Asparagus diseases and common pests: identification & treatment
Asparagus is one of the most prized vegetables of early summer. However, there are some asparagus pests and diseases that can not only reduce the yields but also wipe out an entire crop. Read on to learn more about possible problems and what to do if these pests or diseases strike.
Growing your own asparagus (Asparagus officinalis) is a long-term investment that can be very rewarding. Picking your own asparagus is not only fun but also healthy, thanks to the nutritional value of asparagus. Sadly, asparagus can be attacked by several pests and diseases, which if not properly addressed, can dishearteningly damage the crop after all the hard work of growing it.
In the UK, the most common asparagus diseases you may face include asparagus rust (Puccinia asparagi), grey mould (Botrytis cinerea), and crown or root rot, which are all caused by various fungi.
Asparagus rust is identifiable by orange-yellow spots on the stems and spears, which begin in early summer and later turn red-brown before darkening to almost black. As a fungal disease, asparagus rust can spread rapidly, leading to reduced growth and yields. If you notice a case of it, remove any affected material, especially at the end of the season as the spores can overwinter on the foliage. There are no chemical controls available for the home gardener, so the best way to avoid asparagus rust is to choose cultivars of asparagus that are resistant.
Grey mould (Botrytis cinerea) is a fungal disease that can affect all manner of plants and not just asparagus. Grey mould often enters via a wound, and is encouraged by warm weather, particularly in wet and humid conditions. Symptoms include a layer of grey or brown mould on the asparagus stems or foliage and subsequent wilting, as the fungi impair growth. Unfortunately, there are no fungicides approved for home use if grey mould is suspected. To help prevent grey mould, plant asparagus crowns at the correct spacing. This will increase air circulation and enable you to identify and clear away any infected plants more easily. Plant crowns on ridges to increase drainage. Only use sharp and sanitised tools when pruning. Provide plenty of moisture and nutrients throughout the growing season to promote robust plants that are less susceptible to grey mould. If you do find asparagus spears with grey mould, throw them away rather than eating them.
Crown or root rot
Although perhaps less common for the home grower, crown or root rot can also be a problem when growing asparagus. Violet root rot (Helicobasidium purpureum), Phytophthora root rot and Fusarium pathogens can all affect the crowns or roots of asparagus plants with devastating effects. As soil-borne diseases, they can live in the soil for over ten years and can lead to crooked, bent, or lesioned spears and rotting root systems. Most prevalent in cold, wet and waterlogged conditions, poor drainage and soil contamination often have a large part to play. As with grey mould, controlling any weeds and correctly fertilising asparagus plants can help prevent stress and falling victim to root rot. If you discover root rot, dig up the affected plants and carefully dispose of them. Avoid planting new asparagus plants in the contaminated soil.
Tip: planting asparagus crowns at the recommended spacing and on a new site with free-draining soil can help prevent some of the most common asparagus diseases.
The most common asparagus pests
Pests can also be a problem when growing asparagus. Slugs and snails (Gastropoda), deer, and weeds can all be an issue, but perhaps the most commonly faced and destructive pest is the asparagus beetle (Crioceris asparagi).
When it comes to asparagus plant care, the common asparagus beetle (Crioceris asparagi), even though small, is a serious pest. A member of the leaf beetle family (Chrysomelidae), both the asparagus beetle and its larvae eat the foliage and stem bark. After overwintering, the red-bodied beetles with six cream-coloured spots tend to appear towards the end of the harvesting period in May. They lay their elongated black eggs on the sides of the spears, from which the greyish asparagus beetle larvae hatch shortly after, before maturing and overwintering again. Another asparagus pest, the spotted asparagus beetle (Crioceris duodecimpunctata) is problematic in mainland Europe but currently relatively rare in the UK.
A small number of beetles is unlikely to affect future yields and can be tolerated or picked off by hand. However, if heavy infestations occur, the beetles and their larvae can strip the foliage resulting in brown stems and heavily reduced harvests the following year. Thus, it is important to remain vigilant and if numbers start to increase, it may be necessary to apply an organic contact insecticide that includes natural pyrethrins. Although contact insecticides can prove effective, they only have a short persistence and may need to be used regularly. Before resorting to chemicals, encourage birds into the garden as a first-line defence against asparagus beetles because they feed on both the larvae and adult beetles.
Although more of a problem for rural gardeners, deer have been known to devour the fresh small asparagus spears that emerge in spring. Even if asparagus is not their first choice, there may be little else growing nearby, and an asparagus bed can provide an easy meal. If deer are an issue in your garden, then either a perimeter fence or a barrier around the asparagus may be enough to put them off. Nevertheless, it needs to be high because deer can easily jump well over a metre high.
Slugs and snails
Similar to deer, asparagus may not be a slug or snail’s favourite food, but the fresh tender shoots are certainly vulnerable to being eaten. If slugs and snails are a problem in your garden, you are probably well aware of the damage they can do, especially in spring as the new growth appears.
Preventative measures can include picking them off by torchlight in the evening, encouraging predators such as frogs and birds into the garden, and laying barriers of sharp grit or eggshells around the plants. If these measures fail, biological nematodes can be applied, which can prove surprisingly effective. Solely targeting the molluscs, they are harmless to plants, but do need to be applied correctly and at the right temperature in order to be effective against slugs and snails.
Asparagus plants do not tolerate weeds well, as the weeds compete for not only asparagus’s nutrients but for space as well. If allowed to thrive, weeds reduce airflow, leading to damp ground conditions after heavy rain, as well as harbouring diseases. As the new spears appear in April, annual weeds can germinate and take over rapidly if not kept under control. Hand weeding is the best option as the asparagus crowns and roots under the soil can easily be damaged by hoeing.