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Module 23: Plant Pests, Diseases & Weed Control

When it comes to gardening, particularly if you are new to gardening, these three words can be quite scary to be confronted with, though there is no need to worry! Here is a guide on identifying, treating and preventing diseases, pests and weeds from affecting your plants while not affecting your plants adversely (or the environment where possible.)

Learning Objectives

  • Learn how to identify different plant diseases
  • Identify different plant pests
  • Understand how to remove and prevent weeds in the garden

Diseases

Plant diseases can be detrimental to your plants if allowed to spread, but there are some easy ways to identify and treat these diseases and prevent them from occurring or spreading in the first place. It all depends on what type of plants you have and where they are grown, but there are ways to reduce any harm to your plants while still being conscious of your impact on the environment and on the surrounding wildlife, too. 

How do we identify plant diseases?

The first step in identifying plant diseases is paying attention to how your plant is supposed to look at different times of the year. These live plants will all look different depending on the weather, the seasons and other factors such as their aspect and location. To be able to get a headstart on identifying and treating any diseases which may, unfortunately, crop up, it is best to monitor your plants over time, to ensure that you know what they are supposed to look like.

If you are concerned, we have many useful guides that will explain how different plants look at different times of the year. Some plants bronze over in the winter due to the harshness of the cold weather, others are not supposed to. This is the distinction between whether plants are supposed to look brown or whether there may be some underlying issue that kills off your plants. 

Box Blight

Box Blight is one of those instances where the plants are not supposed to be turning brown and drying up. This disease affects Buxus or box plants, often used as topiary decorations. This fungus disease will cause the box leaves to turn brown and die, falling off and leaving bare patches on the Buxus plant concerned. Some Buxus will bronze over in the winter and turn brown, but their leaves will not drop off, leaving sparse patches, which is the key difference between the two conditions.

Other symptoms of Box Blight include black streaks on stems and, in wet conditions, there may be white spores from the fungus which are visible. You can control Box Blight by pruning off affected foliage and destroying it, or by using fungicides available to the public. Always remove and clear away any affected foliage that has been pruned away. 

Rust

Rust is a disease which affects a wide range of plants. It is also one of the most prevalent fungal diseases that plants can get. Unfortunately, rust diseases will often weaken plants and look unsightly when affected – in some cases, they can also kill the infected plants. The first sign of rust infection on your plants will be pustules/structures that will produce spores. These will emerge from pale leaf spots ranging from orange and yellow to black, brown or white. The most usual colour for rust pustules is rusty-brown, hence the name.

While it may seem quite daunting to face a rust infection in your plants, you can do so by removing affected leaves or limbs and using chemical control in the form of fungicides. Ensure to clean any pruning tools between removing affected foliage and then pruning other plants to prevent cross-contamination. It is always best to destroy the removed foliage as well. 

Mildew

Mildew is another fungus which can be identified by the white, powdery fungus which will appear on the leaves of your plants. This is a fungus which likes to grow in warm, humid areas. The best way to prevent mildew from appearing is to ensure that your plants are grown in a well-aerated space and that they are not being watered on top of the leaves too much, as when the water settles, this can create moisture in between leaves and branches which can then create the perfect environment for mildew to develop. If you do spot mildew on your plants, this can easily be rectified by wiping down the leaves and stems which are affected, with a chemical or non-chemical fungicide (there are lots of home remedies available to make in order to control mildew infections.) Or by removing affected foliage and destroying this. 

Treating diseases such as the three fungal diseases mentioned above doesn’t need to be too stressful, as targeted treatments are widely available for most, including some recipes for homemade fungicides. It is always best to destroy the foliage that you prune away, clean your pruning equipment and do not add the discarded foliage to the compost pile! This could spread the disease to the rest of your plants when you come to add the compost to them. 


