Leek Pests, Diseases and Weed Control

Important leek pests

Thrips (Thrips tabaci, Frankliniella occidentalis)

Thrips are tiny insects that attack a wide range of plant species globally and are a major enemy of leek plants, causing significant yield losses. Their destructive activity could either be direct, by sucking foliar sap, or by transmitting dangerous viruses such as IYSV to infested plants. Thrips are usually found on the underside of leaves, protected from the sun, close to the conjunction point with the stem. Except for detecting the insect directly, the grower may also observe common symptoms like the whitish-silvery discoloration (in patches) and the twist and wilting of leaves.

There are preventive and control measures. Some useful preventive measures are the: 

• Population Monitoring (Should be intensified during spring and early summer so that the conditions are favorable for the insect)
• Pheromone traps 
• Weed and crop residue removal (thrips overwinter in species like alfalfa, clover, and small grains).
• Plant barriers (some farmers plant 2 rows of maize or 1 row of maize and 1 row of wheat around the field 30 days before planting the leeks to serve as barriers and block the movement of thrips).

Conventional farmers may also apply chemical pesticides, always after discussing with a licensed agronomist (e.g., Profenofos, Carbosulfan, Malathion, Delegate, or Fipronil). In this case, spraying with a suitable-registered product should occur when 20-30 adults are counted per plant, or on average 1 thrip per leaf (check 50-100 plants randomly). Insecticidal soaps may also be used. However, it is essential to avoid continuous and excessive use of pesticides (especially with the same mode of action) since thrips can easily develop resistance to them. 

Onion Maggot (Hylemya antiqua)

It is considered the most destructive and common pest in leek crops. The main damage is caused by the feeding activity of the small (1-inch or 2.5 cm long) white-cream worm-larva that can enter the stem and roots, creating characteristic stoae (loggia). Leaves start to turn yellow, the leek shanks (pseudostems) lose their commercial value, and the plants periodically wilt. The problem is more extensive in young leek seedlings. 

The farmer should stay alert throughout the growing season since the insect has 3 generations. If the insect is detected infesting the leek plants, it is already too late since there is no effective control measure for the insect. As a result, leek farmers may invest in preventive measures. Crop rotation and balanced fertilization (no excessive organic matter) can reduce the risk of infestation and insect population build-up. Additionally, the farmer should avoid leaving crop residues at the field, and if onion maggots have been found on the field, a pre-planting application with appropriate insecticides and cultivation of the field during fall (to kill the overwintering pupae) can be helpful. Finally, it is important to remove from the fields the infested leek plants to limit the spread of the problem. 

Major Leek Diseases

White Rot (Sclerotium cepivorum)

White Rot is a fungal, soil-born disease that can damage leek plants (and other alliums) throughout the growing season. The disease is caused by the fungus Sclerotium cepivorum, which survives in the soil. Symptoms include leaf chlorosis, wilting and complete plant death. Usually, the grower may observe the development of a whitish, cotton resembled growth on the part of the stem under the soil, with sclerotinia (small, round orange-brown). Due to favorable conditions, the fungus is more destructive in early spring and autumn. In a field infected by the fungus, the symptoms may appear in groups of plants (patches) rather than in individual plants. 

Disease control begins with proper preventive measures. These include weed control, removal of crop residues, safe distances between plants for better aeration, and improvement of soil drainage. The general condition of the plants (nutrients and water level, sun exposure) can also boost their tolerance. Avoid reinstalling an allium crop if there were symptoms of the disease on the soil the previous years. Moreover, crop rotation with crops like lettuce, carrots, or potatoes can be only partly beneficial since the fungus can survive in the soil for over 10-15 years in the form of sclerotinia in the field. Soil solarization is considered a relatively effective control measure. Chemical treatment is used only if the problem is severe and always under the supervision of a local licensed agronomist. Finally, all equipment should be properly disinfected when moving between fields or from an infected to a healthy field part to avoid the disease spread. 

