Garlic Pests and Diseases
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Garlic Plant Protection – Pest and Disease management in Garlic
In general, garlic has few problems due to pests and diseases compared to other vegetable crops.
Important garlic pests
Thrips (Thrips tabaci, Frankliniella occidentalis)
Thrips are tiny insects (0.5‑2 mm or 0.02-0.08 inches long) that attack a wide range of plant species globally and are a major enemy of garlic, causing significant yield losses. Its 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 of leaves (in patches) and the wilting of leaves and bulbs.
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 garlic to serve as barriers and block the movement of thrips).
Conventional farmers may also apply chemical pesticides, always after consulting a local 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.
Nematodes: Bloat Nematodes (Ditylenchus dipsaci) and Root-knot Nematodes (Meloidogyne spp.)
If found in the field, the grower should avoid planting Allium species like garlic and onion since they are susceptible to nematodes leading to significant yield losses (in extreme cases, even crop failure). Favored by increased soil moisture, humidity, and cool temperatures, bloat Nematodes can enter the plants through injuries and other openings (e.g., stomata). Both nematode species can cause leaf chlorosis, stunned plant and root growth, wilting and shrinking, deformation, and decay of the bulbs. More specifically, a large population of root-knot nematodes can reduce the bulb size up to 50-70%, while galls can usually be observed in the root system. Nematodes are usually “inserted” into a field by infected cloves (planting material). Avoiding any propagation material from an infested plant is also vital. While some suitable nematicides are available in the market, garlic farmers should also consider applying cultural control practices, like deep plowing and soil solarization during the warmest months of the year.
Major Garlic Diseases
White bulb rot (Sclerotium cepivorus)
White Rot is a fungal, soil-born disease that can damage garlic plants throughout the growing season. The disease is caused by the fungus Sclerotium cepivorus, 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, 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 bulbs, 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 garlic 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.
Brown Rust (Puccinia allii)
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, garlic farmers know from experience that planting on time and not too early in the season reduces the risk of infection.
Onion Yellow Dwarf virus (OYDV)
Viruses are either transmitted by infected propagating material or by carriers like aphids and thrips. We may observe chlorotic stripes on the leaves in the infected plants and stunned growth. Since there is no cure for the virus, the only management measures are only preventive and include the selection of diseased-free propagated material (cloves), bought from a legitimate seller, and aphid-thrips control management.
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