require regular watering, but overwatering should be avoided to prevent fungal development.
Leeks are ready to be harvested when the sheaf is 1 to 2 inches in diameter. Although some growers start the harvesting process with machines, most leeks are pulled by hand due to their root systems.
Once harvested, leeks should be washed and stored in a moist, cool environment with temperatures between 32 and 40°F. Wilting will occur if leeks are not stored at proper moisture levels (95 to 100% relative humidity). Leeks are sensitive to ethylene exposure.
Leeks are typically bunched and packed upright in boxes or containers. When trimmed and packed in sealed polyethylene bags and stored in ice, they can last for up to 10 weeks. When stored in a controlled atmosphere at 32°F, leeks may last up to several months.
There is no U.S. grade information specific to leeks at this time.
References: Clemson University Extension, Michigan State University Extension, Oregon State University, University of Maryland Extension, University of Minnesota Extension, Utah State University Extension.