ALLIUM DISEASES
I was encouraged to undertake my colleague's challenge to compile comprehensive information on ALLIUM diseases. I wish I hadn’t.
There are 37 species of fungi, three species of bacteria, and one virus recorded from onions in New Zealand
Alliums, such as scallions, spring onions, hard-necked garlic, leeks, and soft-necked garlic, all have common cultivation requirements. These plants thrive in well-drained, fertile soil that receives adequate sunlight. Both onions and garlic prosper in soil with an optimal pH level, ideally ranging from 6.5 to 7.0.
In the event of pest-related issues, implementing crop rotation serves as an effective preventative measure. By relocating your alliums each season, you can disrupt the lifecycle of pests that may have overwintered in the previous season's onion bed.
Garlic is relatively resilient. In contrast, onions are exposed to a broader range of damaging pests. It is important to note that chives, scallions, ornamental alliums, leeks, and shallots tend to experience a higher incidence of pest-related issues.
Sowing Dates: Schedule planting to avoid peak migration periods of insect pests, ensuring that crops are in a less susceptible growth stage during these times.
Crop Rotation: Implement a rotation system to disrupt the life cycles of soil-borne pests and diseases, thereby preventing their accumulation to epidemic levels in crops.
Hygiene Practices: Regularly remove infested plants to prevent the spread of pests and diseases to healthy specimens. Additionally, destroy crop residues post-harvest to minimize carryover effects into the next planting cycle.
Overview of Allium Fungal Diseases
Allium fungal diseases are prevalent and can significantly impact allium crops. These are the most common fungal diseases affecting alliums, along with recommended management practices.
Botrytis
Botrytis is a fungal disease that primarily affects onion crops throughout all stages of growth. As the plant matures, the fungus colonizes the tissue at the neck of the bulb, leading to neck rot. Early in the growing season, Botrytis allii can result in the death of seedlings, while later stages may cause significant tip dieback of foliage. The most severe impact occurs during storage, where it is a leading cause of neck rot in mature onions. Spoiled bulbs often exhibit signs of this disease. Soft rot presents a considerable challenge for growers due to the limited effective management strategies available. Traditionally, copper sprays have been employed; however, they are known to be toxic to the environment. Environmentally friendly Lost Coast Plant Therapy offers a viable alternative.
White Rot
White rot is a disease caused by the fungus Stromatinia cepivora. It manifests as yellowing leaves that wilt and die, accompanied by roots exhibiting rot and white fungal growth. This disease predominantly affects onion plants during their initial 3-4 months of growth and can also lead to bulb rot during storage. The initial symptoms include a change in leaf color to a deep blue-green, followed by yellowing and wilting. White, fluffy fungal growth may become visible on the roots and stem base. To mitigate the risk of white rot, it is advisable to implement crop rotation and regularly apply fungicides. Lost Coast Plant Therapy is a non-toxic alternative to traditional copper sprays and has been shown to be equally effective. Once a bulb has decayed, it is irretrievable.
Downy Mildew
Downy mildew, caused by the fungus Peronospora destructor, is prevalent in New Zealand and can lead to significant crop losses, particularly during periods of wet weather. Characteristic symptoms include the appearance of small, light-colored spots on the leaves, which subsequently turn yellow and then white. Affected leaves ultimately desiccate, resulting in a burnt appearance of the plant. The disease thrives in conditions of showery weather, accompanied by cool night temperatures and relatively warm daytime temperatures. Effective control of the fungus is typically achieved through the application of chemical sprays, and a protective spray program using Lost Coast Plant Therapy every two weeks as a preventive strategy will dry spores on contact and provide continuous protection.
Stemphylium blight is characterized by the presence of yellow, wet spots on the leaves of affected onions. As the disease progresses, these spots enlarge and transition in color from light brown to dark brown or black. To mitigate the risk of this disease, it is essential to maintain adequate spacing between plants and ensure proper soil drainage. Utilizing Lost Coast Plant Therapy every two weeks provides substantial mitigation.
Fusarium Bulb Rot
Fusarium bulb rot, caused by Fusarium oxysporum, primarily impacts the bulbs and stems of onion plants. Symptoms include soft, weakened necks and bulbs that display browning and decay. It is imperative to promptly remove affected plants, and the remaining healthy specimens should be treated with a fungicide to mitigate the risk of further disease spread. Lost Coast Plant Therapy offers an effective nontoxic alternative avoiding the toxicity associated with copper-based sprays.
Allium rust
Rust is a pervasive fungal disease that affects the entire allium family, including garlic and onions. This disease can lead to the development of smaller-than-normal bulbs and hindered plant growth. Alliums afflicted by rust (Puccinia porri) exhibit small, reddish-orange blisters on their leaves, which ultimately turn yellow and die, resulting in reduced bulb size. Rust can also adversely affect asparagus. Treatment necessitates the application of a fungicide, and we recommend Lost Coast Plant Therapy, alongside maintaining garden hygiene by properly disposing of all plant debris.
Pink Root
The presence of pink roots and stunted, yellow tops indicate an infection of pink root, caused by the fungus Phoma terrestris. This disease leads to the shriveling and death of roots, and unfortunately, there is no cure once it establishes. Affected plants should be destroyed, and proper spacing along with well-drained soil should be ensured to prevent recurrence.
Onion Smut
Onion Smut, Onion Smudge, and Sclerotium reside in soil, and while seed fumigation ensures sterility, soil fumigation is the only course of action for these fungal diseases. The fungus can be eradicated from soil by fumigation with methyl bromide.
Due to the presence of the disease in the Pukekohe district posing a threat to the export trade, all affected areas were fumigated. The use of benomyl-treated seed is still recommended in those areas which previously contained the disease.
Onion Smudge
Onion smudge manifests as dark green or black spots on the bulbs, caused by the same fungus responsible for anthracnose in other crops. This condition typically appears before harvest or during storage. It is imperative to remove all infected bulbs promptly. Yellow and red onion varieties show resistance, while white onions are more susceptible and should be stored separately.
Sclerotium Rolfsii
Losses due to this fungus are normally insignificant except when very warm, wet conditions occur during maturation and curing. Provided onions are well cured at harvest, no problems should occur in storage. Control measures are not considered necessary.
INSECT PESTS ON ALLIUMS
ONION THRIPS
Onion thrips represent a significant insect pest affecting onion crops and potentially other alliums. These pests feed on the foliage of onion plants, leading to the development of silvery scarring and a reduction in the overall green leaf tissue. This adversely impacts the onion's photosynthetic capacity, resulting in smaller bulb sizes and diminished yield. The feeding activity of onion thrips creates open wounds on the plant tissue, which can serve as entry points for various plant pathogens.
Onion thrips are known to disseminate a range of plant pathogens, including viral, bacterial, and fungal agents. Therefore, it is imperative to initiate monitoring of onion crops for thrips at an early stage to detect the onset of population increases.
Lost Coast Plant Therapy is an effective treatment that suffocates the insects and desiccates their eggs; however, it is important to note that it is not systemic, so no resistance emerges. Comprehensive coverage with Lost Coast Plant Therapy is essential, as thrips are frequently located in the densely packed center of the plant.
ALLIUM APHID
Aphids, particularly Onion Aphids, belong to a family that poses a threat to many plants. Lost Coast Plant Therapy has proven to be highly effective in controlling Aphids, whether they are affecting your cherished roses, ornamentals, or vegetable gardens. The organic oils in the spray work by suffocating the insects, preventing the development of insecticide resistance. This economical spray effectively combines drying agents for the eggs with suffocating oils, offering a convenient solution that integrates both insecticidal and fungicidal properties.