Major Onion Pests and Diseases and Sustainable Management Practices

Wikifarmer

Editorial team

8 min read
17/01/2025
Major Onion Pests and Diseases and Sustainable Management Practices

Complete Guide to Onion Pests and Diseases Management

While onion has fewer problems due to pests and diseases compared to other vegetable crops, it is vital that farmers plan and invest in an integrated pest-disease management program to decrease the risk of yield losses and quality deterioration as well as protect the population of beneficial organisms in their field.

Major Onion Pests and their Management

  • Onion Maggot (Onion Fly) - Hylemya antiqua

It is considered the most destructive and common pest in onion crops. The main damage is caused by the small white-cream worm-larva feeding activity that can enter the stem and roots, creating characteristic stoae (loggia). Leaves start to turn yellow, and the plants shrivel and die in heavy infestations. The problem is more extensive in young onion seedlings. The onion bulbs lose their commercial value, while the tissue injuries caused can serve as an entry point for secondary pathogen infection. 

The farmer should stay alert throughout the growing season since the insect has 3 generations. If the insect is detected infesting the onion plants, it is already too late since there is no effective control measure for the insect. As a result, onion 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 in the field. Suppose onion maggots have been found on the field. In that case, 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 essential to remove the infested onion plants from the fields to limit the spread of the problem. 

  • Mole crickets - Gryllotalpa vulgaris or Neoscapteriscus spp.

Mole crickets, specifically species like the southern and tawny mole crickets, are destructive pests of the onion crop. These insects are adapted for burrowing, with enlarged, clawed front legs that allow them to tunnel through soil, which disrupts root systems and dries out the soil, resulting in plant stress. Their tunneling habits create air pockets and pathways that can lead to desiccated soil and roots, weakening plants and sometimes causing visible wilting in onion crops. Infested crops often display signs like uneven plant growth, yellowing, or wilting, as onions’ roots are disturbed and damaged by mole cricket burrows. 

Early control and prevention can be challenging, as mole crickets are hard to spot, but monitoring soil moisture and crop conditions can help. Management strategies include biological controls, such as Steinernema nematodes, which attack the crickets in the soil, and in some cases, baiting with pesticides, though this should be used judiciously to avoid harming beneficial soil organisms. Soil aeration, optimal irrigation, and monitoring pest populations through sound traps or visual inspections can help prevent severe infestations from affecting onion yields effectively.

  • Thrips - Thrips tabaci

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 onion and other Allium plants, 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 (in patches) and the twist and wilting of leaves. The insect can be observed as tiny black spots on the discolored leaves. Remember that depending on the environmental conditions, there might be 3-5 overlapping insect generations per season. 

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

  • Population Monitoring (should be intensified during spring and early summer when the conditions are favorable for the insect)
  • Pheromone traps 
  • Yellow sticky traps (12 traps per hectare or 5 per acre)
  • 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 onions to serve as barriers and block the movement of thrips)

Conventional farmers may also apply chemical pesticides, always after consulting a local licensed agronomist. 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. 

Remember that most of the measures mentioned above can also be effective for other onion pests. In general, an integrated pest management program is expected to be much more effective than applying one single control measure (especially long-term). 

Common Onion Diseases and Management Practices

  • Neck Rot - Botrytis aclada or B. allii

It is an important post-harvest disease of stored onions (onion bulbs and shallots) reported globally in all onion-producing regions. If the production guidelines are not followed, the fungus can cause more than 30% yield losses in stored bulb onions. While the infection starts on the field, the symptoms become apparent after harvest. The first alerting symptoms are the water-soaked appearance of the onion scales in the neck that eventually become brown and covered by the gray mycelium of the fungus. The bulbs start rotting 4-10 weeks after harvest-storage, especially when not treated (dried) correctly.  

The farmer can apply the following preventive measures to avoid yield losses due to Botrytis. So far, there are no resistant onion varieties, but since neck rot can be a seed-born disease, it is vital to use only certified and healthy propagation material. There are also fungicide-treated seeds and sets. Much attention must be given to harvesting only mature onions and cure-drying them properly before storing them. In wet weather, you are advised to apply artificial drying using forced warm air. Later on, the onion bulbs should be stored in optimum conditions (0oC or 32oF and 70-75% relative humidity). If the disease has been recorded in our onion field, we can do a 3-4-year crop rotation to reduce the inoculum on the soil and remove the crop residues. 

  • White Rot - Sclerotium cepivorum

White Rot is a fungal, soil-born disease that can damage onion 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-mounted 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. Furthermore, keeping the plants robust and healthy can boost their tolerance. Avoid reinstalling an allium crop if there were symptoms of the disease on the soil in 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 as 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. Keep in mind that the oospores of the fungus can survive in the soil for many years, serving as a new source of infection. Young plants may be destroyed and die, while older ones stay stunned. Similarly, infected onion bulbs, both in the field but also in storage, can be damaged, shrunk, and rotten. 

Disease control begins with proper preventive measures. 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 to facilitate aeration. Additionally, sprinkler irrigation should be avoided. Other preventive measures mentioned above can also protect the onion 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 with different action modes (to avoid resistance development).

  • Rust - Puccinia allii

Mild temperatures combined with extended periods of high humidity and soil moisture favor the disease. 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, onion farmers know from experience that planting on time and not too early in the season reduces the risk of infection. 

Conclusion

In conclusion, addressing both pests and diseases in onion crops is essential for preserving plant health and maximizing yields. By implementing preventive measures, practicing crop rotation, and adopting integrated pest and disease management techniques, farmers can significantly reduce crop damage, minimize the need for chemical interventions, and enhance the sustainability of onion production.

References

Further information