Pests

Pests are another inconvenience to the gardening community, and a little more difficult to control than diseases in some cases as pests will go through various life stages with lots of them laying eggs which may be undetected until they hatch and create a problem. The best practice is to inspect all of your plants prior to planting and check over them for any possible traces of pests when they are planted out. During the warmer months, this is especially crucial as this is when the pests will generally be causing their havoc.

Weevils

Vine weevils in particular are quite common with a lot of plants and will often target pot-grown plants with succulent stems and leaves, including fruits and ornamental plants as well as indoor plants. Adult vine weevils are about 9mm long and are black with yellow markings on their wing cases. They can cause notches in leaf margins during the summer. Their larvae are often located in plants’ soil and are a creamy-white colour. The eggs, unfortunately, are very difficult to spot in the soil as they are only around 1mm, if not smaller. The best way to get rid of these weevils is to remove them manually – this is best done in the nighttime as they will come out to feed in the night as they are nocturnal. You can also use specifically formulated vine weevil control if a larger population of weevils is found. 

Aphids

Aphids can come in a few different colours, depending on the variety, with a pear-shaped body and ranging from black to pink – the most common colours, though green or brown. They will cause stunted growth in plants after sucking the sap from the plants that they have infested. They will often secrete a sticky substance called honeydew which ants love which can also cause problems with plants being eaten or mould growing on the honeydew if the ants haven’t got to it yet. You can spray aphids with water or knock them into a bucket of soapy water to get rid of them. You can also use soap sprays, neem oil or essential oils. Ladybirds, green lacewings and birds all like to eat aphids and will generally do a lot less damage to the plants concerned, so if biological pest control suits you, this is something to consider. 

For most pests, the best way to control their population is by inspecting your plants frequently and spraying them with a soapy solution, as the pests will find it difficult to cling to the plants afterwards. 

Other Pests

Pests do not just include insects, there are also animals such as birds, cats and other wildlife which may be interested in your plants – particularly those which produce fruits. Ways to prevent this from happening are as follows:

Birds: The old ways are the best ways to deal with them, visual deterrents are the way forward for birds, such as reflective objects and also things like scarecrows, as silly as it may sound!

Cats: Strong smells such as citrus and coffee grounds are good to keep cats away as they cannot stand the smell – you can also get a noise deterrent to help, which will keep away other animals such as foxes. Some plants may also emit a strong enough smell to keep the cats at bay. 

Other wildlife: Fencing can keep out other wildlife and install mesh around the plants, though this may seem unsightly. The best thing to do is identify the culprit first and act accordingly, targeting that specific pest. 


Weeds

Weeds can be a nuisance in the garden; spending hours pulling them up by the roots and using weed killer are some of the ways to get rid of the weeds already there, but some plants that you want to grow may be caught in the crossfire with these methods. 

Removal

The first step to rid your garden of weeds and keep them gone is to remove them by the root – ensure that you dig deep enough if you need to use gardening tools to get the whole weed gone in one go; otherwise, you may spend a long time finding bits of roots in the soil still. You can also use a weed killer if they are not surrounding the plants you wish to keep, which can also affect the plants you want to keep!

Prevention

There are a few ways to prevent the weeds from growing back around your plants:

  • Mulching will help to keep the weeds away as this will leave not as much room for them to grow – using organic mulches such as bark or wood chips will smother weeds and stop them from being able to grow. Ensure that you keep woody-stemmed plants away from the mulch to prevent rotting. 
  • Weed-suppressant materials such as spun/woven materials and also plastic sheeting can be used to prevent the growth of weeds all of the different materials that you can use will have different advantages and disadvantages so it is always good to do your research and ensure that you are comparing all of the options. 

Coming Next

Identifying, treating and preventing diseases, pests and weeds is not a difficult task to be scared of, and there are many options to be able to utilise to ensure that your plants can grow well and unafflicted and hopefully, now you have a better understanding of a few different options that you can implement in your garden to ensure that everything can continue to grow nicely. 

Updated on March 5, 2024

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