Downy Mildew (Peronospora destructor)

Downy mildew is a destructive fungal disease caused by the fungus Peronospora destructor. High moisture levels favor the infection. Some common disease symptoms include white-yellow discolorations of leaves (oval spots). Usually, under optimum temperature and moisture conditions, these spots are covered by the fungus’s whitish-violet furry growth. The pathogen can survive for many years in the soil as oospores. Young plants may be destroyed and die, while older ones stay stunned. Similarly, infected leek shanks, both in the field but also in storage, can be damaged, shrunk, and rotten. 

Disease control begins with proper preventive measures. To facilitate aeration, in areas with high humidity levels, the growers are advised to use larger planting distances and align the plant rows with the direction of the prevailing winds. Additionally, sprinkler irrigation should be avoided. Other preventive measures mentioned above can also protect the leek from this disease. However, many farmers apply a series of regular and preventative sprayings with a fungicide. It is vital, though, to use only registered and suitable crop products.

Rust (Puccinia porri) 

The disease is favored by mild temperatures combined with extended periods of high humidity and soil moisture. 

The main symptoms of the infection include the formation of orange (rust) spots, primarily on the underside of the leaves and periodically on both leaf sides. On top of the orange spots, we may observe the formation of powdery spores of the fungus that eventually become brown-dark. Disease management includes all the controlling measures mentioned before. However, chemical control is not commonly applied. On the other hand, leek farmers know from experience that planting on time and not too early in the season reduces the risk of infection. 

White Tip Disease (Phytophthora porri)

White Tip Disease is a destructive, fungal disease that can appear in a leek field during days of increased temperature (during summer) and humidity levels or after heavy rains. In the case of an infection, the farmer may observe spots on the leaf tips and margins with a water-soaked appearance. Stems can also be infected and begin to wilt. Soft rot may gradually appear on all vegetative organs of the plant. Keep in mind that the pathogen can survive on crop residues. Disease management includes all the controlling measures mentioned before in other fungal diseases. 

Leek Weed Management

Leeks are very sensitive to weeds, especially during the first two months of their growth. As a result, it is essential to perform a suitable and efficient weed management program to offer a clear start (weed-free seedbed) for leeks to grow but also later on during the growing season to allow yield maximization. 

Before planting, many leek farmers plow their fields (up to 40 cm deep) or/and apply the stale/bed technique. Depending on the country and the local legislation, farmers may also use a suitable and registered herbicide. This is especially useful if the leek crop has directly been sown to the field. Moreover, the banking up of the soil around leeks growing in trenches may offer sufficient weed suppression. Cultivation measures can also be applied between the plant rows as the leek crop emerges and grows. However, the farmers should remember that leek plants have fibrous and shallow roots that can be easily damaged if hoeing too close to the plants. 

Some farmers cover the part of the soil that is located between the plant rows with a special black mat or mulch (3-4 inches or 7.5-10 cm thick). They also cover the space between the young plants inside the row with this black mat. The black mat prevents the development of weeds while increasing the soil temperature and preserving soil moisture. Finally, the use of crop rotation and cover crops with species not related to alliums may help reduce the weed population in the field. 

References

https://www.agric.wa.gov.au/leeks/growing-leeks-western-australia?page=0%2C3

https://extension.umn.edu/vegetables/growing-leeks#controlling-weeds-234411

http://www.omafra.gov.on.ca/english/crops/facts/91-004.htm#Nutrition

https://www.dpi.nsw.gov.au/agriculture/horticulture/vegetables/commodity-growing-guides/leeks

https://datcp.wi.gov/Documents/mk_fc_82_web%20pdf%20-%20Adobe%20Acrobat%20Pro.pdf

https://extension.uga.edu/publications/detail.html?number=C852&title=production-and-management-of-garlic-elephant-garlic-and-leek#Fertilization

https://extension.usu.edu/yardandgarden/research/leeks-in-the-garden

15 Interesting Facts about Leeks

Leek: Info, Facts, Nutritional Value & Health Benefits

Leek Plant Information and Variety Selection

How to Grow Leeks at Home

Commercial Leek Farming – How to Grow Leeks for Profit

Leek Soil Requirements, Soil Preparation, and Planting of Leek

Leek Water Requirements and Irrigation Systems

Leek Fertilization Requirements

Leek Harvest, Yields and Storage

Leek Pests, Diseases and Weed Control