The allium family of plants is renowned for its diverse range of flavorful and nutritious crops, including onions, chives, garlic, leeks, and scallions. While they are generally easy to cultivate, simple to harvest, and long-term storage-friendly, these popular edibles can still be susceptible to pests and diseases. Alliums, like any other plant, are not immune to common issues such as damping off, cutworms, aphids, downy mildew, and thrips.
As a result, it’s essential to recognize the signs of trouble and take effective action to address these problems.
Common Allium Pests
When cultivating alliums like scallions, spring onions, garlic, leeks, or ornamental varieties, it’s essential to understand their shared requirements. A well-drained, fertile soil with sufficient sunlight is ideal for all of them. Onions and garlic also thrive in soil with a pH between 6.5 and 7.0, making this range perfect for optimal growth. Providing the right conditions can significantly impact the health of your alliums.
However, even under ideal circumstances, these plants can fall prey to various pests. Fortunately, many of these pests can be effectively managed through crop rotation, which involves moving your alliums each season to a new location. This simple practice confuses overwintering pests that may have targeted last year’s onion bed. While garlic is generally less susceptible to pests, onions and some other allium varieties are more vulnerable.
In fact, onions are particularly prone to damage from certain pests. Chives, scallions, and leeks, on the other hand, tend to attract a wider range of pests compared to garlic and ornamental alliums.
Allium Leafminer
In recent years, North American allium growers have faced a new and formidable foe: the allium leafminer (Phytomyza gymnostoma). This tiny but devastating pest has only recently become a noticeable problem, emerging as a significant issue in 2016. The effects of an infestation are plain to see – a trail of wavy, shrunken, and damaged leaves can be attributed to the feeding habits of these cream-colored grubs.
As they feed on allium crops like garlic, onion, leek, and others, the leafminer larvae spend their time on the plant before dropping off to pupate in the soil. Once fully formed, adult flies emerge, quickly laying eggs on nearby allium plants, perpetuating the cycle. Fortunately, controlling these pests is relatively straightforward – a simple matter of crop rotation and row covers can be employed to prevent adults from laying eggs on your crops.
By moving your alliums each spring and covering young plants with row covers, you can effectively keep allium leafminers at bay.
Onion Maggots
Onion flies (Delia antiqua) are a notorious menace to alliums, with their larvae causing the most damage. These unappealing, quarter-inch long white maggots feed on roots, capable of killing seedlings and mature plants alike. However, they often target bulbs, feeding slowly over time. Unfortunately, affected bulbs become utterly inedible, necessitating prompt destruction of infected plants and their avoidance of compost piles, as onion maggots can thrive in such environments.
To prevent infection, consider implementing crop rotation and utilizing beneficial nematodes. Apply the nematodes to the soil prior to planting your onions, followed by covering young plants with row covers to deter adult onion maggot flies from laying eggs on your crops.
Lesser Bulb Flies
Lesser bulb flies (Eumerus funeralis) may not be as destructive as onion maggots, but they still pose a significant threat to onions and garlic. These pests are larger than onion maggots, with larvae that are about twice the size and a yellowish-grey color. To manage lesser bulb fly infestations, adopt a similar approach to onion maggot control: utilize beneficial nematodes, deploy row covers, and remove affected plants.
A crucial aspect of minimizing damage is clearing every onion out of the garden at the end of the season. This ensures that these pests don’t have access to their preferred food source, allowing them to naturally decline and eventually die off. By leaving an allium or two to rot in the field, you’re inadvertently inviting onion maggots and lesser bulb flies to feast on your plants.
Wireworms
Wireworms, the larvae of click beetles in the Elateridae family, are characterized by their slender and tough-bodied nature. Typically growing no longer than one-and-a-half inches in length, these pests feed on roots and bulbs, causing visible signs of damage such as yellowed and wilted onion tops as they slowly destroy the plant’s root system. While wireworms can be a challenge to manage, utilizing nematodes has been shown to be an effective solution.
Interestingly, the same treatment that controls onion maggots and bulb flies also proves successful in managing wireworm populations.
Common Allium Diseases
Despite their impressive array of anti-viral, anti-biotic, and anti-fungal properties, alliums such as onions, garlic, leeks, and others are susceptible to a wide range of diseases. In fact, they can fall victim to various garden viruses, bacteria, and fungi that have evolved specifically to target these plants. Fungi are the most prevalent cause of allium-related issues, and it’s essential not to assume that your alliums will naturally resist the pathogens that threaten them.
Instead, take proactive steps to protect your crops from these potential threats.
Pink Root
When assessing an allium’s health, it’s not uncommon to encounter a situation where the bulbs and roots appear pinkish in color while the tops are stunted and yellowish. This unappealing display is often a sign of pink root disease (Phoma terrestris), which causes the plant’s roots to shrivel and die. Unfortunately, once this fungal disease takes hold, there is no known cure. The best course of action is to destroy any affected plants completely to prevent further spread of the disease.
To minimize the risk of pink root affecting your alliums, ensure you’re planting them in well-drained soil and provide ample space between each plant. If you’re particularly concerned about the disease, consider applying a copper-based fungicide as a preventative measure. This may help reduce the spread of pink root or prevent a resurgence of the disease.
For those who have experienced issues with pink root in the past, a more effective approach is to choose resistant cultivars like ‘Early White Supreme,’ ‘Tokyo Long White,’ or ‘Super Star’ for your allium plantings.
Fusarium Bulb Rot
Fusarium oxysporum, a fungal disease, is responsible for bulb rot in alliums. While no cultivars are entirely resistant to this affliction, selecting semi-resistant onion varieties like ‘Early Yellow Globe’ or ‘Pulsar’ can significantly minimize the risk of infection. The telltale signs of Fusarium bulb rot include soft, weakened necks and bulbs that have turned brown and rotted with time. If left untreated, the entire plant will eventually succumb to decay.
To prevent the spread of this disease, destroy any affected plants immediately and treat the remaining alliums with a copper-based fungicide to curb its progression.
Onion Yellow Dwarf Virus
The onset of Yellow Dwarf Virus is characterized by the appearance of yellow streaks and crinkled leaves on allium stalks, which are often accompanied by twisted flower stalks that produce few seeds. The virus also causes bulbs to remain undersized. Aphids play a significant role in the spread of this disease, making it crucial to take preventative measures against these pesky insects.
To curb the spread of Yellow Dwarf Virus, gardeners can employ various strategies.
First and foremost, it’s essential to prevent aphid infestations from occurring in the first place. If you do spot aphids in your allium beds, consider spraying them with insecticidal soap or neem oil to disrupt their life cycle. Additionally, using row covers or trap crops can also help to mitigate the issue.
Neck (or Bulb) Rot
When it comes to harvesting allium bulbs, excessive moisture can be a major issue. In fact, it’s not uncommon for too much water right before harvest to lead to a devastating bacterial problem known as neck rot. This condition is caused by the bacterium Pseudomonas viridiflava, which targets the necks of allium bulbs and causes them to rot from the inside out, often accompanied by the development of gray mold.
Damage to the bulbs can create an entry point for the bacteria, making it more common in affected plants. To prevent the spread of this issue, it’s crucial to destroy any infected bulbs you come across. In addition, reducing watering just before harvest and curing bulbs properly before storage can also help minimize the risk of rot developing in storage. As alliums mature, they require less water than younger plants, so make sure to adjust your watering schedule accordingly.
Smut
Allium smut is one of the most significant issues affecting allium crops, leaving young plants with black-streaked stalks filled with a soft, dark brown powder. This fungal disease thrives in cool soil conditions, making it essential to ensure your soil temperature is optimal for growth. To warm up your soil and eliminate pathogens, cover it with clear plastic for a few weeks before planting.
Not only will this increase the soil temperature, but it may also solarize the soil, killing off a range of microorganisms that can harm your crops.
When selecting onion cultivars, consider those that are naturally resistant to smut. Two such options are ‘Evergreen Hardy White’ and ‘Tokyo Long White’, which have been proven to be highly resilient to this disease.
These varieties are particularly suitable for areas with cool spring and early summer temperatures, where smut is a persistent problem.
Onion Smudge
When it comes to alliums like onions and garlic, a common issue is onion smudge, which manifests as dark green or black, ringed spots on the bulbs. This fungal disease, also responsible for anthracnose in other crops, typically appears just before harvest or during storage. To prevent the spread of infection, it’s essential to destroy any affected bulbs. On the plus side, many varieties of yellow and red onions, as well as most garlic, are naturally resistant to onion smudge.
However, white onions are more susceptible to this issue, so it’s crucial to store them separately and opt for disease-resistant varieties if you’ve experienced trouble with smudge in the past.
Purple Blotch
The fungal disease Alternaria porri is characterized by the development of sunken, light-colored spots with concentrically ringed, purple centers on the leaves of alliums. As the spots enlarge and girdle the leaves, they ultimately cause them to wither and fall off. Notably, this disease tends to spread most effectively in cool soil conditions. To combat this issue, consider applying clear plastic to warm up the soil before planting your alliums.
If you do encounter purple blotch on your plants, treat it promptly with a sulfur or copper-based fungicide to prevent further spreading.
Rust
When alliums develop rust, specifically Puccinia porri, small reddish-orange blisters form on their leaves. As the infection progresses, leaves turn yellow and ultimately die off, often resulting in undersized bulbs. It’s essential to note that rust can also affect asparagus plants, making it crucial to keep these two crops separate. To combat rust, apply a copper-based fungicide and maintain a clean garden environment.
Additionally, discard any affected alliums and tops to prevent spores from overwintering. This approach will make it impossible for the infection to persist. If an outbreak occurs, consider crop rotation and solarizing the soil before the next growing season begins to prevent future occurrences.
White Rot
White rot, a fungal disease caused by Stromatinia cepivora, manifests itself in leaves that first turn yellow before wilting and dying back. Meanwhile, beneath the surface, the roots are quietly rotting away, concealed by a characteristic white fungus. To prevent this issue from arising, crop rotation is crucial. While fungicides containing copper can be effective in treating infected crops, it’s essential to apply them regularly.
Unfortunately, once the bulb begins to rot, there is little that can be done to save it.
